1
Introduction
Arsenic is a trace element
found in the earth’s crust at
an average concentration of ∼5 μg/g (ppm). Although its
relative abundance in the earth’s crust is about 54th, arsenic
can become concentrated in some parts of the world because of natural
mineralization. Arsenic is a component of 245 minerals, associated
most frequently with other metals such as copper, gold, lead, and
zinc in sulfidic ores.
1−3
When disturbed by natural processes, such as weathering,
biological activity, and volcanic eruption, arsenic may be released
into the environment. Anthropogenic activities, such as combustion
of fossil fuels, mining, ore smelting, and well drilling, also mobilize
and introduce arsenic into the environment.
Chronic exposure
to arsenic from groundwater has been recognized
to cause the largest environmental health disaster in the world, putting
more than 100 million people at risk of cancer and other arsenic-related
diseases.
4,5
Because of its prevalence in the environment,
potential for human exposure, and the magnitude and severity of health
problems it causes, the United States Agency for Toxic Substances
and Disease Registry (ATSDR) has ranked arsenic as No. 1 on its Priority
List of Hazardous Substances for many years. The recent priority list,
posted in 2011 (http://www.atsdr.cdc.gov/SPL/index.html), shows arsenic as No. 1,
ahead of lead, mercury, and polychlorinated
biphenyls (PCBs).
Epidemiological studies of populations exposed
to high levels of
arsenic due to ingestion from water, including those from Taiwan,
6−8
Argentina,
9,10
Chile,
11,12
West Bengal, India,
13,14
Bangladesh,
15−17
and Inner Mongolia,
China,
18,19
have repeatedly shown strong associations
between the exposure to high concentrations of arsenic and the prevalence
of several cancers,
20−23
most severely bladder, lung, and skin cancers. Arsenic is classified
as a human carcinogen by the International Agency for Research on
Cancer (IARC) and the U.S. Environmental Protection Agency (EPA).
Chronic exposure to elevated concentrations of arsenic has also been
associated with the increased risk of a number of noncancerous effects.
24−27
Although the adverse health effects arising from exposure to arsenic
have been well-recognized, the mechanism(s) of action responsible
for the diverse range of health effects are complicated and poorly
understood.
26−30
It is believed that inorganic arsenate (HAsO4
2-), which is a molecular analogue of phosphate (HPO4
2-), can compete for phosphate anion transporters
and
replace phosphate in some biochemical reactions.
28
For example, generation of adenosine-5′-triphosphate
(ATP) during oxidative phosphorylation can be inhibited by the replacement
of phosphate with arsenate. Depletion of ATP by arsenate has been
observed in cellular systems.
28
However,
the replacement of phosphate in DNA by arsenic is not firmly established.
31−35
The toxicity of trivalent arsenicals likely occurs through
the
interaction of trivalent arsenic species with sulfhydryl groups in
proteins. Arsenic binding to a specific protein could alter the conformation
and function of the protein as well as its recruitment of and interaction
with other functional proteins. Therefore, there has been much emphasis
on studies of arsenic binding to proteins, for the purpose of understanding
arsenic toxicity and developing arsenic-based therapeutics.
This review summarizes various aspects of arsenic binding to proteins.
It discusses the chemical basis and biological implications and consequences
of arsenic binding to proteins. It also describes analytical techniques
and the characterization of arsenic binding, including the binding
affinity, kinetics, and speciation.
2
Chemical
Basis and Biological Implications of
Arsenic Binding to Proteins
Trivalent arsenicals have high
affinity for sulfhydryl groups and
can bind to reduced cysteines in peptides and proteins.
36−60
Figure 1 illustrates schematically the binding
of inorganic arsenite (iAsIII), monomethylarsonous acid
(MMAIII), and dimethylarsinous acid (DMAIII)
to cysteine residues in proteins. This chemical basis has been well-known.
For example, the toxicity of many chemical warfare agents in use during
and after World War I, such as arsine halides, was based on their
binding to protein dithiols.
61−63
Lewisite (β-chlorovinyldichloroarsine)
was known to react with some enzymes by forming a ring involving two
thiol groups in the enzymes.
64
To counteract
the toxicity of these poison gases during World War I, the British
developed dithioglycerol (British anti-lewisite, BAL), which could
bind arsenic to form very stable complexes, preventing arsenic from
binding to proteins.
Figure 1
Binding of inorganic arsenite (iAsIII), monomethylarsonous
acid (MMAIII), and dimethylarsinous acid (DMAIII) to cysteines in a protein.
Arsenic binding to a specific protein could alter the conformation
of the protein, resulting in loss of its function, and affect its
recruitment of and interaction with other proteins and DNA. Figure 2 shows a model
of iAsIII binding to the
Escherichia coli
repressor protein ArsR. In each
subunit, two cysteine residues are within an α-helix, which
must unravel from one end in order to bind iAsIII. Melting
of the helix produces the conformational change that dissociates the
repressor from the DNA, inducing gene expression. This ArsR binds
iAsIII at three cysteine residues, Cys32, Cys34, and Cys37.
Cys32 and Cys34 are within the α1 helix in the DNA binding site.
Their thiolates are oriented such that iAsIII cannot bind
until the helix unwinds, bringing all three thiolates into position
to form the three-coordinate iAsIII binding site. Unwinding
the helix disrupts DNA binding, resulting in dissociation of the repressor
from the operator site and hence gene expression.
65
Figure 2
Binding of iAsIII to the ArsR repressor from
E. coli
plasmid R773 (P15905) results in conformational
change of the repressor. iAsIII binds to Cys32, Cys34,
and Cys37 of the ArsR repressor. Unwinding the helix disrupts DNA
binding, resulting in dissociation of the repressor from the operator
site. Dissociation of the repressor induces gene expression. (Adapted
from ref (65).)
The importance of direct interaction
of arsenic compounds with
proteins has also been highlighted by the observation that the successful
treatment of patients suffering from acute promyelocytic leukemia
(APL) with As2O3 is likely due to the binding
of arsenic to cysteine residues in zinc fingers of the aberrant promyelocytic
leukemia-retinoic acid receptor (PML-RAR) fusion protein expressed
by these patients.
66,67
2.1
Arsenic
Biomethylation
Biomethylation
is the major metabolic pathway for inorganic arsenic (iAs) in nearly
all organisms, including humans and most animal species.
68
Arsenic biomethylation involves specific reductases
27,30,69−73
and methyltransferases.
50,68,74−78
The major methyltransferase responsible for arsenic
biomethylation is arsenic (+3 oxidation state) methyltransferase (AS3MT,
previously named Cty19). This enzyme catalyzes transfer of the methyl
group of S-adenosylmethionine (SAM) to trivalent
arsenic.
50,68,74,75
The crystallographic structure of AS3MT from the
thermophilic eukaryotic red alga
Cyanidioschyzon merolae
has recently been obtained.
75,79
It reveals binding
domains for arsenic and SAM (Figure 3). Although
a crystal structure of human AS3MT is not available, Cys156 and Cys206,
among the 14 cysteine residues, were demonstrated to be the active
sites of human AS3MT.
80−82
Figure 3
Ribbon representation of ligand-free AS3MT (CmArsM) from
the thermophilic
eukaryotic alga
C. merolae
. N and C
indicate the N- and C-terminal domains and are colored blue and red,
respectively. Cysteine residues are shown as balls and sticks and
colored green (carbon) and yellow (sulfur). Three cysteine residues,
C72, C174, and C224, are believed to be involved in arsenic binding.
(Reprinted with permission from ref (75). Copyright 2012 American Chemical Society.)
2.2
Enzyme
Inhibition
Arsenic may inactivate
up to 200 enzymes.
83,84
Most of the enzyme inhibition
studies used iAsIII and some used organic arsenicals, particularly
phenylarsine oxide (PAO). Enzymes that were shown to be inhibited
by arsenic include glutathione reductase,
49,85−87
glutathione S-transferase,
88
glutathione peroxidase,
88
thioredoxin reductase,
89−91
thioredoxin peroxidase,
92
DNA ligases,
93
Arg-tRNA
protein transferase,
94,95
trypanothione reductase,
86
IκB kinase β (IKKβ),
96
pyruvate kinase galectin 1,
97
protein tyrosine phosphatase,
98−101
JNK phosphatase,
102
Wip1 phosphatase,
103
and E3 ligases c-CBL and SIAH1.
104
2.2.1
Pyruvate Dehydrogenase (PDH)
The
enzyme complex pyruvate dehydrogenase (PDH) is one of the most studied
enzyme systems. A search for natural dithiol compounds to mimic BAL
led to the discovery of its cofactor, lipoic acid.
105
PDH complexes from prokaryotes and eukaryotes, which have
molecular weights in excess of 106 Da, are formed by noncovalent
interactions between multiple enzymes. Arsenite inhibits the PDH complex
by binding to the lipoic acid moiety. MMAIII was shown
to be a more potent inhibitor of the PDH complex than iAsIII.
105,106
The PDH complex oxidizes pyruvate to acetyl-CoA,
a precursor to intermediates of the citric acid cycle. The citric
acid cycle degrades the intermediates, providing the mitochondria
with reducing equivalents needed for electron transport and the subsequent
production of ATP. Inhibition of the PDH complex can block the citric
acid cycle and ultimately decrease the production of ATP, resulting
in cell damage and death. On the basis of the fact that As2O3 was far more inhibitory
to the intracellular PDH complex
than to the purified PDH complex (actually similar phenomena were
observed for other enzymes), the PDH complex (and perhaps other dithiol-containing
enzymes) was proposed by Samikkannu et al.
107
to be inhibited by arsenite via reactive oxygen species (ROS) production
rather than direct thiol binding as in the case of PAO. However, a
recent study
105
conducted under conditions
free of ROS generation suggested that all three trivalent arsenic
species (iAsIII, MMAIII, and DMAIII) at biologically relevant concentrations inhibited
the PDH complex
and the 2-oxoglutarate dehydrogenase (KGDH) complex by direct binding.
The reduced form of the lipoic acid moiety appeared to be essential
for the binding. No pentavalent arsenicals (iAsV, MMAV, and DMAV) were found to inhibit
the enzymes.
All enzymes post-translationally modified by the addition of the lipoic
acid cofactor are mitochondrial and are important for respiration.
105
It is conceivable that trivalent arsenic binds
to PDH or KGDH complexes and inhibits their activity, which depletes
the mitochondrial NADH pool and leads to oxidative stress, producing
ROS, which is otherwise suppressed or balanced. Thus, the binding
of arsenic to those enzymes may be the initial point of action to
inhibit the activity of the enzymes. However, subsequently generated
ROS may have further effects on the enzymatic inhibition.
2.2.2
Thioredoxin
The direct binding
between thioredoxin (Trx) and arsenic has been characterized using
mass spectrometry.
108
All of the four trivalent
arsenic species under study (iAsIII, MMAIII,
DMAIII, and PAO) bound to human Trx to form complexes at
room temperature while only MMAIII, DMAIII,
and PAO formed complexes with
E. coli
Trx, consistent with arsenic coordination chemistry and the availability
of cysteine residues in these two proteins for binding. Trx proteins
from both human and
E. coli
have the
highly conserved −CysGlyProCys– region, but human Trx
has three more cysteine residues (Cys62, Cys69, and Cys73). Human
Trx showed slightly higher binding constants than
E.
coli
Trx to the same arsenic species.
108
Pentavalent arsenic species (iAsV, MMAV, and DMAV) could be reduced by excess
Trx at 37 °C to their trivalent counterparts, which in turn could
bind to Trx.
The thioredoxin system consists of two oxidoreductase
enzymes, thioredoxin and thioredoxin reductase.
109
The thioredoxin system complements the function of the
glutathione system in maintaining reductive intracellular redox potential
and protecting against toxicity. Thioredoxin is a small protein (∼12
kDa) containing a highly conserved sequence, −CysGlyProCys–.
It was first identified in
E. coli
and
subsequently found to be present in most eukaryotic and prokaryotic
species.
109
Thioredoxin is the major ubiquitous
disulfide reductase responsible for maintaining proteins in their
reduced state, in which process thioredoxin itself is oxidized and
subsequently regenerated at the expense of NADPH via thioredoxin reductase.
Thioredoxin has been implicated in a multitude of biological activities.
110
As an electron carrier, thioredoxin activates
the catalytic cycles of biosynthetic enzymes, such as ribonucleotide
reductases, methionine sulfoxide reductases, and sulfate reductases.
It protects cytosolic proteins from aggregation or inactivation via
formation of intra- or intermolecular disulfides. Thioredoxin can
regulate the activity of transcription factors such as NF-κB,
AP-1, and p53. Thioredoxin plays a central role in thiol redox control
of cell functions by modulation of the transcription events of target
genes. It can interact with other proteins to form functional protein
complexes. For example, thioredoxin is an essential processivity factor
for bacteriophage T7 DNA polymerase. Its activity has been found outside
of the cell, in the cytoplasm, in the nucleus, and in the mitochondria.
111
As an important cellular dithiol redox
enzyme, thioredoxin is required
for the methylation of arsenic catalyzed by AS3MT.
112,113
Trivalent arsenic compounds, especially the methylated metabolite
MMAIII, were found to inhibit thioredoxin reductase in
cultured rat hepatocytes.
90
Arsenite was
also shown to induce NF-kB activation and to affect AP-1 and NF-κB
DNA binding activity by upregulating the expression of the Trx gene.
114
Thioredoxin peroxidase
II was identified as an arsenic-binding protein in mammalian cells.
92
Thioredoxin stimulates cell growth and has antiapoptotic
properties. Modulation of thioredoxin was implicated in the apoptosis
of APL cancer cells treated with arsenic trioxide (As2O3).
89
2.2.3
DNA
Repair Enzymes
All three trivalent
arsenicals (iAsIII, MMAIII, and DMAIII) can inhibit DNA repair.
115−117
These trivalent arsenicals were
found to inhibit the activities of several DNA repair proteins, including
poly(ADP-ribose) polymerase-1 (PARP-1), formamidopyrimidine-DNA glycosylase
(Fpg), and xeroderma pigmentosum group A protein (XPA), each containing
a zinc finger DNA binding domain.
118,119
The interactions
between these repair proteins and arsenic compounds resulted in the
release of zinc. As the first identified member of the PARP superfamily,
PARP-1 is believed to mediate the major fraction of cellular poly(ADP-ribosyl)ation.
Although its role is not fully understood, there is strong evidence
that PARP-1 contributes to base excision repair (BER), perhaps also
to nucleotide excision repair (NER)
120,121
and to the
nonhomologous end joining pathway of DNA double-strand break (DSB)
repair.
122
PARP-1 contains two zinc fingers,
where each zinc is complexed through three cysteines and one histidine
residue (C3H1). Zinc fingers are essential for the recognition of
damaged DNA. Recently, the direct binding between arsenic and peptides
derived from both zinc fingers of human PARP-1 (apo-PARPzf) was observed
using matrix-assisted laser desorption ionization mass spectrometry
(MALDI-MS).
45,123
A 72-Da m/z shift corresponded to the covalent binding of iAsIII to the PARPzf peptides
after the release of three hydrogen
atoms from the three cysteine residues of the C3H1 zinc finger peptides.
Displacement of Zn2+ from the PARPzf peptides by iAsIII was also demonstrated to be
dependent on the iAsIII concentration.
XPA binds specifically to damaged DNA and plays
a central role in the first step of NER. XPA contains a single C4
zinc finger domain and zinc is complexed by Cys105, Cys108, Cys126,
and Cys129. Trivalent arsenicals (iAsIII, MMAIII, and DMAIII) were shown to release
zinc from the zinc
finger domain of the XPA protein (XPAzf). Methylated trivalent arsenicals
were more efficient than inorganic arsenite in releasing zinc.
115
It was further demonstrated that equimolar
MMAIII could react with XPAzf and yield a mixture of mono-
and diarsenical adducted XPAzf.
124
The
unprotected thiols in the MMAIII-XPAzf were further oxidized
to form an intramolecular disulfide. There was no complex formation
between iAsIII and XPAzf.
2.3
Resistance
to Arsenic
All cells have
regulatory mechanisms for controlling the intracellular concentrations
of metals and metalloids. Metallothioneins, found in both eukaryotes
and prokaryotes, are small proteins (2–7 kDa) high in thiolate
sulfurs that bind metal ions for storage and/or for detoxification.
Phytochelatins are present in plants and some fungi.
125
They are cysteine-rich peptides that bind heavy metals
and metalloids for detoxification, such as in the case of hyperaccumulation
of arsenic.
126−128
By contrast, metallochaperones constitute
a different type of metal-binding protein, serving as an intracellular
shuttles for metal ions. Metallochaperones sequester metals in the
cytoplasm and deliver them to protein targets.
129
The targets can be metalloenzymes, where metals are utilized
as enzyme cofactors, or membrane transporters, which predominantly
transport ATPases. These metallochaperones are involved in metal ion
homeostasis and detoxification.
129
2.3.1
Metallothionein
Metallothioneins
(MT I and II) are highly inducible by metals (such as Zn2+, Cd2+, Co2+, Ni2+, Ag+,
Hg2+, and Bi3+), alkylating agents, oxidizing
agents, glulcocorticoids, and inflammatory signals.
130−138
Arsenic was also reported to induce metallothioneins.
139−144
The mechanism by which arsenic induces metallothioneins remained
unclear until recently when metal-activated transcription factor 1
(MTF1), a member of the zinc finger family, was found to be involved
in Mt1 induction.
130
iAsIII and PAO covalently bound the C-terminal cysteine cluster
(Cys632, Cys634, Cys636, Cys638, and Cys653) of MTF1, which in turn
induced the binding of MTF1 to the metal response elements (MRE) of
endogenous Mt1.
Metallothioneins have also
been found to directly bind to many metals such as Zn2+, Cd2+, Co2+, Ni2+, Fe2+, Cu+,
Ag+, Au+, Pt2+, Bi3+, Hg2+, and TcIVO, either
in vivo or in vitro.
145−148
The direct binding between metallothioneins and arsenic was studied
by several research groups.
149−159
In a large excess of arsenic over metal-free human metallothionein
2 (apo-hMT-2), up to six iAsIII could bind to hMT-2 (Figure 4), on the basis of results
obtained from inductively
coupled plasma atomic emission spectrometry (ICP-AES), UV absorption
spectrometry, MALDI-TOF-MS, and electrospray ionization (ESI) TOF-MS.
149,152,155
Evidence from MALDI-TOF-MS
149
and ESI-TOF-MS
151,152,155
also suggested that the binding of one iAsIII resulted in the loss of three protons
from the protein. The binding
stoichiometry was consistent with the fact that 20 cysteine residues
are contained in mammalian metallothioneins and maximally six iAsIII can bind to the
protein.
Figure 4
A model depicting six arsenic atoms bound
to 18 cysteines in metallothionein.
(Reprinted with permission from ref (155). Copyright 2008 American Chemical Society.)
A detailed binding study between
six arsenic species (iAsIII, MMAIII, DMAIII, iAsV, MMAV, DMAV, and
TMAOV) and rabbit metallothionein
II (rMT II) was conducted by use of HPLC–ICP-MS and ESI-quadrupole-TOF-MS.
151
The binding stoichiometry revealed by the complementary
mass spectrometry data demonstrated that each apo-rMT II molecule
could bind up to six iAsIII, 10 MMAIII, and
20 DMAIII, consistent with the coordinate chemistry of
the individual trivalent arsenicals (Figure 5). Because the reduced product, trimethylarsine,
from TMAOV, has no binding sites for thiol groups, no binding was observed
from the reaction between MT and TMAOV. As expected, no
binding occurred between apo-rMT II and pentavalent arsenic species.
A downward mass shift of MT was observed in the presence of pentavalent
arsenicals, however, implying the oxidation of −SH to disulfide
in MT, associated with reduction of pentavalent arsenicals to their
trivalent counterparts.
Figure 5
Mass spectra showing the number of monomethylarsonous
acid (MMAIII) molecules bound to metallothionein (MT).
The solutions
contained 7 μM metallothionein (rMT-IIa) and increasing concentrations
of MMAIII. The concentration ratios of MMAIII to MT in the solutions were (a) 1:5,
(b) 1:1, (c) 5:1, and (d) 50:1.
The ions carrying 5+ and 4+ charges are shown.
The numbers on the peaks represent the number of MMAIII bound to the MT molecule.
For example, peak 6 represents MT-(CH3As)6. A maximum of 10 MMAIII molecules
were bound to a single MT that contained 20 cysteines. (Adapted from
ref (151).)
2.3.2
Ars Operon
Resistance
to arsenite
and antimonite in many prokaryotes is conferred by an ars operon,
which often contains three genes, arsR, arsB, and arsC. Some operons have two extra
genes, arsD and arsA, first identified in the
E. coli
plasmid R773 ars operon.
160−162
Cells expressing the arsRDABC operon are more resistant to iAsV and iAsIII than those
expressing the arsRBC operon.
The first two genes, arsR and arsD, encode regulatory proteins. The arsA gene encodes
the transport ATPase and the arsB gene encodes the
membrane translocase subunit of an arsenic pump. The last gene of
the operon, arsC, encodes an arsenate reductase that
is required to catalyze the reduction of iAsV to iAsIII prior to extrusion. Figure
6 shows
arsenic detoxification in
E. coli
by
the plasmid R773 arsRDABC operon.
Figure 6
Schematic representation of arsenic interaction
with the arsRDABC
operon and related enzymes in
E. coli
. ArsR is an AsIII-responsive transcriptional repressor
that binds to the ars promoter, repressing transcription. Binding
of iAsIII to ArsR results in dissociation of the repressor
from the DNA and hence gene expression. iAsV is taken up
by the phosphate transporter, while iAsIII is taken up
by the aquaglyceroporin GlpF. iAsV is reduced to iAsIII by ArsC. Intracellular iAsIII
is extruded from
the cells by ArsB alone or by the ArsAB ATPase. (Modified from ref (163).)
ArsR is a homodimer of two 117-residue monomers with high
affinity
for the arsRDABC promoter. ArsR binds to the DNA
and represses expression of the operon to a basal level in the absence
of iAsIII. But in the presence of arsenic, the binding
of iAsIII to ArsR changes its conformation and thus ArsR
dissociates from the DNA, enabling transcription. ArsD also functions
as a homodimer of two 120-residue subunits. ArsD and ArsR bind to
the same operator site but the affinity of ArsD is 2 orders of magnitude
lower than that of ArsR. Hence, ArsD has a weaker ars operon repressor
activity and it binds to the ars operon when produced in high concentrations,
such as after prolonged stimulation of transcription of the arsD gene following induction
by the toxic iAsIII. It has been proposed that ArsD controls the upper level of expression
of the operon, preventing the buildup of ArsB, which at high levels
appears to be toxic to the cell. Although the affinity of ArsD for
iAsIII is an order of magnitude lower than that of ArsR,
ArsD binds to high levels of iAsIII and then dissociates
from the DNA, allowing the operon to transcribe. Thus, ArsR and ArsD
together form a regulatory circuit that controls the expression of arsRDABC operon
at both high and low levels of iAsIII.
162,163
Recently another role of arsD was identified.
164
It serves as an arsenic metallochaperone that
delivers iAsIII to the arsA ATPase. Interaction with ArsD
increases the affinity of ArsA for iAsIII, resulting in
increased efflux and resistance at environmental concentrations of
arsenic.
In almost all the encoded proteins of the arsRDABC,
arsenic–thiol
interaction plays an essential role for their induction or function.
161
ArsR, ArsD, and ArsA each contain critical
cysteine residues that interact with AsIII. Coordination
of AsIII with some critical cysteine thiolates in ArsR
(Cys32, Cys34, and Cys37) and ArsD (Cys12, Cys13, Cys112, and Cys
113) results in a conformational change in the repressors that disrupts
their interaction with the operator DNA. Three critical cysteine residues
(Cys12, Cys13, and Cys18) were also reported to be required of ArsD
to exert its metallochaperone activity.
164
Cys113, Cys172, and Cys422 are involved in the metallostimulation
of the ArsA ATPase. The strong arsenic–thiol interactions provide
an excellent way to sense arsenic and to turn on the genes transcriptionally
or the ArsA ATPase allosterically. In the ArsC protein, Cys12 is involved
in forming a covalent thioester intermediate with arsenate. The thioester
is further reduced by glutaredoxin and glutathione, producing a Cys12–S–AsIII intermediate,
from which arsenite is released upon hydrolysis.
165
ArsB contains a single cysteine residue that
was shown not to be essential for its function. Low-affinity oxyanion
binding instead of a very strong arsenic–thiol interaction
seems to be appropriate for this membrane transporter protein to be
effective, binding the substrate on one side of the membrane and releasing
it on the other.
162
2.4
Arsenic Binding to Hemoglobin and Accumulation
of Arsenic in Rat Blood
There are noticeable differences
in the half-life of arsenic in blood between animal species. Most
arsenic can be rapidly eliminated from blood in humans, with a half-life
of about 1 h. Studies on hemodialysis patients indicate that part
of the arsenic is bound to transferrin.
166
However, arsenic is retained in rat blood considerably longer than
in blood of other species. The accumulation of arsenic in rat red
blood cells was reported more than 50 years ago.
167
The cat can also accumulate arsenic in blood, albeit to
a lesser extent than does the rat.
168
Binding
of arsenic to hemoglobin in red blood cells was proposed as the site
of accumulation in rat blood.
169
Hemoglobin makes up to about 97% of the red blood cell’s dry
content in mammals and, therefore, is a reasonable target protein
of arsenic in red blood cells. Preferential accumulation of arsenic
in rat blood was attributed to arsenic binding to hemoglobin.
169
Rat hemoglobin showed higher affinity for trivalent
arsenic species than human hemoglobin (Figure 7). Both hemoglobins are tetramers,
each consisting of two α-chains
and two β-chains. Rat hemoglobin has three cysteines in each
α-chain (Cys13, Cys104, and Cys111) and two cysteines in each
β-chain (Cys93 and Cys125). Human hemoglobin has only one cysteine
in each α-chain (Cys104) and two cysteines in each β-chain
(Cys93 and Cys112). Although previous in vivo studies
170
showed that DMAV was detected in
rat hemoglobin, further investigations revealed that DMAIII is the actual binding
form of arsenic to rat hemoglobin and Cys13
is the preferential binding site for arsenic retention.
60,171,172
It is interesting that cat hemoglobin
also contains Cys13.
60
Whether this coincidence
is responsible for arsenic accumulation in the cat has not been investigated.
Figure 7
Comparison
of rat hemoblobin (rHb) and human hemoblobin (hHb) binding
to three trivalent arsenicals (iAsIII, MMAIII, and DMAIII). Higher percentages of
the trivalent arsenicals
are bound to rHb than to hHb. (Reprinted with permission from ref (169). Copyright
2004 American
Chemical Society.)
2.5
Arsenic-Induced
Apoptosis
Arsenic
(in the form of As2O3) has been successfully
used as a chemotherapeutic agent in treating certain types of cancers,
especially relapsed or all-trans-retinoic acid (ATRA)-resistant acute promyelocytic
leukemia (APL).
As2O3-induced apoptosis was suggested to be
the mechanism of the therapeutic effect of As2O3.
173
Although apoptosis can be triggered
by many different pathways, direct binding of iAsIII to
cysteine residues in zinc fingers (ZFs) located in the promyelocytic
leukemia protein (PML) was demonstrated to be one of the plausible
modes of action leading to APL cell apoptosis.
66,67
The conserved cysteines in both zinc fingers contained within PML-R
and PML-B (Cys60, Cys77, Cys80 in ZF1 and Cys72, Cys88, Cys91 in ZF2)
were indicated to be coordinated with iAsIII. Apo-PML-R
(PML-R without zinc) could bind one or two arsenic atoms at a clinically
relevant dose of As2O3, determined by MALDI-TOF-MS.
Arsenic binding induces PML oligomerizaiton, which facilitates the
SUMOylation (a reversible post-translational modification involving
conjugation to small ubiquitin-related modifier proteins) and subsequent degradation
of PML-RARα
(a fusion protein of PML and retinoic acid receptor alpha) and/or
PML proteins essential for the growth of APL cells (Figure 8
).
Figure 8
A proposed mechanism for the treatment
of acute promyelocytic leukemia
(APL) using inorgaic arsenic. Binding of arsenic to the promyelocytic
leukemia (PML) retinoic acid receptor alpha (RARα) fusion protein
triggers SUMOylation and ubiquitination and ultimately leads to the
degradation of the oncoprotein and cell death. (Reprinted with permission
from ref (66) and Kogan,
S.C. 10.1126/science.1189198.
Copyright 2010 American Association for the Advancement of Science.)
Mitochondria play a vital role
in the cell, providing most of the
cell’s energy and participating in Ca2+, redox,
and pH homeostasis. A major mitochondrial dysfunction is likely to
cause cell death.
174,175
With few exceptions, mitochondria
represent an essential component of many apoptotic pathways.
176,177
Proteins present in the mitochondrial intermembrane space have been
shown to induce apoptosis in response to a variety of apoptotic stimuli.
174
Mitochondrial membrane permeabilization is
critical for the release of proapoptotic factors from mitochondria.
The mitochondrial permeability transition pore (MPTP), a multiprotein
complex that is located at the contact sites of the inner and outer
membranes, is responsible for mitochondrial permeabilization. It is
comprised of the voltage-dependent anion channel (VDAC) and the adenine
nucleotide translocator (ANT), the two most abundant MPTP components
of the inner and outer mitochondrial membranes, respectively, in association
with other proteins. ANT is a 30 kDa protein and has three cysteines
(Cys57, Cys160, and Cys257). PAO and some trivalent arsenic–glutathione
derivatives have been shown to cross-link the critical thiols (Cys160
and Cys257) in ANT and therefore lock ANT into an open configuration,
leading to apoptosis.
178−181
Since there are numerous redox-sensitive mitochondrial proteins
that are arsenic binding targets,
182−186
including PDH, VDAC, tubulin, glucocorticoid
receptor, thioredoxin, thioredoxin reductase, thioredoxin peroxidase,
and glutathione S-transferase, their contribution
to arsenic-induced apoptosis cannot be precluded. Thiol modification/redox
regulation in mitochondria was acknowledged to be relevant in inducing
apoptosis.
187,188
For instance, inhibition of
thioredoxin reductase has been suggested to be responsible for APL
cell apoptosis induced by As2O3.
89
Both the N-terminal dithiols and the C-terminal
selenothiol may participate in the reaction with the arsenic compound.
Arsenic may also mimic glucocorticoids to induce apoptosis by binding
to mitochondrial glucocorticoid receptor.
189
2.6
Arsenic Binding to Other Proteins
Trivalent
arsenicals also bind many other proteins, including tubulin,
190,191
actin,
192
glucocorticoid receptor,
193,194
and estrogen receptor α.
42,195
It is notable
that iAsIII binds Kelch-like ECH-associated protein 1 (Keap1),
an antioxidant sensing protein, at low K
d values.
134
Actin-tethered Keap1 that
binds to nuclear factor erythroid 2-related factor 2 (Nrf2) constitutes
a protein triad that is indispensable for induction of the phase II
response, a major cellular reaction to oxidative/electrophile stress.
Nrf2 is involved in inducing phase II and antioxidative proteins and
regulates both the basal and inducible expressions of many cytoprotective
genes through a common antioxidant response element (ARE). Reaction
of oxidative/electrophile inducers, such as iAsIII, with
Keap1 releases Nrf2 for nuclear translocation and activation of ARE.
196
The primary sensors for these inducers are
the reactive cysteine thiols of Keap1.
Two cysteine residues
are frequently found separated by two other amino acid residues (CXXC),
a motif known to be present in a variety of metal-binding proteins
and oxidoreductases.
197
There is also a
positive correlation between the occurrence of cysteine in proteins
and the complexity of organisms,
197
indicating
that evolution takes advantage of increased use of cysteine residues.
In humans, several hundred good binding sites for trivalent arsenicals
were proposed to be present in each organ.
42,198
3
Determination of Arsenic Species Bound to Proteins
Arsenic can exist in different oxidation states (commonly +V, +III,
+I, 0, and −III), depending on the redox status and pH of the
environment and biological activities. More than 50 arsenic species
have been identified in the natural environment and biological systems.
199
It has been recognized that environmental behavior,
physical and chemical properties, toxicity, mobility, and biotransformation
differ enormously between individual arsenic species.
74
For example, Figure 9 shows cell
viability as a function of the concentration of arsenic in the form
of six arsenic species.
200
The relative
cytotoxicity of these six arsenic species varies by up to 5 orders
of magnitude. Therefore, it is essential to identify and quantify
individual arsenic species.
Figure 9
Toxicity of six arsenic compounds to HL-60 cells
after a 48-h incubation.
(Adapted from ref (200).)
3.1
Fractionation of Protein-Bound
Arsenic
Dialysis, ultrafiltration, gel filtration chromatography,
and protein
precipitation have been employed to differentiate the protein-bound
arsenic from the free arsenic species. Dialysis and ultrafiltration
are both membrane-based techniques, aimed at separating high molecular
weight molecules from small molecules. The molecular weight cutoff
of the membranes is typically 10 or 25 kDa.
201−206
After ultrafiltration or dialysis, the protein fraction is retained
for analysis. Compared to ultrafiltration, which can be accelerated
by centrifugation or pressurization, dialysis usually takes longer
to complete and requires frequent change of the buffer outside of
the dialysis sac. Moreover, dialysis usually results in sample dilution,
and thus, subsequent sample concentration or redissolution after freeze-drying
is sometimes needed to enable detection of trace amounts of arsenic.
Ultrafiltration is a convenient technique to evaluate the binding
percentage when radiolabeled arsenic is used in an assay system, because
the radioactivity retained in the ultrafiltration apparatus after
washing can be taken as a measure of the extent of radiolabeled arsenic
binding to proteins and expressed as a percentage of the total radionuclide
in the assay system.
207
It should be pointed
out that washing residues and repeated ultrafiltration can remove
free arsenic more efficiently, but this could result in a loss of
arsenic-protein binding.
166
Conceivably,
centrifugation and filtration could be applied to studying arsenic
binding to nonsoluble proteins, which would not be amenable to chromatographic
methods of analysis.
Gel filtration chromatography enables separation
of the unbound
arsenic species from the protein–arsenic complexes. Absorbance
at 280 nm is typically used for protein detection, while arsenic species
are detected by either atomic spectrometry or radiochemical methods.
Peaks appearing at the same elution times on both protein and arsenic
detectors are assigned to the arsenic–protein complexes, although
orthogonal separation techniques, each relying on a different separating
mechanism, such as molecular size or charge, may be needed to avoid
coelution and erroneous peak assignment.
208,209
Gel filtration chromatography can be used to fractionate arsenic-binding
proteins, giving additional information on the molecular weight range
of the proteins. Both denaturing and nondenaturing polyacrylamide
gel electrophoresis (PAGE) methods have been used as alternatives
to determine the molecular weights of arsenic-binding proteins.
201,210,211
Although generally regarded
as a crude fractionation method, ammonium
sulfate fractionation can also be used to maximize fractionation efficiency
and partially purify arsenic-binding proteins prior to gel electrophoresis
or gel filtration chromatography.
211
The
binding between radiolabeled arsenite and rabbit liver cytosolic proteins
was examined, both in vitro and in vivo, by combining ammonium sulfate
fractionation and gel electrophoresis. Protein precipitation is not
only an effective way of separating proteins from contaminating small
molecules but also serves as a preconcentration step for proteins
from dilute solutions. Trichloroacetic acid (TCA) and acetone
210,212−218
are commonly used to precipitate arsenic-bound proteins. Similar
to nonspecific retention of arsenic on filter membranes during ultrafiltration,
202,207,219,220
nonspecific adsorption of arsenic to sample tubes was also acknowledged
in an in vitro binding study with radiolabeled arsenic
219
and needs to be accounted for when determining
the arsenic content in the protein pellet.
211
Furthermore, the precipitating agents, such as 10% TFA, could affect
the stability of arsenic–protein complexes.
210,221
3.2
Release of Arsenic Species from Proteins
Treatment with thiol-reactive agents or chelators, such as N-ethylmaleimide, 5,5′-dithio-bis(2-nitrobenzoic
acid), β-mercaptoethanol, dithiothreitol, and 2,3-dimercaptopropanol,
failed to quantitatively release most protein-bound arsenicals.
220
Because arsenic–protein complexes are
sensitive to NaOH treatment,
213
alkaline
conditions have been explored to release arsenic from proteins. However,
interconversion of iAsIII to iAsV was observed,
during the treatment with a strong base, tetramethylammonium hydroxide
(TMAH).
222
Likewise, oxidation with concentrated
H2O2 at room temperature was able to release
all the protein-bound arsenic species as free pentavalent arsenicals.
223−225
Acidic conditions have also been explored to release arsenic
from proteins,
213,226
although the use of TCA or HCl
alone could not quantitatively release protein-bound arsenicals.
213,226
By including CuCl in HCl (pH 1), Styblo et al.
220
have developed a method of liberating protein-bound arsenicals.
CuCl was originally used by Reinke et al.
227
to reduce iAsV to iAsIII, which was subsequently
extracted and analyzed polarographically for the determination of
iAsIII and iAsV in fish and shellfish. This
method was later modified
202
by increasing
the treatment temperature from 37 to 100 °C to shorten the treatment
time. The reaction mechanism of how CuCl releases arsenic from proteins
has not been investigated. It is possible that complexation of the
cuprous ion with proteins is responsible for displacing arsenic from
the cysteine residues in the proteins. Complexation of the cuprous
ion with proteins is the basis for protein determination by the bicinchoninic
acid (BCA) and Lowry methods.
A systematic study
226
was conducted
to examine release of arsenic species from liver proteins by four
different acids (2 M HCl, 20% TCA, 2 M H2SO4, and 2 M HNO3), alone or in combination
with heat treatment.
The releasing capability of the four acids was in the descending order
of HNO3 > H2SO4 > TCA >
HCl. HNO3 (2 M) and 1 min heating at 110 °C proved
to quantitatively
(>99%) release all arsenic from the liver protein fraction. Similar
to alkaline treatments, acidic treatments can change the oxidation
status of arsenic species. For example, MMAIII and DMAIII were oxidized to their pentavalent
counterparts upon TCA
treatment, and iAsV was partially reduced to iAsIII upon TFA treatment.
226
Although TFA at
4 °C did not influence the chemical form of DMAV and
TMAO, a hot 2 M HNO3 treatment resulted in detectable loss
of methyl groups from MMAV and DMAV. In the
presence of proteins (rabbit liver cytosol), partial methylation of
MMAV to DMAV was also observed after the 2 M
HNO3 treatment.
226
These observations
suggest the importance of recognizing the integrity and artifacts
of arsenic species during sample treatment.
Lu et al.
228
have recently developed
a method based on proteolytic digestion to release protein-bound arsenicals,
aimed at preserving both the original methylation and oxidation status
of the bound arsenic. Under mild alkaline conditions (pH 8.0), no
oxidation of iAsIII and MMAIII was observed
and up to 61% DMAIII (the least stable arsenic species)
remained unoxidized.
228
The recovery ranged
from 93% to 106% with a digestion time as short as 30 min. Thus, the
method not only quantitatively releases protein-bound arsenicals but
also minimizes species conversion (at least in the AsIII state). The recent identification
of a pentavalent arsenic–peptide
complex (the glutathione conjugated form of dimethylmonothioarsinic
acid) in cabbage emphasizes the importance of preserving the oxidation
state of protein-bound arsenicals.
229
The
protease digestion method can be expected to achieve this goal because
of its mildness toward arsenicals. However, it remains to be seen
if this method can be extended from model proteins to complex protein
samples from diverse biological sources, where protease inhibitors
can be present. Removal of protease inhibitors will be necessary in
those cases prior to protease digestion.
3.3
Determination
of Arsenic Species Released
from Proteins
After release of arsenicals from proteins,
the arsenicals can be determined using a range of speciation techniques,
as discussed in several reviews.
230−233
The most commonly used techniques
for speciation of arsenic in solution involve a combination of high-performance
liquid chromatography (HPLC) separation with inductively coupled plasma
mass spectrometry (ICP-MS) and/or electrospray ionization mass spectrometry
(ESI-MS) detection.
234−238
3.4
Direct Analysis of Arsenic Species by X-ray
Spectroscopy
Direct speciation of arsenic protein fractions
without releasing arsenic can be achieved by using X-ray absorption
spectroscopy,
239
especially EXAFS (extended
X-ray absorption fine structure) and XANES (X-ray absorption near
edge spectroscopy). These techniques can reveal the electronic environment
around the arsenic atom and therefore provide information as to whether
arsenic is trivalent or pentavalent and whether arsenic is bound to
sulfur, oxygen, or carbon.
240−247
Therefore, XAS is invaluable for probing the kinetically labile
arsenic–protein complexes that are prone to dissociation or
ligand exchange during sample handling.
235
Complete identification of arsenic species is best achieved by complementing
XAS information with structural information obtained by using other
techniques.
4
Identification of Arsenic-Binding
Proteins
4.1
Autoradiography
Radioactive isotopes
of 73As and 74As (73As has a longer
half-life and lower γ-emission) have been used, although organic
arsenic compounds have labeling options with radioisotopic carbon.
202,248
Whole-body autoradiography of mice after intravenous injection of 73As compounds provided
information on the tissue distribution
of arsenic.
170,249
The studies on arsenic distribution
and binding involving the use of radiolabeled arsenic compounds in
animals and humans have been thoroughly reviewed.
250
In general, keratin-rich tissues (such as the hair, nails,
and skin) with high cysteine content exhibit the highest concentrations
and longest retention. At the cellular level, arsenic-protein binding
occurs mainly in the cytosol, most notably in liver and kidney. Greater
retention of arsenic was observed with arsenite compared to arsenate,
and differences in arsenic accumulation were noted in the liver, kidney,
and skin. In the bloodstream, arsenic is distributed between erythrocytes
and plasma and binds to proteins in both compartments.
Although
the radiotracer can be monitored and its activity measured typically
with a scintillation detector equipped with a thallium-activated sodium
iodide crystal [NaI(Tl)],
251
identification
of arsenic-binding proteins requires more complementary techniques
for separation and characterization. Since the heme moiety strongly
absorbs light at 420 nm, simultaneous detection of both 74As and the heme moiety after
size exclusion chromatography (SEC)
revealed that arsenic was bound to hemoglobin in packed blood cells
of rabbits intraperitoneally injected with 74As-labeled
arsenate.
204
This binding was also observed
in the red blood cells of rats, as SEC showed that the 74As was predominantly associated
with a protein that had a molecular
mass of ∼60 kDa and a strong absorption band at 420 nm.
209
To study potential arsenic binding partners
in serum, cation and anion exchange separation schemes with fast protein
liquid chromatrography were applied to serum that had been incubated
in vitro with 74As-labeled arsenate. The protein fractions
so isolated were further separated by isoelectric focusing and identified
as asialotransferrin, sialotransferrin, and albumin.
209
Other useful radiolabeled arsenic species include
phenylarsine
oxide derivatives. N-[3H]acetyl-4-aminophenylarsine
oxide and 4-[125I]iodophenylarsine oxide were synthesized
and employed as covalent affinity reagents to study arsenic–protein
interaction.
190
4.2
Fluorescent
Arsenical Probes
Wang
et al.
252
identified two naturally occurring
tetracysteine-containing sequences that bind to FlAsH [4,5-bis(1,3,2-dithioarsolan-2-yl)fluorescein]
by selecting against the
Shewanella oneidensis
and
E. coli
proteomes. SlyD, a metallochaperone
in the assembly of [Ni–Fe] clusters in the hydrogenase biosynthetic
pathway, was identified as a high-affinity biarsenical binding protein
in these two prokaryotic organisms. Fluorescent biarsenical affinity
probes FlAsH and its many analogues, such as ReAsH (see structures
in Table S1 of Supporting Information),
were initially developed by Tsien and co-workers
253,254
to selectively label a range of proteins in both eukaryotic and
bacterial organisms fused to an engineered tetracysteine binding sequence,
CysCysXXCysCys, where the central XX is preferentially ProGly. The
biarsenical dyes undergo a dramatic enhancement of fluorescence upon
binding to the proteins. Because endogenous proteins that would bind
FlAsH are rare, fluorescent biarsenical probes permit the easy detection
of the peptide-tagged proteins over the background. These fluorescent
biarsenicals could also be used to detect endogenous cysteine-rich
proteins.
66,255,256
More recently, a naphthalimide-based fluorescent monoarsenical
probe, NPE (see the structure in Table S1 of Supporting
Information), has been developed by Huang et al.
257
to detect and image vicinal dithiol proteins
both in vitro and in living cells.
4.3
Affinity
Chromatography
4.3.1
Protein-Specific Immunological
Affinity
Chromatography
Plasma from rabbits intraperitoneally injected
with 74As-labeled arsenate was analyzed by transferrin-specific
affinity chromatography by using an N-hydroxysuccinimide
antirabbit transferrin HiTrap column.
204
Up to 18% of total plasma 74As was found to bind to transferrin.
Similarly, a transferrin-specific affinity column was used to further
confirm the identity of transferrin in serum of patients on continuous
ambulatory peritoneal dialysis.
166
Transferrin
is capable of binding to a wide range of synergistic anions at its
iron binding site, and formation of an iron–arsenate–transferrin
complex is presumably responsible for arsenate–transferrin
binding.
204
4.3.2
Arsenical
Affinity Chromatography
Chromatography using trivalent arsenicals
as affinity ligands was
first devised by Hannestad et al.
258
to
retain and separate mono- and dithiols, making use of interactions
between the thiols and trivalent arsenicals. The affinity adsorbent
was prepared by directly binding p-aminophenylarsine
oxide to CNBr-activated Sepharose 6B. The arsenic affinity chromatography
was later modified by several research groups to study covalent arsenical–protein
interaction or to purify proteins. In their investigation into the
complex effects of the trivalent arsenical PAO on insulin-dependent
hexose uptake in 3T3-L1 adipocytes, Hoffman and Lane
190
prepared affinity resins by covalently coupling agarose,
derivatized with a 10-atom spacer arm having a terminal hydroxysuccinimide
active ester, to 4-aminophenyldithioarsine of β-mercaptoethanol
(β-ME). The captured proteins were eluted off from this arsenic
affinity column using β-mercaptoethanol in 1% Triton X-100 or
SDS, further separated on SDS–PAGE, and detected by selective
staining with 4-[125I]iodophenylarsine oxide. Several proteins
from 3T3-L1 adipocytes were identified to be able to bind arsenic,
including the insulin-responsive glucose transporter GLUT4 and tubulin.
Zhou et al.
259
covalently linked 4-aminophenylarsine
oxide to carboxymethyl (CM)-Bio-Gel A activated through an N-hydroxysuccinimide ester.
They used this column to purify
human plasma lecithin:cholesterol acyltransferase (LCAT). LCAT is
a dithiol enzyme that has two reduced cysteine residues (Cys31 and
Cys184) within its catalytic site. Binding of these cysteines to the
arsenical affinity column enabled the capture and subsequent elution
(with a buffer containing 2,3-dimercaptopropanesulfonic acid), affording
a resultant 13-fold increase in specific activity of the enzyme and
an overall yield of 11%. Kalef et al.
260
prepared affinity resins based on Sepharose 4B linked with aminohexanoyl-4-aminophenylarsine
oxide to purify proteins containing vicinal dithiols from L1210 murine
leukemia lymphoblasts. A one-step purification of the l-triiodothyronine
recombinant rat c-erb Aβ1 T3 receptor synthesized in yeast was
achieved with this affinity chromatography, resulting in a 62-fold
increase in specific activity and a yield of 11%. CNBr-activated Sepharose
4B was derivatized with 6-aminohexanoic acid to constitute a spacer
arm prior to coupling with freshly prepared 4-aminophenylarsine oxide
(high solubility in DMSO) via carbodiimide. Elution was performed
with β-mercaptoethanol or dithiothreitol (DTT) in buffers. The
dithiol proteins were identified by specific labeling with N-iodoacetyl-3-[125I]-iodotyrosine
([125I]IAIT). A different method of preparing PAO-Sepharose 4B was reported
by Berleth et al.
261
to partially purify
ubiquitin-protein ligase (E3). This method was subsequently adopted
by Boerman and Napoli
262
to partially purify
a microsomal retinol dehydrogenase (RoDH).
Arsenic affinity
chromatography was also used by Winski and Carter
263
to characterize arsenic binding proteins in rat red blood
cells. Proteins retained on the column were displaced by sequential
washes with cysteine, glutathione, and dimercaptopropanesulfonate
(DMPS) after removing nonspecific binding materials with buffer and
ethylene glycol. Hemoglobin seemed to be the main arsenic-binding
protein in the rat red blood cell, which was captured on the arsenic
affinity column and eluted with 100 mM glutathione. Using the same
arsenic affinity chromatographic strategy, Menzel et al.
192
identified two arsenic-binding proteins to
be tubulin and actin on the basis of their molecular weights. A highly
arsenite-inducible protein, heme oxygenase 1 (HO1), failed to bind
to the arsenic affinity column because it is devoid of cysteines.
Chang et al.
92
used commercial PAO-agarose
resins to isolate AsIII-modulated proteins from Chinese
hamster ovary (CHO) and SA7 (arsenic-resistant CHO) cells. Following
washes with 2 M NaCl in Tris buffer to remove the nonspecific binding
proteins, the specific binding proteins were sequentially eluted with
20 mM β-ME and 20 mM DTT. Galectin 1 (Gal-1; in the β-ME-eluted
fraction from CHO cells), glutathione S-transferase
P-form (GST-P), and thioredoxin peroxidase II (TPX-II) were identified
by partial amino acid sequence analysis. TPX-II was preferentially
expressed in SA7 cells (which were routinely cultured and maintained
in AsIII-containing medium), but not in CHO or SA7N (a
revertent of SA7 cells cultured in regular medium). In contrast, Gal-1
was specifically identified in CHO and SA7N cells, but not in SA7
cells. The specific binding of Gal-1 and TPX-II to AsIII was verified by both coimmunoprecipitation
from mammalian cells
treated with AsIII and coelution of recombinant Gal-1 and
TPX-II with AsIII from arsenic-treated transfected
E. coli
.
Mizumura et al.
264
prepared three different
arsenic-bound Sepharose resins (AsV-immoblized Sepharose,
type A; AsIII-diglutathione-immobilized Sepharose, type
B; and AsIII-immobilized Sepharose, type C) to investigate
the binding of hepatic cytosolic proteins to pentavalent, glutathione-conjugated,
and trivalent arsenicals. They synthesized AsV-immoblized
Sepharose by conjugating NHS-activated Sepharose 4 Fast Flow gel with
p-arsanilic acid. This gel was further converted to AsIII-diglutathione-immobilized
Sepharose gel after reduction with GSH,
which could be stabilized in the presence of GSH. The AsIII-diglutathione-immobilized
Sepharose gel was hydrolyzed to produce
AsIII-immobilized Sepharose gel in the absence of GSH.
Heptatic cytosolic proteins bound to each Sepharose were eluted with
GSH, DTT, or LDS (lithium dodecyl sulfate) sample buffer. SDS–PAGE
followed by EZBlue staining showed no proteins bound to pentavalent
arsenic. MALDI-MS/MS was able to identify the binding of protein disulfide
isomerase-related protein 5 (PDSIRP5) and peroxiredoxin 1/enhancer
protein (PRX1/EP) to trivalent arsenic. Four proteins, peroxiredoxin
2, cytosolic pyrophosphatase, phosphoglycerate kinase 1, and KM-102-derived
reductase-like factor (equivalent to thioredoxin reductase), were
identified to specifically bind to glutathione-conjugated trivalent
arsenic, possibly involving substitution of a cysteine residue of
the proteins for glutathione at the As-SG site. He and Ma
130
conjugated 4-amino-phenylarsine oxide to Affigel
to make PAO affinity beads, which were used to probe binding of arsenic
to Kelch-like ECH-associated protein 1 (Keap1), nuclear factor erythroid
2-related factor 2 (Nrf2), and the carboxyl half of metal-activated
transcription factor 1 (MTF1). The PAO affinity materials have also
been used to isolate some other enzymes, such as adenine nucleotide
translocase (ANT) and protein tyrosine phosphatases (PTP), from cell
lysates.
188,265
In all these identified proteins,
multiple cysteine residues are present for arsenic binding.
Reversible oxidation on proteins of vicinal thiols to form disulfides
is believed to be an important means of redox-based regulation of
cell signaling, metabolism, and gene expression. Gitler et al.
266
introduced a general method to identify and
enrich vicinal thiol proteins (VTPs) present in intact cells in the
oxidized, disulfide state. They first blocked unoxidized thiols in
situ by incubation of murine leukemia L1210 cells with cell permeable N-ethylmaleimide
(NEM) and then reduced oxidized thiols
with DTT. These proteins containing reduced thiols could be enriched
by using PAO affinity chromatography. It was a challenge to the use
of PAO affinity chromatography for the isolation of readily oxidizable
VTPs because it required a delicate balance between maintaining the
thiols in the reduced state necessary for PAO binding and the possible
competition of DTT for the binding. Conceivably, the use of DTT could
have led to an underestimation of the proteins able to bind PAO in
the previous studies. By substituting tris(2-carboxyethyl)phosphine
(TCEP), a nonthiol reducing agent, for DTT, Foley et al.
267
improved the capture of VTPs from a Triton
X-100-soluble rat brain extract by using PAO-affinity chromatography.
After unoxidized thiols were alkylated with iodoacetamide (IAM), the
oxidized thiols in proteins were reduced with TCEP prior to loading
the protein sample onto a PAO affinity column. The eluted fractions
were analyzed by SDS–PAGE, and protein bands were subjected
to in-gel tryptic digestion and MALDI-MS/MS analysis for protein identification.
The two most abundant proteins were identified as albumin and triose
phosphate isomerase (TPI). The catalytic subunit of protein phosphatase
2A (PP2Ac) from rat brain was also revealed, by the improved method,
to contain reversibly oxidized thiols.
268
Similarly, protein phosphatase 2B (Calcineurin, CaN) and thioredoxin
were isolated and identified from a cytosolic fraction of polymorphonuclear
leukocytes (PMN) as involved in redox regulation, along with four
other proteins (glutathione S-transferase P1-1, calgranulin, l-plastin, and cofilin).
269
Our group
270
has developed a targeted
proteomic approach to identify arsenic-binding proteins in human cells
involving arsenical affinity chromatography. The affinity resins were
synthesized by reaction of Eupergit C beads with 4-aminophenylarsine
oxide (NAPOIII). After being eluted with increasing levels
of DTT in buffer with or without 1% SDS, the bound proteins were subjected
to in-gel or in-solution digestion prior to LC–ESI-MS/MS analysis.
This approach demonstrated that arsenic-binding proteins could be
identified in the presence of a large excess of nonspecific proteins.
Fifty proteins in the nuclear fraction and 24 proteins in the membrane/organelle
fraction were identified in the A549 human lung carcinoma cell line.
4.4
Biotinylated Arsenical Pull-Down
Zhang et al.
271
designed an in-solution
pull-down approach to identify arsenic-binding proteins in human breast
cancer MCF-7 cells. An arsenic–biotin conjugate was synthesized
by coupling the pentafluorophenol ester of biotin with p-aminophenylarsine oxide.
Improved synthesis of arsenic–biotin
conjugates in a one-pot procedure was reported.
272
Arsenic-binding proteins were pulled down with streptavidin
resins and subsequently released, separated by SDS–PAGE, and
identified by MALDI-MS/MS. Analyses of MCF-7 cells resulted in tentative
identification of 50 proteins. Two proteins, β-tubulin and pyruvate
kinase M2, were confirmed further by Western blotting and molecular
modeling. The same approach was applied to demonstrate that arsenic
bound to PML and PML-RARα fusion protein, but not to RARα
alone.
66
This direct binding was shown
to be important in the anticancer activity of As2O3 in patients with acute promyelocytic
leukemia (APL).
Donoghue et al.
273
devised a membrane-impermeable
trivalent arsenical to identify closely spaced thiols in mammalian
cell-surface proteins undergoing redox reactions. Because GSH is constitutively
secreted by mammalian cells but is not taken up by these cells, an
arsenic probe for the cell-surface proteins could be developed by
conjugating GSH to arsenic. Aminophenylarsine oxide was therefore
coupled to the thiol of GSH to produce 4-(N-(S-glutathionylacetyl)amino)-p-phenylarsine
oxide (p-GSAO), which was shown to bind tightly to
dithiols but not to monothiols. To identify cell-surface proteins
that contain closely spaced thiols, they attached a biotin moiety
through a spacer arm to the primary amino group of the γ-glutamyl
residue of GSAO to afford GSAO-B. The biotin moiety was expected to
prevent cleavage of the γ-glutamyl residue by membrane-bound
γ-glutamyl transpeptidases.
274
Cells
were labeled with GSAO-B in the absence or presence of 2,3-dimercaptopropanol
(DMP), lysed, and incubated with streptavidin-agarose beads to collect
biotin-labeled proteins. The bound proteins were eluted with 50 mM
DTT in buffer, resolved on SDS–PAGE, transferred to PVDF membranes,
and blotted with specific antibodies against candidate proteins. Alternatively,
after labeling with GSAO-B, proteins were resolved on SDS–PAGE,
transferred to PVDF membranes, and blotted with streptavidin-peroxidase
to detect the GSAO-B label. There were 10 distinct proteins on the
surface of bovine aortic endothelial (BAE) cells and 12 on the surface
of human fibrosarcoma (HT1080) cells that incorporated GSAO-B. Protein
disulfide isomerase (PDI) was labeled with GSAO-B on the cell surfaces
of both cell types. The pattern of labeling of the cell lysate with
GSAO-B was very different from the pattern of labeling of the cell
surface proteins, reflecting the membrane-impermeable nature of GSAO-B.
The interaction of GSAO-B with PDI and thioredoxin was also demonstrated
in vitro with purified proteins. The adenine nucleotide translocase
(ANT) was captured by GSAO-B, from isolated rat liver mitochondria,
when the arsenical moiety of GSAO was either at the ortho (o-GSAO-B) or para (p-GSAO-B)
position on
the phenyl ring.
275
The GSAO-B-bound proteins
were then pulled down by streptavidin- or avidin-coated beads and
eluted with DTT. By a similar strategy, GSAO-B has been demonstrated
to be retained in the cytosol predominantly by covalent reaction with
a dithiol in the 90 kDa heat shock protein (Hsp90), followed by eukaryotic
translation elongation factor 2 and filamin A.
276
Apart from GASO-B, three novel biotinylated PAO compounds
with spacers of varying length were also synthesized as bifunctional
reagents to examine spatially close thiols in Torpedo nicotinic receptors.
277
Aside from identifying arsenic binding
proteins, arsenical probes
and PAO affinity chromatography have been employed to confirm the
interaction of arsenic and the proteins in question.
66,134,196,278,279
They can further be used to
ascertain the critical thiols for binding in proteins after site-directed
mutagenesis.
134,161,280
4.5
Metalloproteomic Approaches
General
metalloproteomic approaches combining efficient separation of metalloproteins
by liquid chromatography or electrophoresis with highly sensitive
elemental detection can be applied to identification of arsenic-binding
proteins in biological samples.
201,281
Several excellent
review papers have been dedicated to the analytical chemistry approaches
in metalloproteomic or metallomic research.
232,282−288
Proteomics approaches using mass spectrometry (with MALDI and ESI)
remain the state-of-the-art for high-throughput protein identification.
289,290
5
Characterization of Protein Binding to Arsenic
Species
Although numerous cysteine-containing proteins that
can bind arsenic
have been identified, not many arsenic-binding proteins have been
thoroughly characterized in terms of their binding sites, binding
stoichiometry, and binding affinity. Part of the reason for this paucity
is that purified proteins are not readily available in sufficient
amounts for structural determination and binding characterization.
One early study that employed a fractional occupancy assay with a 73As tracer indicated
that the average dissociation constant
(K
d) for all proteins capable of binding
arsenite in rabbit liver was 18.4 μM.
211
5.1
Binding Sites
Cysteines and histidines
are known to be most frequently involved in binding metals such as
iron, zinc, and copper.
291,292
The binding between
arsenic and histidines, however, has never been unequivocally established.
In an NMR spectroscopic study of complexation of arsenicals to rabbit
hemoglobin,
293
a direct interaction of
AsIII with histidine was discounted because there was no
NMR spectral modification observed to a buffered solution of histidine
when AsIII was added. AsIII seemed to bind to
Cys93 but not to other neighboring amino acids of Hb. Similarly, histidine
tags in recombinant Trx and ArsD were shown not to bind iAsIII.
108,164
Rosen and co-workers
162,294−296
frequently substituted serine residues for
cysteine residues in their site-directed mutagenesis studies to define
the role of the cysteine residues in arsenic binding, on the premise
that the interaction of serine with arsenite is weak. So far, the
only binding ligand of arsenic identified in the ArsRDABC operon proteins
is cysteine thiolates. In a recent examination of amino acid constituents
that can potentially bind arsenic, Kitchin and Wallace
42
showed that amino acids other than cysteine
in their synthetic peptides did not bind arsenite at all. Adequate
cysteine content and vicinal cysteine residues were shown to be required
for high-affinity arsenite binding with the peptides.
42,43
Arsenite was shown to preferentially bind zinc finger peptides containing
three or four cysteine residues and interact selectively with zinc
finger proteins containing a C3H1 or C4 but not with a C2H2 motif
in releasing zinc and reducing DNA-binding capacity of the proteins.
45
In sum, cysteine (and selenocysteine as in selenoproteins,
considering its lower pK
a and therefore
increased nucleophilicity
297
) is the only
naturally occurring amino acid that has been demonstrated to be responsible
for trivalent arsenic–protein binding. Other amino acid residues
in close proximity to cysteine may serve as proton acceptors and enhance
thiol reactivity toward arsenic.
298
Li and Pickart
94
demonstrated that vicinal
thiols were not involved in the binding of phenylarsine oxide to Arg-tRNA
protein transferase. They proposed that structural features other
than cysteines and/or serines were responsible for the binding. Arg-tRNA
protein transferase is an enzyme containing 15 cysteine residues,
and multiple cysteine residues are essential for its activity. Despite
using site-directed mutagenesis to mutate specific cysteine residues,
it was challenging to rule out the role in the arsenic binding of
any remaining cysteine residues that could be brought into spatial
proximity, even in the presence of urea because “unfolded proteins
are conformationally mobile and structurally accommodating”.
299
The final possibility of “redundancy
in the abilities of cysteine residues of the transferase to bind arsine
oxide” could not be excluded but was dismissed as unlikely
by the authors.
94
However AsIII has lately been demonstrated to be able to distort a polypeptide
structure so that AsIII can capture thiols in the reorganized
structure in order to satisfy its desire for thiolate coordination.
299−301
Therefore, it is possible that the inhibition of Arg-tRNA protein
transferase occurs due to conformational distortion of the enzyme
induced by arsenic cross-linking of thiol groups.
Besides sulfhydryl
groups, hydroxyl groups in small molecules were
demonstrated to have some potential to form a ring structure with
phenylarsine oxide in the gas phase.
258
In a crystal structure of the arsenite–dithiothreitol complex,
AsIII bound with two sulfur thiolates and one hydroxyl
oxygen.
302
Therefore, in proteins, hydroxyl-containing
serine appears to be another possible arsenic binding ligand. In support
of this possibility, several studies
303−305
have implicated the
role of the active site serine bound by arsenic in the inhibition
of enzymes, including those containing no cysteine. In these studies
the enzymes were all serine hydrolases and the arsenicals interacting
with the active site serine were all pentavalent. The only exception
is cholinesterases, of which arsenite but not arsenate is a potent
inhibitor.
306,307
A tripartite oxyanion hole in
proximity to the active site is usually required to stabilize the
transition state complex between pentavalent arsenic and the catalytic
serine.
305
5.1.1
X-ray
Crystallography and Nuclear Magnetic
Resonance (NMR) Spectroscopy
X-ray crystallography and NMR
spectroscopy are the two primary techniques employed to determine
three-dimensional structures of the metal binding sites of proteins.
The first example of covalent modification of serine by pentavalent
arsenic was observed with X-ray crystallography.
304
In the study, the crystal structures of the acetyl esterase
HerE and its complexes with an inhibitor, dimethylarsinic acid, were
determined and a covalent bond via the Oγ of Ser160 seemed to
be formed between the enzyme and arsenic. For this experiment, HerE
was crystallized in the presence of 100 mM cacodylate (dimethylarsinic
acid) without DTT. This was different from the DTT-mediated reaction
of cysteine with the arsenic center of the cacodylate present in the
crystallization buffer (usually containing100 mM cacodylate and 5
mM DTT) observed with several proteins. In the latter case, the presence
of DTT results in the reduction of cacodylate by DTT and subsequent
reaction with exposed cysteine residues.
308−312
X-ray crystallography was also employed by Rosen and co-workers
to study the mechanism of arsenic binding in arsenate reductase ArsC.
165
The crystal structures of ArsC and its complexes
with arsenate (substrate) and arsenite (product) were determined.
Arsenate forms a thioester with the catalytic residue Cys12. Arsenite
also binds the Cys12 thiol to form an unusual bicoordinate thiarsahydroxyl
intermediate ArsC-Cys12-As+-OH, which is essential to prevent
the suicide inhibition of the enzyme by its reaction product (arsenite).
Rosen and co-workers have also recently obtained the X-ray crystallographic
and NMR information
313,314
about the ArsD arsenic metallochaperone
in attempts to examine the structural changes in the protein in response
to metalloid binding and upon interaction with ArsA. Molecular modeling
was additionally performed to probe the metal binding sites of ArsD
to incorporate induced changes upon arsenic binding since the apoprotein
(without bound arsenic) was used in their structural determinations.
Conserved cysteine residues Cys12, Cys13, and Cys18 form a tricoordinate
binding site for AsIII, which corroborated their previous
determination of AsIII binding sites on this protein by
using site-directed mutagenesis combined with arsenic analysis.
315
The coordination of AsIII with three
sulfur atoms in ArsD was further demonstrated by X-ray absorption
spectroscopy.
164
Compared with X-ray
crystallography, NMR generally offers a lower-resolution structure,
but it does not require crystallization. After transmetalation to
CdII, which served as an NMR spectroscopic probe, the ZnII enzyme alkaline phosphatase
was studied on its binding with
arsenate.
316
Arsenate, an enzyme inhibitor,
bridged Ser102 and zinc-binding sites. This binding geometry was confirmed
by a subsequent X-ray crystallographic study of alkaline phosphatase.
317
5.1.2
Molecular Modeling and
Site-Directed Mutagenesis
Molecular modeling, a term for
computational protein structure
prediction, can contribute to the protein determination process and
deal with large proteins that are impossible with either X-ray crystallography
or NMR.
318,319
Many review articles and monographs
318,320,321
have been dedicated to the topic
of molecular modeling, and it has been used to determine the arsenic
binding site in the example of ArsD mentioned above.
313
The site-directed mutagenesis approach is generally
applicable to identify the roles of specific residues in ligand binding,
specificity, and catalysis, which in turn is guided by knowledge of
the 3-D structures of proteins. The importance of site-directed mutagenesis
and subsequent quantitative kinetic analysis of mutant enzymes has
been highlighted in structural biology in the case of adenylate kinase,
where a discrepancy between the X-ray model and the NMR model existed.
322
Site-directed mutagenesis has been intensively
used by Rosen and co-workers to identify arsenic binding sites of
Ars operon proteins. For instance, a combination of site-directed
mutagenesis and gel permeation chromatography analysis revealed that
ArsA had a high-affinity metalloid binding site composed of Cys113
and Cys422.
323
The 2.3 Å crystal structure
of ArsA crystallized in the presence of SbIII and MgADP
showed that ArsA had two other bound metalloid atoms, one bound to
Cys172 and His453 and the other bound to His148 and Ser420. Further
site-directed mutagenesis analysis revealed that His418 and Ser420
had little effect on metalloid binding while the C172A single mutant
and C172A/H453A double mutant exhibited significantly decreased affinity
for SbIII, suggesting that C172 is the third residue that
participates in high-affinity binding.
296
A site-directed mutagenic approach was also employed in search for
a third arsenic ligand in ArsR, Cys37, which forms a tricoordinate
complex with AsIII together with highly conserved Cys32
and Cys34.
161
The As-S coordination environment
in ArsR was confirmed by X-ray absorption spectroscopy of ArsR stoichiometrically
treated with arsenite. Cys95, Cys96, and Cys102, the only three cysteine
residues in an ArsR family protein in
Acidithiobacillus
ferrooxidans
, were also identified to form a tricoordinate
AsIII binding site.
295
5.1.3
Electrospray Ionization Mass Spectrometry
(ESI-MS)
The development of soft ionization methods (MALDI
and ESI) for mass spectrometry has facilitated the study of metal
ion binding to macromolecules, including proteins. With ESI-MS, noncovalent
protein–metal interactions in solution can be maintained during
desolvation and transfer to the gas phase. Therefore, solution-phase
complex formation can be detected and monitored by mass spectrometry
in simple “mix-and-measure” experiments, which makes
ESI-MS a more natural choice than MALDI-MS.
324
Binding stoichiometries can be determined directly from the measured
mass. In favorable cases binding affinities and binding kinetics may
be obtained from the measured intensity ratios of the free and bound
protein ions.
324
Structural information
such as binding sites may also be obtained using tandem MS (MS/MS)
or top-down strategies. In combination with chemical modifications,
the capability of binding site determination by ESI-MS can be extended.
325
As further evidence that trivalent arsenicals
have a strong preference to bind to cysteine residues in proteins,
ESI-MS studies revealed that trivalent arsenicals failed to bind purified
hemoglobin and thioredoxin after reactive cysteine sulfhydryl groups
were blocked with N-ethylmaleimide (NEM) or monobromobimane.
108,169
ESI-MS has enabled identification of a preferential binding
cysteine in rat hemoglobin (rHb).
60
In
vitro binding of trivalent arsenicals to hemoglobin generated the
corresponding arsenic–hemoglobin complexes with all possible
stoichiometries depending on the relative concentrations of arsenicals
and hemoglobin.
169
In vivo, however, a
DMAIII–rHb complex was exclusively observed on the
α-chain with a stoichiometry of 1:1 in rats exposed to iAsV, MMAV, or DMAV. A mass
shift of 104
Da was indicateive of the binding of one DMAIII [(CH3)2AsOH, FW 122] to the α-unit
with the loss
of one H2O molecule. No complexes of iAsIII or
MMAIII with rHb were detected from any of the treated rats.
It has been known that in the rat iAsIII and MMAIII can be excreted into the bile
in the form of As(GS)3 and
MeAs(GS)2, respectively, while DMAIII is released
into the bloodstream and efficiently taken up by red blood cells.
326,327
The identity of DMAIII bound to rHb was confirmed by
HPLC analysis of arsenic species released from the protein: only DMAIII and its oxidation
product DMAV were observed.
60
Collision-induced dissociation (CID) tandem
mass spectrometry
172
revealed two characteristic
fragment ions associated with a DMAIII-tagged cysteine
residue that were equally observable from DMAIII complexes
with cysteine and glutathione. From measuring the accurate masses
of the DMAIII-tagged internal ions (di-, tri-, and tetrapeptides)
and comparing with the theoretical masses for peptides encompassing
the three cysteine residues (Cys13, Cys104, and Cys111) in the rHb
α-unit, Cys13 was determined to be the only binding site of
DMAIII. The observation of exclusively DMAIII-tagged fragment ions at Cys13 was not
due to differential production
by CID fragmentation, because the internal ions from all three cysteine
residues could be detected from rHbα–(DMAIII)3 that was formed by incubating rHb with
a 100-fold molar
excess of DMAIII. The unbiased fragmentation was further
confirmed by CID MS/MS analysis of DMAIII complexes with
three synthetic nonapeptides embedding each of the cysteine residues
in the α-chain.
172
The relative reactivity
of cysteine residues in rHb α-chain was determined to be in
the order of Cys13 ≫ Cys111 > Cys104. The preferential binding
of Cys13 to DMAIII is probably because Cys13 in rat Hb
is located in an open hydrophobic pocket that provides easy access
and a favorable binding microenvironment for moderately hydrophobic
DMAIII.
60
In the β-chain,
Cys125 is more reactive than Cys93, and glutathione preferentially
binds Cys125.
5.2
Binding Stoichiometry
In order to
obtain information on metal-binding sites from X-ray spectroscopy,
it is necessary to have access to well-defined structures of stoichiometric
preparations of metal–protein complexes. For X-ray crystallography,
sufficient amounts of the ligand have to be present to ensure the
formation of a fully occupied stoichiometric complex obtained either
by cocrystallization or by soaking the ligand into a fully grown crystal.
328
Mass spectrometry offers a rapid and precise
determination of metal–protein binding stoichiometry, providing
integer numbers of bound metal ions per protein molecule, even when
not all protein molecules are fully metal-loaded. Titration-based
methods, isothermal titration calorimetry included, may yield noninteger
numbers of metal ions (the average metal content) per protein molecule
under similar circumstances.
329
The binding
stoichiometries of AsIII were shown by using either ESI-MS
or MALDI-MS to be consistent with the number of available cysteines
in proteins such as metallothionein (shown in Figures 4 and 5) and hemoglobin examined
at
saturating arsenic concentrations.
149,152−155,169
5.3
Binding
Affinity
The binding affinity
(association constant, K
a; dissociation
constant, K
d) of arsenic–protein/peptide
binding can be determined by measuring changes in either the properties
of the protein/peptide or in the partition of arsenic. Commonly adopted
procedures to determine K
d include absorbance
shift of protein/peptide or arsenical,
254,330,331
intrinsic tryptophan fluorescence quenching,
164,299,332
the reduction of thiol content
due to arsenic binding,
273
and the changes
in the protein electrophoretic mobility measured by capillary electrophoresis.
97
Alternatively, partition of arsenic in free
and protein/peptide-bound forms after reaching binding equilibrium
can be quantified to determine the binding affinity after separating
the two forms by using size exclusion liquid chromatography,
108,169
dialysis,
248,333−335
or filtration.
43,198
Use of radioactive or fluorescently
labeled arsenic compounds offers a fast and convenient determination
of arsenic-protein binding affinity.
254,336
As a technique
that directly measures the heat released or absorbed during a biomolecular
binding event, isothermal titration calorimetry (ITC) can simultaneously
determine several binding parameters (binding stoichiometry, binding
constant, binding enthalpy, and entropy) in one experiment and has
been used to study arsenic–protein binding.
337
Information readily obtained from ITC, such as formation
of multiple titration species fitting a sequential-binding model,
was proven to be useful for data modeling, distinguishing this technique
from other titration methods. For example, a stability constant (K
a = 1/K
d) of 2 ×
106 was determined with ITC for the 1:3 arsenite complex
with GSH,
338
for which unusually high values
of 2 × 1033 and 1 × 1032 were reported
by Rey et al.
339
on the basis of their
near-UV absorption and potentiometric data, respectively. The binding
affinities between arsenic and several thiol-containing biomolecules
were also determined with ESI-MS by Schmidt and co-workers,
212,340−342
but their values were inconsistent. One
issue with quantifying both the unbound and bound species using ESI-MS
is that these species may have different ESI-MS responses and their
intensities may not be calibrated against a single species. Thus,
ESI-MS should be used to determine relative binding affinities rather
than absolute values.
341,343,344
K
d values obtained by MS should be used
with caution in the absence of validation from other solution-phase
techniques.
343
The arsenic–protein
binding affinity can also be determined indirectly in inhibition studies
by measuring the inhibitory effect of arsenic on ligand binding or
enzyme activity. When measured this way, the dissociation constant
of binding of the inhibitor is termed the inhibition constant, K
i (equivalent to K
d). The inhibition of enzyme activity by arsenic compounds was shown
to be either competitive or noncompetitive (mixed),
345−356
demonstrating the ability of arsenic to bind to free enzymes.
357
A majority of the binding constants for arsenic–protein
binding were obtained indirectly. IC50, the half-maximal
inhibitory concentration, which can often be related to K
i by the Cheng–Prusoff equation [K
i = IC50/(1 + [S]/K
m)],
358
is not a direct indicator of affinity
and may vary with experimental conditions such as substrate concentrations.
Therefore, IC50 values reported in the literature are not
included in this review. When arsenic acts as the substrate for an
enzyme, K
m, the Michalis constant, can
be determined using Michaelis–Menten kinetics. K
m is the concentration of substrate at which enzyme activity
is half-maximal. Its values include not only the affinity of the substrate
for the enzyme but also the rate at which the enzyme-bound substrate
is converted to the product in the catalytic reaction. Thus, K
m can be interpreted as a crude measurement
of the affinity of the substrate for the enzyme.
359
A large K
m indicates weak affinity. K
m values reported in the literature for arsenic-containing
compounds are included in Table S3 of the Supporting
Information. A schematic comparison with statistical analysis
is shown in Figure 10 on the K
d (or K
i) and K
m values for trivalent and pentavalent arsenicals separately. K
d values of the binding between trivalent arsenicals
and small peptides/thiols are included as well (see Supporting Information, Table
S4). Strict comparison of the
listed binding constants cannot always be made since different arsenic
compounds, methods, and assay conditions have been used. For example,
in the absence of tetrahedral oxyanions, such as sulfate, oxidative
inactivation of ArsC (arsenate reductase) encoded by
Staphylococcus aureus
, a highly oxygen sensitive
redox enzyme, was observed during the course of the enzymatic assay,
leading to significantly lower V
max and K
m values.
355,360
Figure 10
A comparison
of binding affinity of trivalent and pentavalent arsenicals
to proteins.
Several interesting
inferences can be made from the enzymatic studies
showing K
m or K
i values: (A) Reflective of product inhibition in enzymatic reactions,
arsenite is a potent inhibitor of arsenate reductase, for which arsenate
is the substrate. Similarly, MMAIII is an inhibitor of
iAsIII methylation by arsenic methyltransferase from rabbit
liver, suggesting a possibility that MMAIII is the enzymatic
reaction product of arsenic methyltransferase from iAsIII.
361,362
(B) Arsenic reductases (arsenate reductase
and GSTO1-1) generally have higher K
m values
than arsenic methyltransferases (including AS3MT), indicating that
the reduction step is rate-limiting for the overall process of arsenic
metabolism. (C) Pentavalent arsenicals are substrates (showing K
m) or inhibitors (showing K
i) of many enzymes, as shown in Table S3 (Supporting Information), probably as a result
of their mimicking a tetrahedral phosphorus atom in substrate or product
analogues. These enzymes include phosphorylases,
363−368
phosphatases,
335,348,369−375
phosphoenolpyruvate mutase,
376
glycerol-3-phosphate
dehydrogenase,
377
adenylate kinase,
378
enolase,
379
ethanolamine-phosphate
cytidylytransferase,
380
triosephosphate
isomerase,
381−383
ornithine carbamoyltransferase,
384,385
acetyl esterase,
304
chymotrypsin,
305,386
subtilisin,
305,386
and angiotensin-converting enzyme.
387
Ubiquitin, which has esterase, carbonic anhydrase,
and phosphatase activities, was also inhibited by arsenate.
388
The majority of the enzymes mentioned above
fall into the category of serine hydrolases. Serine hydrolases form
covalent intermediates with their substrate through the hydroxyl group
of the active site serine, and the catalytic action can be nearly
irreversibly inhibited by organophosphorus compounds, accounting for
the notoriously toxic effects of organophosphorus compounds.
389
The anomaly manifested by cholinesterases (acetylcholinesterase
and butylrylcholinesterase) for which arsenite but not arsenate is
an inhibitor, may be related to their unique structures compared to
other serine hydrolases. They contain a deep and narrow active site
groove with two separate ligand binding sites: an acylation site or
A-site at the base of the groove and a peripheral anionic site or
P-site at the groove mouth, with a strong electrostatic dipole aligned
with the groove leading to its active site.
390,391
Ligand binding to the P-site can result in steric blockade of the
A-site and inhibit substrate hydrolysis.
392
The anionic nature of the P-site
393
allows
entry of cationic but not anionic (e.g., arsenate) ligands to the
active site groove. Inhibition of acetylcholinesterase by arsenite
was shown to be pH-dependent,
394
possibly
reflecting the change with pH of the dissociated state of arsenious
acid (neutral). Arsenobetaine (AsB) and arsenocholine (AsC) were shown
to inhibit butyrylcholinesterase.
395
(D)
Arsenate is a potent inhibitor of alkaline or acid phosphatase (K
i < 1 mM) while arsenite inhibits the same
enzyme with K
i ≥ 17 mM, indicating
that reasonably low K
i (or K
d) values can differentiate specific from nonspecific
binding between arsenic and proteins. However, it is difficult to
establish a universal benchmark K
i (or K
d) below which a specific binding is defined
because of different methods and assay conditions used across the
studies. (E) With regard to possible arsenic–DNA interactions,
the dissociation constant (K
d) of arsenite–DNA
binding was determined spectroscopically to be around 8–90
μM
396,397
and can be even lower in the
case of arsenic binding to DNA aptamers.
398
However, no specific binding of either arsenate or arsenite to DNA
was observed by using vacuum filtration binding assays,
198
and the nature of arsenic–DNA binding
is noncovalent.
399
5.4
Binding
Kinetics
A few studies have
reported rate constants (association rate constant, k
on; dissociation rate constant, k
off) for the binding between arsenicals and proteins/peptides
(Table S5 in Supporting Information). Various
kinetic methods can be employed for the determination of rate constants.
Radioactive arsenic offers an extremely sensitive yet discontinuous
assay,
44
requiring off-line separation
of bound from free arsenic, which needs large sample size, excessive
analysis time, and time profile reconstruction. Assays based on UV–vis
absorption or fluorescence spectroscopy are most convenient since
the reaction can be monitored continuously and thus remain the major
methods for studying the reaction kinetics between arsenic and proteins/peptides.
The application of these assays requires optical changes accompanying
the reaction. These changes can result from ligand to metal charge-transfer,
330,400,401
use of chromophoric substrates,
87,254,301,331,336,394
or intrinsic tryptophan quenching.
299
Obviating the need for chromophoric probes, time-resovled ESI-MS
has been shown to be a powerful tool for kinetic studies in online
experiments.
402,403
Capable of simultaneous monitoring
of multiple intermediate species, ESI-MS affords accurate rate parameters
of biochemical reactions and provides insights into reaction mechanism.
403
Stillman and co-workers have acquired k
on values and depicted mechanisms for the binding
between arsenite and metallothioneins using time-resolved ESI-MS.
152−155
Figure 11 shows an example of reaction kinetics
between iAsIII and metallothionein. Current restriction(s)
with this technique lie(s) in the incompatibility of the electrospray
process with nonvolatile components, such as high concentrations of
salts, that may be required in some biochemical reactions. Under such
circumstances online microdialysis for rapid desalting of the reaction
mixture immediately prior to ESI-MS analysis is necessary and can
be achieved with an automated microfluidic system, without significantly
compromising the time resolution.
404
Figure 11
Kinetic data
showing the relative abundance of the various arsenic–metallothionein
species detected using electrospray mass spectrometry. The solution
was analyzed following reaction of 9 μM apo-rfMT with 108 μM
arsenite at 23.5 °C. The experimental data have a relative standard
error of 7%. (Reprinted with permission from ref (153).)
The kinetics of the binding between arsenic and protein thiols
have also been investigated at the single molecule level. By constructing
transmembrane protein pores formed by mutated α-hemolysin in
a planar bilayer, Shin et al.
405,406
studied the interaction
of 4-sulfophenylarsonous acid with a strategically positioned cysteine
in the protein. The k
on, k
off, and K
d values were derived
from recorded fluctuations in the ionic current flowing through a
single transmembrane protein pore after the arsenical was added to
the trans side of the bilayer.
Generally speaking, arsenic binds
both proteins and peptides with k
off values
of similar orders of magnitude (10–3–10–6 s–1). Arsenic binds to proteins
with much lower k
on values than to small
peptides, possibly reflecting that
an additional energy barrier has to be overcome in restructuring proteins
for arsenic binding. This energetic expense to disrupt preorganized
conformations has been evidenced in the case of arsenite binding to
human metallothionein α-domain with a slower than expected k
on for the first arsenite binding event when k
off was assumed to be constant and insignificant.
155
6
Summary and Perspectives
The biological effects of arsenic are diverse and the mechanisms
of arsenic toxicity and carcinogenicity are complicated. Arsenic–sulfur
interactions are the chemical basis of most of these effects. Trivalent
arsenicals bind to thiols that are contained in numerous intracellular
and cell-surface proteins, and this arsenic–protein binding
often triggers cellular responses. For example, arsenic binding to
PML or some mitochondrial proteins is a key step leading to cell differentiation
or apoptosis induced by As2O3. A low baseline
cellular glutathione content and a high expression level of the primary
membrane transporter aquaglyceroporin 9 that mediates uptake of As2O3 into cells predetermine
the selectivity of APL
toward As2O3.
407
Generation
of cellular ROS is often implicated in the inhibition of various enzymes
by arsenic; however, it cannot supplant the role of direct protein
binding by trivalent arsenicals. ROS generation itself may result
from thiol chelation or complexation in dithiol-containing enzymes,
such as pyruvate dehydrogenase (PDH), by trivalent arsenic metabolites.
Direct protein binding by pentavalent arsenicals is mainly manifested
in phosphate utilizing enzymes that normally alkylate, acylate, or
phosphorylate the phosphate,
408
with conceivable
biological implications. Serine hydrolases are often the proteins
that bind pentavalent arsenicals at the active site serine residue:
sulfhydryl groups do not seem to play an important role in their enzymatic
activities. Whether pentavalent arsenicals bind to these proteins
in a covalent way or as a coordination complex of certain thermodynamic
and kinetic properties needs to be examined. Future arsenic–protein
binding studies should include a variety of arsenicals on selected
proteins in order to fully understand the nature and diversity of
arsenic binding to proteins.
The issue of nonspecific association
needs to be addressed in protein
binding to arsenicals. Stable and specific interaction between arsenic
and biomolecules may be necessary to sustain the biological impact
of arsenic. For example, arsenite binds peptides containing three
(C3) or four (C4) cysteine residues more stably than those containing
two cysteine (C2) residues,
45,46
which may explain the
finding that cellular arsenic exposure resulted in loss of zinc from
C3 or C4 (PARP-1 and XPA), but not C2 (SP-1, APTX, and PARP-1 zinc
finger mutants) zinc finger proteins.
45
Similarly, nonspecific association between arsenite and DNA
198
demonstrates that arsenic does not directly
react with DNA and may account for the fact that inorganic arsenic
is inactive or extremely weak in its ability to induce gene mutations.
409,410
With the development of integrated metallomic approaches,
more
arsenic-binding proteins are expected to be identified under physiologically
and pathologically relevant conditions. However, binding specificity
should be enshrined as the key concept and scrutinized when identifying
novel arsenic-binding proteins with putative arsenic-binding domains
demonstrated.