Arsenic is known to be a human carcinogen as well as one of the most effective drugs
for treatment of patients with acute promyelocytic leukemia. The intermediate metabolites
of arsenic, monomethylarsonous acid (MMA(III)) and dimethylarsinous acid (DMA(III)),
are formed by methylation reactions, and they are more reactive and toxic than the
inorganic precursor arsenite (iAs(III)); however, the detailed mechanism of toxicity
is poorly understood. Here, we studied the effects of three arsenic compounds (i.e.,
iAs(III), MMA(III), and DMA(III)) on mitochondrial permeability transition pore (mPTP)
and release of apoptotic cytochrome c (Cyt c) after incubating with rat liver mitochondria.
Inorganic iAs(III) had no effect on mitochondrial swelling even at higher concentrations
ranging up to 100 μM, but swelling was significantly induced in the presence of Ca(2+).
Additionally, mitochondrial swelling was strongly induced by exposure to the methylated
forms of MMA(III) and DMA(III) in a dose-dependent manner in the absence of Ca(2+),
suggesting that the methylated forms may have potent effects on cellular mitochondria.
Although mitochondrial swelling was completely inhibited in the presence of cyclosporin-A
(an inhibitor of mitochondrial permeability transition) or ruthenium red (an inhibitor
of Ca(2+) uniporter) following exposure to methylated arsenicals, the release of apoptotic
Cyt c from mitochondria was not inhibited, indicating that release of Cyt c is probably
not dependent on mPTP opening. In addition, inhibitors of Bax (e.g., Bax-inhibiting
peptide) did not reduce the release of Cyt c from the mitochondria by formation of
Bax-voltage-dependent anion channel (VDAC) complex, whereas the recombinant Bcl-x(L
)proteins significantly reduced the release of Cyt c after exposure to DMA(III), suggesting
that dimethylated DMA(III) directly interacted with the VDAC in mitochondria and caused
the release of Cyt c from mitochondria.