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      Arsenic detoxification and evolution of trimethylarsine gas by a microbial arsenite S-adenosylmethionine methyltransferase.

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      Journal: Proceedings of the National Academy of Sciences of the United States of America
      Keywords: Arsenic, metabolism, pharmacology, Arsenicals, Arsenites, Cloning, Molecular, Drug Resistance, genetics, Escherichia coli, drug effects, enzymology, Gene Expression Regulation, Bacterial, Inactivation, Metabolic, Methylation, Methyltransferases, chemistry, Rhodopseudomonas, S-Adenosylmethionine, Transcription, Genetic, Volatilization

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          Abstract

          In this article, a mechanism of arsenite [As(III)]resistance through methylation and subsequent volatization is described. Heterologous expression of arsM from Rhodopseudomonas palustris was shown to confer As(III) resistance to an arsenic-sensitive strain of Escherichia coli. ArsM catalyzes the formation of a number of methylated intermediates from As(III), with trimethylarsine as the end product. The net result is loss of arsenic, from both the medium and the cells. Because ArsM homologues are widespread in nature, this microbial-mediated transformation is proposed to have an important impact on the global arsenic cycle.

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          Bacterial mercury resistance from atoms to ecosystems.

          L. Miller, Tamar Barkay, Anne O Summers … (2003)
          Bacterial resistance to inorganic and organic mercury compounds (HgR) is one of the most widely observed phenotypes in eubacteria. Loci conferring HgR in Gram-positive or Gram-negative bacteria typically have at minimum a mercuric reductase enzyme (MerA) that reduces reactive ionic Hg(II) to volatile, relatively inert, monoatomic Hg(0) vapor and a membrane-bound protein (MerT) for uptake of Hg(II) arranged in an operon under control of MerR, a novel metal-responsive regulator. Many HgR loci encode an additional enzyme, MerB, that degrades organomercurials by protonolysis, and one or more additional proteins apparently involved in transport. Genes conferring HgR occur on chromosomes, plasmids, and transposons and their operon arrangements can be quite diverse, frequently involving duplications of the above noted structural genes, several of which are modular themselves. How this very mobile and plastic suite of proteins protects host cells from this pervasive toxic metal, what roles it has in the biogeochemical cycling of Hg, and how it has been employed in ameliorating environmental contamination are the subjects of this review.
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            Comparative toxicity of trivalent and pentavalent inorganic and methylated arsenicals in rat and human cells.

            Miroslav Stýblo, Luz M Del Razo, Libia Vega … (2000)
            Biomethylation is considered a major detoxification pathway for inorganic arsenicals (iAs). According to the postulated metabolic scheme, the methylation of iAs yields methylated metabolites in which arsenic is present in both pentavalent and trivalent forms. Pentavalent mono- and dimethylated arsenicals are less acutely toxic than iAs. However, little is known about the toxicity of trivalent methylated species. In the work reported here the toxicities of iAs and trivalent and pentavalent methylated arsenicals were examined in cultured human cells derived from tissues that are considered a major site for iAs methylation (liver) or targets for carcinogenic effects associated with exposure to iAs (skin, urinary bladder, and lung). To characterize the role of methylation in the protection against toxicity of arsenicals, the capacities of cells to produce methylated metabolites were also examined. In addition to human cells, primary rat hepatocytes were used as methylating controls. Among the arsenicals examined, trivalent monomethylated species were the most cytotoxic in all cell types. Trivalent dimethylated arsenicals were at least as cytotoxic as trivalent iAs (arsenite) for most cell types. Pentavalent arsenicals were significantly less cytotoxic than their trivalent analogs. Among the cell types examined, primary rat hepatocytes exhibited the greatest methylation capacity for iAs followed by primary human hepatocytes, epidermal keratinocytes, and bronchial epithelial cells. Cells derived from human bladder did not methylate iAs. There was no apparent correlation between susceptibility of cells to arsenic toxicity and their capacity to methylate iAs. These results suggest that (1) trivalent methylated arsenicals, intermediary products of arsenic methylation, may significantly contribute to the adverse effects associated with exposure to iAs, and (2) high methylation capacity does not protect cells from the acute toxicity of trivalent arsenicals.
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              Microbial arsenic: from geocycles to genes and enzymes.

              Rita Mukhopadhyay, Simon Silver, Kieu Phung … (2002)
              Arsenic compounds have been abundant at near toxic levels in the environment since the origin of life. In response, microbes have evolved mechanisms for arsenic resistance and enzymes that oxidize As(III) to As(V) or reduce As(V) to As(III). Formation and degradation of organoarsenicals, for example methylarsenic compounds, occur. There is a global arsenic geocycle, where microbial metabolism and mobilization (or immobilization) are important processes. Recent progress in studies of the ars operon (conferring resistance to As(III) and As(V)) in many bacterial types (and related systems in Archaea and yeast) and new understanding of arsenite oxidation and arsenate reduction by respiratory-chain-linked enzyme complexes has been substantial. The DNA sequencing and protein crystal structures have established the convergent evolution of three classes of arsenate reductases (that is classes of arsenate reductases are not of common evolutionary origin). Proposed reaction mechanisms in each case involve three cysteine thiols and S-As bond intermediates, so convergent evolution to similar mechanisms has taken place.
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                Author and article information

                Journal
                PubMed ID:: 16452170
                PMC ID:: 1413689
                DOI:: 10.1073/pnas.0506836103

                ScienceOpen disciplines: Chemistry
                Keywords: Arsenic,metabolism,pharmacology,Arsenicals,Arsenites,Cloning, Molecular,Drug Resistance,genetics,Escherichia coli,drug effects,enzymology,Gene Expression Regulation, Bacterial,Inactivation, Metabolic,Methylation,Methyltransferases,chemistry,Rhodopseudomonas,S-Adenosylmethionine,Transcription, Genetic,Volatilization
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                ScienceOpen disciplines: Chemistry
                Keywords: Arsenic, metabolism, pharmacology, Arsenicals, Arsenites, Cloning, Molecular, Drug Resistance, genetics, Escherichia coli, drug effects, enzymology, Gene Expression Regulation, Bacterial, Inactivation, Metabolic, Methylation, Methyltransferases, chemistry, Rhodopseudomonas, S-Adenosylmethionine, Transcription, Genetic, Volatilization

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