Arsenolysis and Thiol-Dependent Arsenate Reduction

Toxicological Sciences, Oct 2010

Conversion of arsenate to arsenite is a critical event in the pathway that leads from inorganic arsenic to a variety of methylated metabolites. The formation of methylated metabolites influences distribution and retention of arsenic and affects the reactivity and toxicity of these intermediates. Indeed, some of the toxic and carcinogenic effects associated with exposure to arsenate or arsenite are probably mediated by methylated arsenicals. Recent work has demonstrated a biologically plausible role for phosphorolytic-arsenolytic enzymes in a reaction scheme in which an “activated” arsenate ester is readily reduced by thiols to arsenite. Thiol-dependent reduction of arsenate esters formed by arsenolysis may be one of several functionally reductant processes that control the flux of arsenic into the cellular pathway for arsenic methylation. Integrating these reductive processes into a conceptual model for arsenic metabolism may provide new insights into the cellular machinery for handling this toxic metalloid.

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Arsenolysis and Thiol-Dependent Arsenate Reduction

David J. Thomas 0 0 Pharmacokinetics Branch, Integrated Systems Toxicology Division, National Health and Environmental Effects Research Laboratory, Office of Research and Development, U.S. Environmental Protection Agency , MD B143-1, 109 Alexander Drive, Research Triangle Park, North Carolina 27711 Conversion of arsenate to arsenite is a critical event in the pathway that leads from inorganic arsenic to a variety of methylated metabolites. The formation of methylated metabolites influences distribution and retention of arsenic and affects the reactivity and toxicity of these intermediates. Indeed, some of the toxic and carcinogenic effects associated with exposure to arsenate or arsenite are probably mediated by methylated arsenicals. Recent work has demonstrated a biologically plausible role for phosphorolytic-arsenolytic enzymes in a reaction scheme in which an ''activated'' arsenate ester is readily reduced by thiols to arsenite. Thiol-dependent reduction of arsenate esters formed by arsenolysis may be one of several functionally reductant processes that control the flux of arsenic into the cellular pathway for arsenic methylation. Integrating these reductive processes into a conceptual model for arsenic metabolism may provide new insights into the cellular machinery for handling this toxic metalloid. - arsenate in this unstable species is quickly reduced to arsenite (see, Gregus et al., 2009). Arsenolysis is a biochemical phenomenon with a distinguished lineage that reflects the chemical similarities of arsenate and phosphate. Early in the 20th century, biochemists elucidating the role of phosphorous in cellular energetics found that arsenate could disrupt phosphate metabolism. By midcentury, it was clear that arsenate could substitute for phosphate in many reactions, including formation of arsenate esters, which were far less stable than corresponding phosphate esters. For example, replacement of phosphate with arsenate in reactions catalyzed by bacterial sucrose phosphorylase converted sucrose to glucose not glucose1-phosphate (Doudoroff et al., 1947). These investigators postulated that glucose-1-arsenate formed in this reaction was unstable; the rapid decomposition of this arsenate ester was termed arsenolysis. Subsequent studies of oxidative phosphorylation in partially purified rat liver mitochondria showed that arsenate reduced the rate of ATP generation by arsenolysis of an unstable ADP-arsenate complex and coincidentally provided the first indirect evidence that arsenate was reduced to arsenite in a mitochrondrial-enriched assay system (Crane and Lipmann, 1953). In the new paper, the investigators first examined the reactions catalyzed by recombinant Escherichia coli purine nucleoside phosphorylase (E.C. 2.4.2.1, PNPase). In the presence of polyA, arsenate and GSH, PNPase catalyzed a reaction in which arsenite was a final reaction product. Although formation of AMP-arsenate was unaffected by the presence of GSH, arsenite production depended on the presence of a mono- or dithiol. Experiments involving sequential addition of arsenate and GSH to reaction mixtures indicated that formation of AMP-arsenate facilitated thiol-dependent reduction of arsenate to arsenite. These studies of the role of arsenolysis in the reduction of arsenate were extended to examine the reduction of arsenate in mitochondria. Nemeti and Gregus (2002) had reported that isolated rat liver mitochondria efficiently reduce arsenate to arsenite and extrude arsenite. In a series of studies that link back to the work of Crane and Lipmann (1953), they examined the role of ATP synthase activity of mitochondria in reduction of arsenate in this organelle. Because ATP synthase activity depends on the structural integrity of mitochondria, studies were performed in in vitro systems containing isolated rat liver mitochondria. Hence, these studies were not wholly amenable to the tools used by an enzymologist to study catalysis by a purified enzyme, and some conclusions must be qualified by the uncertainties surrounding results obtained in complex systems. Given these caveats, results reported in this paper are consistent with a prominent role for mitochondrial ATP synthase in reduction of arsenate. In particular, depletion of intramitochondrial GSH markedly reduced production of arsenite, suggesting that formation of ADP-arsenate and thiol-dependent reduction of arsenate to arsenite probably occurred in the organelle. As noted by the authors, other phosphorolyticarsenolytic enzymes in mitochondria might also contribute to the organelles capacity to reduce arsenate, although their contribution to reductive capacity is likely to be small relative to the role of ATP synthase. These findings merit consideration from several perspectives. First, what is the importance of arsenate reduction in the metabolism of arsenic? Second, how is thiol-dependent arsenolytic reduction of arsenate related to other pathways for reduction of arsenate? T (...truncated)


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David J. Thomas. Arsenolysis and Thiol-Dependent Arsenate Reduction, Toxicological Sciences, 2010, pp. 249-252, 117/2, DOI: 10.1093/toxsci/kfq224