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)