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METHYLATION OF ARSENIC BY RECOMBINANT HUMAN AS3MT/287M AND AS3MT/287T POLYMORPHS
Citation:
Ding, L., J. Saunders, M. Styblo, AND D. J. THOMAS. METHYLATION OF ARSENIC BY RECOMBINANT HUMAN AS3MT/287M AND AS3MT/287T POLYMORPHS. Presented at Society of Toxicology (SOT) Annual Meeting, Washington, DC, March 06 - 10, 2011.
Impact/Purpose:
This abstract describes the effect of genetic polymorphism in human AS3mt that alters a single amino acid (Met287Thr). This change alters the catalytic properties of the enzyme and affects the pattern of metabolites formed in the reaction sequence.
Description:
Arsenic (+3 oxidation state) methyltransferase (AS3MT) is the key enzyme in the pathway for methylation of inorganic arsenic (iAs). AS3MT polymorphism is, in part, responsible for interindividual differences in iAs metabolism. AS3MT/M287T polymorphism that is found in ~ 10% of Caucasian and Hispanic populations has been shown to affect the profiles of iAs metabolites in urine of subjects chronically exposed to iAs. Previous work using a suboptimal in vitro system containing glutathione (GSH) showed a significant difference between the rates of iAs methylation by recombinant human AS3MT/287M and AS3MT/287T. We compared the catalytic properties of recombinant human AS3MT/287M and AS3MT/287T variants in an in vitro reaction mixture containing arsenite (iAslll) , S-adenosylmethionine and an endogenous reducing system consisting of thioredoxin (Trx), TRx reductase (TR), and NADPH. The AS3MT variants methylated 0.1-1 uM iAsIII with similar efficiencies to yield primarily dimethylarsinate (DMAsv) and dimethylarsinite (DMAslll) ; methylarsonate (MAsv) and methylarsonite (MAslll) were only minor metabolites. However, MAsv and MAslll were the predominant methylated metabolites in reaction mixtures containing 10 uM iAslll. Addition of 1-10 mM GSH into the reaction mixture resulted in a 2-to 3-fold increase in the rate of iAs methylation by either variant, favoring production of trivalent methylated metabolites, MAslll and DMAslll. Under these conditions, the AS3MT/287T variant produced significantly more DMAs lll than did AS3MT/287M variant. Notably, in the absence of the TRx/TR/NADPH system, addition of GSH was not sufficient to support iAsIlI methylation by either AS3MT/287T or AS3MT/287M. These results suggest that: (1) GSH is not essential for iAs methylation but is an important modulator of AS3MT activity in human tissues; (2) the carriers of AS3MT/287T genotype may be more susceptible to adverse effects of iAs exposure due to an increased production of DMAslll. (This abstract does not reflect U.S. EPA policy