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METHYLATION OF ARSENIC BY RECOMBINANT HUMAN AS3MT/287M AND AS3MT/287T POLYMORPHS
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.
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
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.
Record Details:Record Type: DOCUMENT (PRESENTATION/ABSTRACT)
Organization:U.S. ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF RESEARCH AND DEVELOPMENT
NATIONAL HEALTH AND ENVIRONMENTAL EFFECTS RESEARCH LAB
INTEGRATED SYSTEMS TOXICOLOGY DIVISION