Methylation of arsenic by recombinant human wild-type arsenic (+ 3 oxidation state) methyltransferase and its methionine 287 threonine (M287T) polymorph: Role of glutathione

Abstract

Arsenic (+3 oxidation state) methyltransferase (AS3MT) is the key enzyme in the pathway for methylation of arsenicals. A common polymorphism in the AS3MT gene that replaces a threonyl residue in position 287 with a methionyl residue (AS3MT/M287T) occurs at a frequency of about 10% among populations worldwide. Here, we compared catalytic properties of recombinant human wild-type (wt) AS3MT and AS3MT/M287T in reaction mixtures containing S-adenosylmethionine, arsenite (iAsIII) or methylarsonous acid (MAsIII) as substrates and endogenous or synthetic reductants, including glutathione (GSH), a thioredoxin reductase (TR)/thioredoxin (Trx)/NADPH reducing system, or tris (2-carboxyethyl) phosphine hydrochloride (TCEP). With either TR/Trx/NADPH or TCEP, wtAS3MT or AS3MT/M287T catalyzed conversion of iAsIII to MAsIII, methylarsonic acid (MAsV), dimethylarsinous acid (DMAsIII), and dimethylarsinic acid (DMAsV); MAsIII was converted to DMAsIII and DMAsV. Although neither enzyme required GSH to support methylation of iAsIII or MAsIII, addition of 1mM GSH decreased Km and increased Vmax estimates for either substrate in reaction mixtures containing TR/Trx/NADPH. Without GSH, Vmax and Km values were significantly lower for AS3MT/M287T than for wtAS3MT. In the presence of 1mM GSH, significantly more DMAsIII was produced from iAsIII in reactions catalyzed by the M287T variant than in wtAS3MT-catalyzed reactions. Thus, 1mM GSH modulates AS3MT activity, increasing both methylation rates and yield of DMAsIII. AS3MT genotype exemplified by differences in regulation of wtAS3MT and AS3MT/M287T-catalyzed reactions by GSH may contribute to differences in the phenotype for arsenic methylation and, ultimately, to differences in the disease susceptibility in individuals chronically exposed to inorganic arsenic.