Glutathione Modulates Recombinant Rat Arsenic (+3 Oxidation State) Methyltransferase-Catalyzed Formation of Trimethylarsine Oxide and Trimethylarsine

Abstract

Humans and other species enzymatically convert inorganic arsenic (iAs) into methylated metabolites. Although the major metabolites are mono- and dimethylated arsenicals, trimethylated arsenicals have been detected in urine following exposure to iAs. The AS3MT gene encodes an arsenic (+3 oxidation state) methyltransferase, which catalyzes both the oxidative methylation of trivalent arsenicals and the reduction of pentavalent arsenicals. In reaction mixtures containing recombinant rat AS3MT (rrAS3MT) and radiolabeled arsenite, mono- and dimethylated arsenicals and a third radiolabeled product can be resolved by thin-layer chromatography. Hydride generation atomic absorption spectrometry and electrospray ionization mass spectrometry identified the third reaction product as trimethylarsine oxide. The addition of glutathione to reaction mixtures containing radiolabeled arsenite and rrAS3MT increased the yield of methylated and dimethylated arsenicals but suppressed the formation of trimethylarsine oxide. Although a dimethylarsenic-glutathione complex was rapidly converted to trimethylarsine oxide, the addition of a molar excess of glutathione to dimethylarsenic suppressed the production of trimethylarsine oxide. The nonquantitative recovery of radioarsenic from reaction mixtures suggested that AS3MT catalyzed the formation of a volatile arsenical. This volatile species was identified as trimethylarsine. Thus, AS3MT catalyzes the formation of all products in a reaction sequence leading from an inorganic to a volatile methylated arsenical. The regulation of this pathway by intracellular glutathione may be an important determinant of the pattern and extent of formation of arsenicals.