Endogenous Reductants Support the Catalytic Function of Recombinant Rat Cyt19, an Arsenic Methyltransferase

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Abstract
The postulated scheme for the metabolism of inorganic As involves alternating steps of oxidative methylation and of reduction of As from the pentavalent to the trivalent oxidation state, producing methylated compounds containing AsIII that are highly reactive and toxic. S-Adenosyl-l-methionine:AsIII methyltransferase purified from rat liver catalyzes production of methyl and dimethyl arsenicals from inorganic As. This protein is encoded by the cyt19 gene orthologous with cyt19 genes in mouse and human. The reductants dithiothreitol or tris(2-carboxylethyl)phosphine support catalysis by recombinant rat cyt19 (rrcyt19). Coupled systems containing an endogenous reductant (thioredoxin/thioredoxin reductase/NADPH, glutaredoxin/glutathione/glutathione reductase/NADPH, or lipoic acid/thioredoxin reductase/NADPH) support inorganic As methylation by rrcyt19. Although glutathione alone does not support rrcyt19's catalytic function, its addition to reaction mixtures containing other reductants increases the rate of As methylation. Aurothioglucose, an inhibitor of thioredoxin reductase, reduces the rate of As methylation by rrcyt19 in thioredoxin-supported reactions. Addition of guinea pig liver cytosol, a poor source of endogenous As methyltransferase activity, to reaction mixtures containing rrcyt19 shows that endogenous reductants in cytosol support the enzyme's activity. Methylated compounds containing either AsIII or AsV are detected in reaction mixtures containing rrcyt19, suggesting that cycling of As between oxidation states is a component of the pathway producing methylated arsenicals. This enzyme may use endogenous reductants to reduce pentavalent arsenicals to trivalency as a prerequisite for utilization as substrates for methylation reactions. Thus, cyt19 appears to possess both AsIII methyltransferase and AsV reductase activities.