Comparative in vitro methylation of trivalent and pentavalent arsenicals

The time course and extent of methylation of 1 microM arsenite (iAsIII), arsenate (iAsV), methylarsenite (MeAsIII), methylarsenate (MeAsV), and MeAsIII-diglutathione complex (MeAsIII(GS)2) were examined in an in vitro assay system that contained rat liver cytosol. Precursor arsenicals and methylated metabolites were analyzed by thin-layer chromatography (TLC) or by hydride generation-atomic absorption spectrophotomoetry (HG-AAS). More than 90% of iAsIII was converted to a dimethylated species (Me2As) during a 90-min incubation at 37 degrees C; the amount of monomethylated metabolite was maximal at 15 min. In contrast, only 40% of iAsV was dimethylated during a 90-min incubation. Comparison of the yields of methylated species in the whole in vitro assay system as determined by HG-AAS and in an ultrafiltrate prepared from the in vitro assay system as determined by TLC indicated that nearly 70% of the dimethylated metabolite (possibly Me2AsIII) that was produced during a 90-min incubation was bound to proteins (> 10 kDa). The percentage of protein-bound arsenic in the assay system incubated at 0 degree C with trivalent arsenicals was three-to fivefold greater than the binding of corresponding pentavalent species. This indicated that both iAsIII and trivalent organoarsenicals interact avidly with proteins. Both MeAsIII prepared by metabisulfite-thiosulfate reduction of MeAsV and a MeAsIII(GS)2 were quantitatively converted to Me2As during 90-min incubation. In contrast, only 3% of MeAsV was dimethylated during this interval. These results suggest that trivalent arsenicals are preferred substrates for methylation reactions and that the reduction of As from pentavalent to trivalent states may be a critical step in the control of the rate of metabolism of As.