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COMPARATIVE IN VITRO METHYLATION OF TRIVALENT AND PENTAVALENT ARSENIC SPECIES
Citation:
Styblo, M., H. Yamauchi, AND D. Thomas. COMPARATIVE IN VITRO METHYLATION OF TRIVALENT AND PENTAVALENT ARSENIC SPECIES. U.S. Environmental Protection Agency, Washington, D.C., EPA/600/J-96/044, 1995.
Description:
The time course and extent of methylation of 1 uM arsenite (iAsIII), arsenate (iAsV), methylarsenite (MeAsV), methylarsenate (MeAsV) and MeAsIII - diglutathione complex (MAsIII(GS)2) were examined in an in vitro assay system that contained rat liver cytosol. recursor arsenicals and methylated metabolites were analyzed by thin layer chromatography (TLC) or by hydride generation-atomic absorption spectrophotometry (HG-AAS). ore than 90% of iAsIII was converted to a dimethylated species (Me2As) during a 90-minute incubation at 37 degrees C; the amount of monomethylated metabolite was maximal at 15 minutes. n contrast, only 40% of iAsV was dimethylated during a 90-minute incubation. omparison 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 Me2As ) that was produced during a 90 minute-incubation was bound to proteins (>10 kDa). he percentage of protein-bound arsenic in the assay system incubated at O degrees C with trivalent arsenicals was 3 to 5-fold greater than the binding of corresponding pentavalent species. his indicated that both iAsIII and trivalent organoarsenicals interact avidly with proteins. oth MeAsIII prepared by metabisulfite-thiosulfate reduction of MeAsv and a MeAsII(GS)2 were quantitatively converted to Me2As during 90-minute incubation. n contrast, only 3% of MeAsv was dimethylated during this interval. hese 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.