Determination of monomethylarsonous acid, a key arsenic methylation intermediate, in human urine

In this study we report on the finding of monomethylarsonous acid [MMA(III) I in human urine. This newly identified arsenic species is a key intermedia te in the metabolic pathway of arsenic biomethylation, which involves stepw ise reduction of pentavalent to trivalent arsenic species followed by oxida tive addition of a methyl group. Arsenic speciation was carried out using i on-pair chromatographic separation of arsenic compounds with hydride genera tion atomic fluorescence spectrometry detection. Speciation of the inorgani c arsenite [As(III)], inorganic arsenate [As(V)], monomethylarsonic acid [M MA(V)], dimethylarsinic acid [DMA(V)], and MMA(III) in a urine sample was c omplete in 5 min. Urine samples collected from humans before and after a si ngle oral administration of 300 mg sodium 2,3-dimercapto-1-propane sulfonat e (DMPS) were analyzed for arsenic species. MMA(III) was found in 51 out of 123 urine samples collected from 41 people in inner Mongolia 0-6 hr after the administration of DMPS. MMA(III)in urine samples did not arise from the reduction of MMA(V) by DMPS. DMPS probably assisted the release of MMA(III ) that was formed in the body. Along with the presence of MMA(III), there w as an increase in the relative concentration of MMA(V) and a decrease in DM A(V) in the urine samples collected after the DMPS ingestion.