Exploring the in vitro formation of trimethylarsine sulfide from dimethylthioarsinic acid in anaerobic microflora of mouse cecum using HPLC-ICP-MS and HPLC-ESI-MS

Although metabolism of arsenicals to form methylated oxoarsenical species has been extensively studied, less is known about the formation of thiolated arsenical species that have recently been detected as urinary metabolites. Indeed, their presence suggests that the metabolism of ingested arsenic is more complex than previously thought. Recent reports have shown that thiolated arsenicals can be produced by the anaerobic microflora of the mouse cecum, suggesting that metabolism prior to systemic absorption may be a significant determinant of the pattern and extent of exposure to various arsenic-containing species. Here, we examined the metabolism of 34S labeled dimethylthioarsinic acid (34S-DMTAV) by the anaerobic microflora of the mouse cecum using HPLC-ICP-MS and HPLC-ESI-MS/MS to monitor for the presence of various oxo- and thioarsenicals. The use of isotopically enriched 34S-DMTAV made it possible to differentiate among potential metabolic pathways for production of the trimethylarsine sulfide (TMASV). Upon in vitro incubation in an assay containing anaerobic microflora of mouse cecum, 34S-DMTAV underwent several transformations. Labile 34S was exchanged with more abundant 32S to produce 32S-DMTAV, a thiol group was added to yield DMDTAV, and a methyl group was added to yield 34S-TMASV. Because incubation of 34S-DMTAV resulted in the formation of 34S-TMASV, the pathway for its formation must preserve the arsenic-sulfur bond. The alternative metabolic pathway postulated for formation of TMASV from dimethylarsinic acid (DMAV) would proceed via a dimethylarsinous acid (DMAIII) intermediate and would necessitate the loss of 34S label. Structural confirmation of the metabolic product was achieved using HPLC-ESI-MS/MS. The data presented support the direct methylation of DMTAV to TMASV. Additionally, the detection of isotopically pure 34S-TMASV raises questions about the sulfur exchange properties of TMASV in the cecum material. Therefore, 34S-TMASV was incubated and the exchange was monitored with respect to time. The data suggest that the As-S bond associated with TMASV is less labile than the As-S bond associated with DMTAV.