In vitro biotransformation of dimethylarsinic acid and trimethylarsine oxide by anaerobic microflora of mouse cecum analyzed by HPLC-ICP-MS and HPLC-ESI-MS

Abstract

The capacity of the anaerobic microflora from a mouse cecum to metabolize dimethylarsinic acid (DMA^V) and trimethylarsine oxide (TMAO) was examined in an in vitro assay system containing cecal contents in modified VPI buffer. Samples were incubated under anaerobic conditions at 37 °C for up to 24 hours and metabolic products were analyzed by HPLC-ICP-MS and HPLC-ESI-MS/MS. Under these conditions, DMA^V was thiolated to dimethylthioarsinic acid (DMTA^V) and dimethyldithioarsinic acid (DMDTA). The identities of DMTA^V (m/z 154), DMDTA (m/z 170), and trimethylarsine sulfide (TMAS, m/z 152) were confirmed with HPLC-ESI-MS/MS. Three chromatographic separations were utilized to verify the lack of co-elution prior to quantification by ICP-MS. The predominant arsenical in reaction mixtures at 24 hours was DMDTA. The presence of TMAS in reaction mixtures after a six hour incubation implies a metabolism route from DMA^V to TMAS possibly via reduction to dimethylarsinous acid, methylation to yield TMAO, and thiolation to TMAS. Addition of TMAO to in vitro assay systems containing cecal contents demonstrated that TMAO was almost quantitatively converted to TMAS within one hour. These combined results indicate that ingested arsenicals can undergo substantial metabolism mediated by the microflora of the gastrointestinal tract. Finally, metabolism of arsenicals occurring before absorption across the gastrointestinal barrier could be a modifier of exposure and dose.