Proposal for novel metabolic pathway of highly toxic dimethylated arsenics accompanied by enzymatic sulfuration, desulfuration and oxidation

The International Agency for Research on Cancer (IARC) has concluded that dimethylarsinic acid [(CH3)2AsO(OH), DMAV], a main metabolite of inorganic arsenic, is responsible for carcinogenesis in urinary bladder and lung in rodents, and various modes of carcinogenic action have been proposed. One theory concerning the mode of action is that the biotransformation of dimethylarsinous acid [(CH3)2AsOH, DMAIII] from DMAV plays an important role in the carcinogenesis by way of reactive oxygen species (ROS) production. Furthermore, dimethylmonothioarsinic acid [(CH3)2AsS(OH), DMMTAV], a metabolite of DMAV, has also been noted because of its higher toxicity. However, the metabolic mechanisms of formation and disappearance of DMAIII and DMMTAV, and their toxicity are not fully understood. Thus, the purpose of the present study was to clarify the mechanism of metabolic formation of DMMTAV and DMAV from DMAIII. The in vitro transformation of arsenicals by treatment with liver homogenate from rodents and sulfur transferase was detected by HPLC-ICP-MS and HPLC-tandem MS. DMMTAV is produced from DMAIII but not DMAV by cellular fractions from mouse liver homogenates and by rhodanese from bovine liver in the presence of thiosulfate, a sulfur donor. Not only DMMTAV thus produced but also DMAIII are re-converted into DMAV by an in vitro addition of S9 mix. These findings indicate that the metabolic process not only of DMAIII to DMAV or DMMTAV but also of DMMTAV to DMAV consists of a complicated mode of interaction between monooxygenase including cytochrome P450 (CYP) and/or sulfur transferase.