Generation of thioarsenicals is dependent on the enterohepatic circulation in rats

Abstract

Three minor sulfur-containing arsenic metabolites: monomethylmonothioarsonic acid (MMMTA^V), dimethylmonothioarsinic acid (DMMTA^V), and dimethyldithioarsinic acid (DMDTA^V) were recently found in human and animal urine after exposure to inorganic arsenic. However, it remains unclear how the thioarsenicals are formed in the body and then excreted into the urine. It is hypothesized that the generation of thioarsenicals occurs during enterohepatic circulation. To address this hypothesis, male Sprague Dawley (SD) rats and Eisai hyperbilirubinuric (EHB) rats (with deficiency of multidrug resistance-associated protein 2) were orally administered a single dose of inorganic arsenite (iAs^III) at 3.0 mg kg⁻¹ of body weight. Five hours after dosing, less than 1.0% of the dose was recovered in the bile of EHB rats, while more than 27% of the dose was recovered in the bile of SD rats, with the majority being monomethylarsinodiglutathione [MMA(SG)₂] with a small amount of arsenic triglutathione [iAs(SG)₃]. During the early time periods (3 h and 6 h) the arsenic levels in the liver, red blood cells (RBCs) and plasma of EHB rats were higher than those of SD rats, and approximately 76% and 87% of the dose was recovered in the RBCs of SD and EHB rats, respectively, at day 5 after dosing. However, there were no significant differences in arsenic concentration in urine between the two types of animal. Regarding the arsenic species in the urine of both types of rat, significant levels of thiolated arsenicals MMMTA^V and DMMTA^V were detected in SD rat urine, however in EHB rat urine only low levels of DMMTA^V were detected. The present result of the metabolic balance and speciation study suggests that the formation of MMMTA^V and DMMTA^V in rats is dependent on enterohepatic circulation. In addition, in vitro experiments indicated that arsenicals excreted from bile may be transformed by gastrointestinal microbiota into MMMTA^V and DMMTA^V, which are then absorbed into the bloodstream and finally excreted into the urine.