The metabolism of benzimidazoline-2-thione (I) and the 1-methyl (11) and 1,3-dimethyl (III) derivatives was studied to elucidate the mechanisms of hepatic oxidation for this class of thionosulfur-containing xenobiotics. NADPH-dependent metabolism of I, II, and III to the corresponding benzimidazoles Ia, IIa, and IIIa, respectively, was observed in dexamethasone-pretreated rat hepatic microsomes. III was the only thiocarbamide converted to an amide metabolite (IIb). The effects of heat and 1-aminobenzotriazole pretreatment suggested that rat hepatic microsomal metabolism of I was catalyzed by the flavin-containing monoxygenase (FMO) only and that of II and III by both FMO and cytochrome P450 isozymes (P450). Addition of 5.0 mM glutathione (GSH) blocked formation of all metabolites from I, II, and III. Highly purified hog liver FMO catalyzed formation of all metabolites observed in rat hepatic microsomal systems. Incubation of III with either rat liver microsomes or with highly purified hog liver FMO in the presence of [O-18] water led to ca. 50 % incorporation of [O-18] into IIIb. When [O-18] molecular oxygen was used, ca. 8 % incorporation of [O-18] into IIIb was observed. Highly purified hog liver FMO also converted I-III to chemically reactive species that covalently bound to protein thiols. In the presence of hog liver FMO, the covalent binding pattern of radiolabeled I-III to bovine serum albumin was essentially identical to that observed for rat hepatic microsomes. The formation of Ib and concomitant extrusion of the sulfur moiety of the molecule to species that covalently bind to microsomal proteins suggested a novel oxidation mechanism for III by both microsomal monooxygenases in which sulfur monoxide is a reactive species produced. While rat liver microsomal FMO alone catalyzed the metabolism of I, these studies suggested that P450 and FMO catalyzed formation of the same reactive intermediates from II and III. Apparently, the reactivity of the S-oxygenated species is a major determinant in the ultimate formation of products and covalent binding to proteins. The results of these studies are inconsistent with a general role for the formation of atomic sulfur as a reactive species and hydrodisulfide protein adducts during microsomal metabolism of thiocarbamides.
