Activation of 14C-toluene to covalently binding metabolites by rat liver microsomes.

14C-Toluene was incubated with rat liver microsomes in the presence of an NADPH-generating system and metabolites were concentrated on cyclohexyl cartridges. The metabolites were separated by reverse phase HPLC and identified by comparing the retention time to standards. 14C-Toluene was converted to 14C-benzylalcohol, 14C-cresols, and an unidentified 14C-metabolite. Some of the radioactivity was found to bind covalently to microsomal macromolecules, preferentially to proteins. The binding was proportional to incubation time and microsomal protein concentration and required NADPH and molecular oxygen. The binding was greatly diminished when microsomes were heat denatured. The binding process was partially inhibited by carbon monoxide and SKF 525-A. When microsomes from phenobarbital- and 3-methylcholanthrene-treated rats were employed, binding was enhanced by 8- and 4-fold, respectively. The binding process was effectively diminished by the presence of reduced glutathione or cysteine in the incubation mixture and was not affected by lysine. Styrene oxide greatly enhanced binding. UDP-glucuronic acid, superoxide dismutase, and ascorbic acid also diminished the binding to some degree. It was concluded that toluene undergoes a hepatic microsomal monooxygenase-mediated activation, and the resultant reactive metabolites binds covalently to microsomal proteins.