Metabolism of dihalomethanes to carbon monoxide. II. In vitro studies

Dibromomethane was metabolized to carbon monoxide and inorganic bromide by a rat liver microsomal fraction requiring both NADPH and molecular oxygen. This biotransformation was characterized with respect to time course, microsomal protein concentration, pH, and temperature. The metabolism of dihalomethanes to carbon monoxide followed the halide order; thus, diiodomethane yielded the greatest amount of carbon monoxide, whereas dichloromethane yielded the smallest amount. A KM of approximately 16 mM was established for dibromomethane while the Vmax was found to be about 8 nmol of CO per mg of microsomal protein per min. Cytochrome P-450 was found to bind dibromomethane to produce a type I binding spectrum. Pretreatment with phenobarbital increased both microsomal cytochrome P-450 levels and the rate of conversion of dibromomethane to carbon monoxide. Pretreatment with cobaltous chloride or storage of microsomal preparations at 4 degrees C resulted in parallel reductions of both cytochrome P-450 levels and the rate of formation of carbon monoxide from dibromomethane. SKF 525-A, ethylmorphine, and hexobarbital inhibited the conversion of dibromomethane to carbon monoxide. Microsomal preparations from rat lung metabolized this substrate at about 18% of the rate found in liver microsomes. The requirement for both NADPH and molecular oxygen, the inhibitory effects of SKF 525-A, ethylmorphine, and hexobarbital on the production of carbon monoxide from dibromomethane, and the correlation established between microsomal cytochrome P-450 levels and the rate of metabolism of dihalomethanes to carbon monoxide after treatments that alter the cytochrome to P-450 content suggest that these compounds are metabolized to carbon monoxide via a cytochrome P-450-dependent system.