Studies on the O-demethylation of misonidazole by rat liver microsomes

The role of rat liver microsomes in the O-demethylation of misonidazole to desmethylmisonidazole was studied. The rate of the microsomal-dependent formation of desmethylmisonidazole was linear up to a protein concentration of 2 mg/ml and over a 10-minute interval. The metabolism was optimal in a system comprised of microsomes, O2, and NADPH. Metabolism in incubation mixtures continuously flushed with N2 was inhibited by 78%. The O-demethylase activity was competitively inhibited by the addition of SKF 525-A, with a Ki of approximately 1 x 10(-5) M. The Km and Vmax values of normal microsomes were 1.87 +/- 0.30 mM and 413 +/- 14 pmols/minute/mg of microsomal protein, respectively. Pretreatment of rats with phenobarbital for 7 days prior to preparation of the microsomes resulted in no significant change in the Km, but the Vmax was considerably increased to 1033 +/- 203 pmols/minute/mg of microsomal protein. The results demonstrate that the O-demethylation of misonidazole is mediated by cytochrome P-450.