Inhibition of Cytochrome P-450-Dependent Monooxygenase Activities by a Synthetic Androgen Danazol in Mouse Hepatic Microsomes

The inhibitory effect of danazol, a synthetic androgen, on the hydroxylations of testosterone as a model for endogenous steroids and the dealkylations of aminopyrine and 7-ethoxycoumarin as models for xenobiotics in mouse hepatic microsomes in vitro was studied. Danazol inhibited these enzyme activities in a dose-dependent fashion. The inhibition constants (Ki) of danazol for these enzyme activities were 1-4 orders of magnitude lower than those of cimetidine, while there was a great difference among the inhibitory potencies of danazol for each enzyme activity. Addition of danazol to microsomal preparation resulted in a reverse type I difference spectrum and the spectrophotometric analysis revealed that danazol had a high affinity for cytochrome P-450 with dissociation constants (Ks) of 0.9 and 4.2 microM, which were 2 orders of magnitude lower than those of cimetidine. On the other hand, danazol did not significantly affect reduced nicotinamide adenine dinucleotide phosphate cytochrome c reductase activity and levels of cytochrome P-450 in the microsomes. These results suggest that danazol is a highly potent inhibitor for several cytochrome P-450-mediated metabolisms of testosterone and xenobiotics in mouse hepatic microsomes.