Abstract: Isolation and identification of the liver microsomal cytochrome P-450 isoenzymes responsible for the formation of diazepam main metabolites-nordiazepam and temazepam in rats were studied. The effects of P-450 inducers and inhibitors on the protein contents in SDS-poly-acrylamide gel electrophoresis and thin layer chromatography to the corresponding diazepam metabolizing activities of rat liver microsomes were observed. The P-450 contents were dramatically reduced by ip diazepam, cimetidine or propranolol. Diazepam and propranolol inhibited temazepam formation, high dose of propranolol also inhibited nordiazepam formation. Phenobarbital increased the P-450 contents and induced the production of both nordiazepam and temazepam. It also induced proteins with molecular weight (m) of 51 and 59 kDa in SDS-PAGE and those with m ranging from 45 to 55 kDa and from 55 to 65 kDa in TLC. Propranolol inhibited both fractions, especially that of m 55~65 kDa, whereas diazepam tended to inhibit the fraction of 45~55 kDa. The protein of m 51 kDa could be mainly involved in diazepam C3-hydroxylation, whereas those of m 59 kDa could be responsible for the N-demethylation of diazepam in rats.

Key words: Diazepam, Nordiazepam, Temazepam, Demethylase, Hydroxylase, Cytochrome P-450, Liver microsomes