BCNU-induced quantitative and qualitative changes in hepatic cytochrome P-450 can be correlated with cholestasis.

Male Sprague-Dawley rats were given single i.p. injections of 1,3-bis(2-chloroethyl)-1-Nitrosourea (BCNU) to investigate changes in hepatic microsomal cytochrome P-450 content and metabolic activity. On day 14 after treatment (20 mg/kg), cytochrome P-450 content had decreased by approximately 25% and ethylmorphine N-demethylase activity (nmol product/nmol P-450/min) had decreased by 36%. In contrast, ethylmorphine O-deethylase and 7-ethoxycoumarin O-deethylase activities were not significantly decreased by BCNU treatment. Hepatic delta-aminolevulinic acid synthetase activity was only 60% of control values, and microsomal heme oxygenase activity was slightly but not statistically elevated. Cytochrome P-450 content in control and BCNU-treated rats increased in a similar manner after phenobarbital or beta-naphthoflavone induction. Electrophoretic analysis of cytochrome P-450 proteins isolated from hepatic endoplasmic reticular membranes of treated and control rats suggested that alterations in these proteins occurred in BCNU-treated rats. These changes in cytochrome P-450 content and activity are very similar to those reported in isolated systems exposed to bile acids or in rats with experimentally produced cholestasis. BCNU has been shown to produce cholestasis, which precedes its effects on microsomal mixed-function oxygenase activity. Thus, the delayed effects of BCNU on microsomal drug metabolism are probably secondary to its interference with bile formation.