Immunochemical quantification of cytochrome P4501A and IIB subfamilies in the livers of metyrapone-treated rats

Polyclonal antibodies to the major /8-naphthoflavone (BNF)-inducible form of cytochrome P-450 (P4501A) and to the major phenobarbitone (PB)-inducible form (P45011B) have been used to quantify the contribution of these subfamilies to the total amount of cytochrome P-450 in rat livers and rat hepatocyte cultures treated with PB, BNF and metyrapone for 24 and 72 h. The P4501A and IIB subfamilies were not detectable (less than 5 pmol/mg of microsomal protein) in the livers of control rats, but administration of BNF resulted in the P4501A subfamily comprising more than 80% of the total hepatic cytochrome P-450. Administration of PB and metyrapone to rats did not elevate the level of this subfamily but elevated the levels of the P450IIB subfamily to 60% and 30% respectively of the total. Thus metyrapone is a 'PB-like' inducer. However, in contrast with their effects in vivo, treatment with PB and metyrapone of rat hepatocytes did not elevate the proportion of the P4501IB subfamily relative to that in untreated cells but rather, like BNF, increased the P450IA subfamily. This would account for the ability of metyrapone to produce in hepatocyte culture, like BNF, a pronounced induction of ethoxyresorufin O-de-ethylase activity, but it does not account for why of all inducers studied only metyrapone can maintain the total cytochrome P-450 content of cultured hepatocytes, or the activity of ethylmorphine N-demethylase. This activity is generally considered to be associated with the P4501IB subfamily, but the lack of effect of metyrapone on this subfamily in hepatocyte culture must suggest that metyrapone is able to prevent the loss of the total amount of the cytochrome by increasing the expression of other cytochromes P-450.