Qualitative alteration in hepatic microsomal cytochrome P-450 apoproteins associated with bile duct ligation, and the administration of ethinyl estradiol, phenobarbital and 3-methylcholanthrene

Abstract

We have investigated in rat liver whether different forms of cytochrome P-450 are altered in hepatic disorders associated with impaired drug metabolism. Total hepatic cytochrome P-450 is decreased after either bile duct ligation or the administration of ethinyl estradiol. In contrast, phenobarbital administered alone increases hepatic content of cytochrome P-450, and when administered with ethinyl estradiol the reduction in cytochrome P-450 was prevented. Microsomal ethylmorphine N-demethylase activities paralleled changes in cytochrome P-450 content, except in bile duct ligation. where it is diminished to a greater extent. Four forms of microsomal cytochrome P-450 apoproteins. ranging in molecular weight from 50,000 to 58,000, were tentatively identified in a sodium dodecyl sulfate (SDS)-6 M urea polyacrylamide gel electrophoresis system by their responsiveness to pharmacological agents, turnover and benzidine peroxidase staining. Phenobarbital administration increased primarily band IV (50,000 daltons); in contrast only band III (53,000 daltons) was responsive to 3-methyl-cholanthene. Bile duct ligation was associated with a marked reduction in bands I, III and IV while bands I and III were decreased with ethinyl estradiol administration. Simultaneous administration of phenobarbital and ethinyl estradiol demonstrated a return of band I and an increase in density of bands II and IV. The mechanism of this reversal by phenobarbital was determined by the double-isotope technique and demonstrates that phenobarbital increases the relative synthesis rates of P-450 apoproteins in ethinyl estradiol-treated rats. These sludies support the hypothesis that mulliple forms of cytochrome P-450 are present in liver microsomal membranes and that alterations in specific apoproteins may be associated with an increase or a decrease in the functional properties of cytochrome P-450.