The relationship between increases in the hepatic content of cytochrome P-450, form 5, and in the metabolism of aromatic amines to mutagenic products following treatment of rabbits with phenobarbital.

Treatment of rabbits with phenobarbital is followed by increases in the hepatic microsomal concentration of cytochrome P-450, form 5, and in the hepatic microsomal metabolism of aromatic amines to mutagenic products. Inhibition by antibodies to form 5 of the activation of 2-aminoanthracene and 2-aminofluorene demonstrates that these increases are directly related. The extent of the apparent induction of form 5 by phenobarbital is determined from single radial immunodiffusion, immunostaining of form 5 on nitrocellulose sheets containing microsomal proteins transferred from polyacrylamide gels, and the amount of antibody required for 50% of maximal inhibition of activity. The results indicate that phenobarbital increases the hepatic microsomal concentration of cytochrome P-450, form 5, to the same extent that it increases form 5-mediated metabolism of aromatic amines to mutagenic products: 10- to 12-fold. In contrast to the effects of phenobarbital, treatment of rabbits with beta-naphthoflavone decreases the hepatic microsomal concentration of cytochrome P-450, form 5, to less than detectable levels and has little effect on the metabolism of aromatic amines to mutagenic products. Our findings, along with the known effects of phenobarbital on cytochrome P-450, form 2, and the known catalytic activity of cytochrome P-450, form 4, lead to the following conclusions: (a) treatment of rabbits with phenobarbital results in increases in the hepatic microsomal concentrations of at least two structurally, immunochemically, and catalytically distinct isozymes of cytochrome P-450, forms 2 and 5; (b) the metabolism of aromatic amines to mutagenic products in rabbit hepatic microsomal preparations depends on the relative concentrations of at least two isozymes of cytochrome P-450, forms 4 and 5, that change in response to different inducers.