Selective Induction of Two Different Molecular Species of Cytochrome P-450 by Phenobarbital and 3-Methylcholanthrene

Two forms of cytochrome P-450, P-450PB and P-450MC, were purified to homogeneity from the liver microsomes of phenobarbital (PB)-treated and 3-methylcholanthrene (MC)-treated rats, respectively. Rabbit antibodies against P-450PB and P-450MC were prepared and the monospecificity of each antibody preparation was confirmed by various lines of evidence. By the use of these antibodies, the contents of P-450PB and P-450MC in microsomes could be determined separately by quantitative immunoprecipitation. P-450PB and P-450MC each amounted to about 10% of total cytochrome P-450 in the microsomes of normal rats. However, each of them was selectively and substantially increased by the appropriate inducer, and became a predominant component of cytochrome P-450 in the microsomes of drug-treated animals. The increase of each specific molecular species of cytochrome P-450 was about 10- and 20-fold within 48 h, whereas the increase of total cytochrome P-450 was only about 2- to 3-fold even after maximal induction by the drugs. The drug oxidation activities of P-450PB and P-450MC were also significantly altered by the drug treatments. The administration of MC to PB-treated rats induced a drastic decrease in the P-450PB-dependent oxidations of benzo(a)pyrene and 7-ethoxycoumarin, while the corresponding activities of P-450MC increased sharply, and these changes were much more rapid than the change of the amount of each form of cytochrome P-450. However, the oxidation of benzphetamine was almost exclusively P-450PB-dependent even after extensive induction by MC. These observations suggest that the specific activities of p-450PB and P-450MC in the oxidations of various drugs are quite differently affected by the induced states of animals.