Phenobarbital- and 3-Methylcholanthrene-Induced Synthesis of Two Different Molecular Species of Microsomal Cytochrome P-450 in Rat Liver1

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Abstract
The syntheses of two different molecular species of cytochrome P-450, P-450(PB), and P-450(MC), were examined using normal and drug-treated rats, and the rate of synthesis was correlated with the drug-induced increase of the amounts in the liver microsomes of treated animals. Phenobarbital (PB) and 3-methylcholanthrene (MC) were used as inducers. The syntheses of cytochrome b 5 and NADPH-cytochrome P-450 reductase (fp T ) were also examined. In vivo incorporation of L-[ 3 H]leucine indicated a large increase of P-450(PB) synthesis in the livers of PB-treated animals, which reached a plateau value about 18-fold higher than the control level at 12 h. The synthesis of fp T was also stimulated by PB showing a peak value of 3 times the control at 12 h after PB administration, but it returned to the control level afterwards. On the other hand, the syntheses of P-450(MC) and cytochrome b 5 did not change at all. Similarly, MC administration selectively induced the synthesis of P-450(MC), which was about 24 times the control at 6 h, whereas those of P-450(PB), cytochrome b 5 and fp T were not affected by MC. Analysis of nascent peptides and in vitro translation of polysomes and total liver RNA prepared from control and drug-treated animals were also carried out, and the results of these in vitro experiments confirmed the observations in in vivo incorporation studies. It was found that both free and membrane-bound ribosomes participate in the syntheses of P-450(PB) and P-450(MC) in the case of control animals. PB and MC induced the syntheses of P-450(PB) and P-450(MC) by bound ribosomes but not by free ribosomes, and the contribution of bound ribosomes to the syntheses of these two species of cytochrome P-450 was predominant in the case of drugtreated animals. The molecular sizes of in vitro synthesized P-450(PB) and P-450(MC) were the same as those of authentic samples prepared from liver microsomes.