Molecular Pharmacology, Vol 17, 218-224, Copyright © 1980 by the American Society for Pharmacology and Experimental Therapeutics

Identification of the Form of Cytochrome P-450 Induced in Neonatal Rabbit Liver Microsomes by Phenobarbital

¹ Department of Biochemistry, Scripps Clinic and Research Foundation, La Jolla, California 92037

Transplacental induction of cytochrome P-450 by phenobarbital was studied in neonatal rabbit liver microsomes. This treatment increased a single form of cytochrome P-450 identified by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate, peptide mapping and immunological characterization as cytochrome P-450, form 2. Form 2 is the predominant species of the cytochrome in adult liver microsomes from rabbits pretreated with phenobarbital. This assignment was further supported by examining the kinetics of 7-ethoxycoumarin O-deethylation. Reconstituted form 2 and phenobarbital-induced neonatal microsomes showed a similar monophasic dependence on 7-ethoxycoumarin concentration. Phenobarbital-induced adult microsomes, however, exhibited a biphasic dependence on substrate concentration. This is apparently due to the presence of other forms of cytochrome P-450 in these microsomes participating in the O-deethylation of this substrate. Previously an age difference was found to exist in the induction by 2,3,7,8-tetrachlorodibenzo-p-dioxin of two forms of cytochrome P-450, forms 4 and 6, in rabbit liver microsomes. Form 6 is the sole form induced in the neonate, whereas form 4 is the major form and form 6 is the minor form induced in the adult rabbit liver. Thus, phenobarbital, like 2,3,7,8-tetrachlorodibenzo-p-dioxin, induces only one form of cytochrome P-450 in neonatal liver microsomes. Unlike 2,3,7,8-tetrachlorodibenzo-p-dioxin, phenobarbital induces the same major form in both adults and neonates.

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ACKNOWLEDGMENTS We thank Ms. Maryann Zounes, Ms. Kendis Cox and Ms. Rita Dunning for their skillful technical assistance.