Molecular Pharmacology, Vol 20, 234-235, Copyright © 1981 by the American Society for Pharmacology and Experimental Therapeutics

Bromobenzene Metabolism in the Rabbit

Specific Forms of Cytochrome P-450 Involved in 2,3- and 3,4-Epoxidation

¹ Department of Pharmacology, University of Michigan Medical School, Ann Arbor, Michigan 48109

Previous studies in our laboratory indicated that phenobarbital treatment of rats caused a significant increase in both 2,3- and 3,4-epoxidation of bromobenzene in their hepatic microsomes and that 3-methylcholanthrene or -naphthoflavone caused a selective increase in the 2,3-epoxidation pathway. Sodium dodecyl sulfate, polyacrylamide gel electrophoresis of microsomes revealed multiple forms of cytochrome P-450, in keeping with the notion that different species of the heme protein catalyzed the "nontoxic" 2,3-epoxidation and the "toxic" 3,4-epoxidation of this environmental chemical. The present study describes the metabolism of bromobenzene with highly purified cytochrome P-450 and P-448 isolated from rabbit hepatic microsomal preparations. This study involved the enzymatic conversion of bromobenzene to o-bromophenol via 2,3-epoxidation and p-bromophenol via 3,4-epoxidation in a reconstituted mixed-function oxygenase system. Evidence is presented that purified rabbit cytochrome P-450 (LM₂) prepared from animals treated with phenobarbital specifically catalyzes the 3,4-epoxidation of bromobenzene to p-bromophenol. Furthermore, evidence is given that purified rabbit cytochrome P-448 (LM₄) prepared from animals treated with -naphthoflavone specifically catalyzes the 2,3-epoxidation of bromobenzene to o-bromophenol. These data represent an interesting example of two epoxidation pathways involved in the metabolism of a common substrate, one of which leads to cellular damage, i.e., phenobarbital-inducible 3,4-epoxidation; the other, i.e., -naphthoflavone-inducible 2,3-epoxidation of bromobenzene, is not particularly detrimental. Each epoxidation pathway preferentially requires a different and specific form of the heme protein.

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ACKNOWLEDGMENTS The authors wish to thank Dr. Minor J. Coon, Mr. L. Gorsky, Mr. L. Cooke, and Ms. S. Pawlowski of the Department of Biochemistry, University of Michigan, for their generous gift of purified rabbit cytochrome P-450 LM₂ and LM₄, NADPH-cytochrome c reductase, and dilauroylglyceryl-3-phosphorylcholine.