Molecular Pharmacology, Vol 6, 631-640, Copyright © 1970 by the American Society for Pharmacology and Experimental Therapeutics

Role of Phospholipids in the Hepatic Microsomal Drug-Metabolizing System

¹ Department of Pharmacology, University of Minnesota, Minneapolis, Minnesota 55455

Cytochrome P-450, the terminal oxidase that functions in the oxidation of many drugs and other foreign compounds, is thought to depend in large degree for its physical and biochemical properties on its association with microsomal phospholipids. Drugs have been classified into two groups, depending upon whether they form a type I or a type II difference spectrum when they combine with cytochrome P-450. The type I binding site is thought to be located in an undetermined hydrophobic region of cytochrome P-450 protein or in lipids of the microsomal membrane, whereas the type II binding site is thought to be associated with the CO-binding site of the hemoprotein. Phospholipase C (EC 3.1.4.3), which specifically hydrolyzes phosphatidylcholines and phosphatidylethanolamines to corresponding phosphoryl products, was used in the current studies in an attempt to evaluate the role of phospholipids in the microsomal drug-metabolizing system. Untreated and phospholipase C-treated microsomes from the livers of rats were used to study the oxidation and binding of two type I drugs, ethylmorphine and hexobarbital, and the type II compound, aniline. Treatment of microsomes with phospholipase C destroyed the type I binding site, but microsomes lost only about 40% of their ability to oxidize ethylmorphine and hexobarbital. The loss of aniline oxidation was only about 15%, and the binding of aniline to hemoprotein was increased significantly. Phospholipase C treatment caused about a 20% conversion of cytochrome P-450 to cytochrome P-420, which remained with the microsomes. These studies provide further evidence that the type I and type II binding sites differ and that the type I binding site is associated with membrane phospholipids. The studies also show that type I binding is not required for the oxidation of type I compounds by microsomal enzymes.

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ACKNOWLEDGMENT The authors gratefully acknowledge the able technical assistance of Mrs. Janice Shoeman.