Molecular Pharmacology, Vol 11, 818-823, Copyright © 1975 by the American Society for Pharmacology and Experimental Therapeutics

A Re-evaluation of the Optical Titrations of the 430 and 455 nm Chromophores of Ethyl Isocyanide Complexes of Mammalian Hepatic Cytochrome P-450

¹ Departments of Pharmacology and Molecular Biology, Albert Einstein College of Medicine, Yeshiva University, Bronx, New York 10461, and Department of Pharmacology, College of Medical Sciences, University of Minnesota, Minneapolis, Minnesota 55455

A computational method is presented from which one may calculate the pK for the pH-induced spectral changes of the 430 and 455 nm chromophores of ethyl isocyanide complexes of rat liver microsomal cytochrome P-450. For the rat liver heme protein from control and phenobarbital- or 3-methylcholanthrene-treated animals, the pK for the loss of the 430 nm absorption is equal to the pK for the gain of the 455 nm absorption, establishing that the two chromophores are in pH equilibrium with one another in all three preparations. This equilibrium is maintained even when the change in pH occurs after the addition of ethyl isocyanide. It is concluded that the appearance of 430 nm absorption after the addition of ethyl isocyanide to reduced cytochrome P-450 does not necessarily represent conversion of this cytochrome to cytochrome P-420.

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ACKNOWLEDGMENT The authors wish to acknowledge the technical assistance of Miss Viola Abbott.

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