An Analysis of the Optical Titrations of the 430 and 455 NM Chromophores of Ethyl Isocyanide Complexes of Mammalian Hepatic Cytochrome P-450

Summary

A computational method is presented from which one may calculate the pK for the spectral changes of the 430 and 455 mm chromophores of ethyl isocyanide complexes of rabbit and rat liver microsomal cytochrome P-450. For the rat liver protein from control or phenobarbital-treated animals, the pK for the loss of the 430 nm absorption is approximately equal to the pK for the gain of the 455 nm absorption, confirming that the two chromophores are in pH equilibrium with one another. For a soluble preparation in which the chromophores exhibit the pH equilibrium property, this equilibrium is also maintained even after the addition of ethyl isocyanide. It is concluded that the appearance of a 430 nm absorption after the addition of ethyl isocyanide to reduced cytochrome P-450 does not necessarily represent conversion of the protein to cytochrome P-420.

This investigation was supported in part by United States Public Health Service Research Grant HL-13399 from the Heart and Lung Institute and is Communication 323 from the Joan and Lester Avnet Institute of Molecular Biology.

Recipient of Public Health Service Research Career Development Award 2-K3-GM-31, 156 from the National Institute of General Medical Sciences.