SPIN STATES AND REDOX POTENTIALS OF HEPATIC CYTOCHROME P450

Abstract

The chapter describes the spin states and redox potentials of hepatic cytochrome P-450. It has been demonstrated that both substrate free and substrate bound cytochrome P-450 from pseudomonas, adrenal mitochondria, and hepatic sources exist as a thermal—as opposed to quantum—mixture of high- and low-spin forms. In a study described in the chapter, the free-energy linkages of spin, redox, and substrate equilibria were examined in a partially purified hepatic P-450 obtained from untreated rats. When data obtained in this study from the pseudomonas camphor hydroxylase is presented together with data from a partially purified P-450 from untreated rat liver, excellent agreement between theory and experiment is found. The results clearly indicate that the principle effect of drug-binding alteration of the cytochrome redox potential is through modulation of the ferric heme spin equilibrium. Such a model could also account for observed substrate dependent reduction rates of P-450. If the equilibration of spin states is fast compared to electron transfer, the initial velocity for cytochrome reduction is k 3 K a / (1 + K a ) whereas in the opposite case, interconversion between the high- and low-spin forms could be limiting in total turnover.