Spin state control of the hepatic cytochrome P450 redox potential.

Abstract We have measured the oxidation-reduction potential of isolated and partially purified cytochrome P450 from uninduced rat liver, both in the presence and absence of Type I substrates. Native P450 is found to have a potential of −300 mV with respect to the standard hydrogen electrode, while the addition of benzphetamine or hexabarbital increases the redox potential to −237 mV and −225 mV respectively. Quantitation of the thermally induced S = 1 2 to S = 5 2 spin transition of the ferric heme iron for this P450 preparation both substrate free as well as with bound benzphetamine and hexabarbital indicate an increase in the high spin ( S = 5 2 ) fraction on the binding of Type I substrates. The relevant oxidation-reduction equilibria of the heme chromaphore are presented in terms of a thermodynamic model for the control of the observed cytochrome P450 redox potential through this modulation of the spin configuration of the five d-electrons of the ferric heme iron induced by the binding of Type I substrates.