On the mechanism of action of cytochrome P-450. Oxidation and reduction of the ferrous dioxygen complex of liver microsomal cytochrome P-450 by cytochrome b5.

The effects of cytochrome b5 on the decay of the ferrous dioxygen complexes of P-450LM2 and P-450LM4 from rabbit liver microsomes were studied by stopped-flow spectrophotometry. The P-450 (FeIIO2) complexes accept an electron from reduced cytochrome b5 and, in a reaction not previously described, donate an electron to oxidized cytochrome b5 to give ferric P-450. A comparison with the electron-transferring properties of ferrous P-450 under anaerobic conditions allowed determination of the limiting steps of the two reactions involving the oxygenated complex. The rate of decay of the dioxygen complex was increased in all cases with b5 present; however, with oxidized b5 a large increase in the rate was observed with P-450 isozyme 4 but not with isozyme 2, whereas the opposite situation was found when reduced b5 was used. The reactions between b5 and ferrous dioxygen P-450 were not at thermodynamic equilibrium under the conditions employed. From the results obtained, a model is proposed in which the ferrous dioxygen complex decomposes rapidly into another species differing from ferric P-450 in its spectral properties and from the starting complex in its electron-transferring properties. A scheme is presented to indicate how competition among spontaneous decay, cytochrome b5 oxidation, and cytochrome b5 reduction by the ferrous O2 complex may influence substrate hydroxylation.