Quantitative analysis of the spin equilibrium of cytochrome P-450 LM-2 fraction from rabbit liver microsomes

The LM-2 fraction of cytochrome P-450 from rabbits in the presence and in the absence of substrate (benzphetamine) is shown to be a thermal mixture of a high spin (S = 5/2) and a low spin (S = 1/2) form each of which exhibiting its individual optical basic spectrum with the Soret maxima at 387 nm and 417 nm for the high spin form and the low spin form, respectively. The equilibrium constants and thermodynamic parameters describing the spin transition and the substrate binding have been evaluated from the temperature and substrate difference spectra, respectively. These two interacting equilibria are presented in terms of a thermodynamic model, which provides a clear quantitative description of the properties of the cytochrome P-450 substrate system. From the thermodynamic model also the cause of the substrate difference spectra can be explained. The importance of the spin shift in the presence of substrate with respect to the reduction rate is discussed.