The low-spin/high-spin transition equilibrium of camphor-bound cytochrome P-450. Effects of medium and temperature on equilibrium data.

The spin state of camphor-bound cytochrome P-450 is shown to depend largely on medium and temperature in aqueous as well as in mixed organic buffer. At sub-zero temperatures a variation of paH, ionic strength or camphor concentration modifies the spin equilibrium from nearly pure high-spin form to nearly pure low-spin form. Since the apparent pKa of transition is a linear function of log I, the spin state seems to be controlled by the electrostatic potential in the heme proximity. K+ is found to have a specific effect on the spin state. The change of enthalpy, deltaH, of the spin transition depends on the same parameters as the equilibrium constant, in the organic cosolvent as well as in aqueous buffer. As the cosolvent effect is reflected by higher deltaH values, and KCl and pH tend to lower deltaH, the cosolvent effect can easily be compensated. Therefore kinetic studies of the spin conversion might well be undertaken at sub-zero temperature in this solvent.