The reduction kinetics of cytochrome P-450 are known to be biphasic, with a rapid initial phase and a slower subsequent phase, both of which appear linear in semilogarithmic plots. The present report demonstrates that these biphasic reduction kinetics can be described in terms of a preequilibrium between high- and low-spin ferric states. Computer simulations are used which express the rate and extent of the fast phase or burst as being due to the initial proportion of high-spin cytochrome P-450. According to this simplified sequential model, the slow phase of reduction is controlled by the rate of formation of high-spin cytochrome P-450. The substrate-induced alterations in the reduction kinetics are likewise consistent with the model, which indicates that type I compounds exert their effect by virtue of a decrease in the rate constant controlling the shift from high-spin to low-spin ferric cytochrome. The model is further supported by the influence of temperature on the spin equilibrium and reduction kinetics. Potential influences of other intermediate steps in the reduction and assumptions in the hypothesis are described.
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