[Kinetics of formation of the enzyme-substrate complexes of cytochrome P-450 in rat liver microsomes].

A spectrophotometric study of the kinetics of binding of I and II type substrates to cytochrome P-450 from rat liver microsomes has been carried out. The rates of binding of the substrates of type I to a low spin form of the enzyme and those of substrates of type II to a high spin form are approximately the same. The interaction of type II substrates with a low spin form of cytochrome P-450 occurs at a 15--20-fold lower rate. The dependence of initial reaction rates on substrate concentration describing the Michaelis equation suggests that the reaction occurs in more than one step both for type I and II substrates. The data obtained allowed to develop a two-step scheme of reactions, according to which the substrate in the first step rapidly and irreversibly interacts with the binding centers, which can be located both in the microsomal membrane and in cytochrome P-450. At the second stage the substrate in transferred into the active center of cytochrome P-450, resulting in a formation of complexes with specific optical properties.