Studies on the association of cytochrome P-450 and NADPH-cytochrome c reductase during catalysis in a reconstituted hydroxylating system

Abstract

The interaction between cytochrome P-450 and NADPH-cytochrome c reductase during catalysis has been investigated with a reconstituted monooxygenase system composed of the two purified enzyme components and synthetic phospholipid. Steady state kinetic data are consistent with a scheme in which the formation of a binary complex between the two proteins precedes catalysis. The formation of this binary complex is described by a simple mass action equation. In agreement with this equation, the observed Vmax for benzphetamine N-demethylation was found to be directly proportional to the calculated concentration of the cytochrome P-450 . reductase complex. Furthermore, with appropriate reductase/cytochrome P-450 mole ratios, the Vmax could be shown to be linearly dependent on either the reductase or the cytochrome P-450 concentration alone. In contrast, the Km parameter is independent of the complex concentration, indicating that no change in the rate-limiting step has occurred. Thus a distinction should be made between a rate-limiting enzyme component and the rate-limiting step in this multienzyme system.