Competition between cytochrome P-450 isozymes for NADPH-cytochrome P-450 oxidoreductase affects drug metabolism

NADPH-cytochrome P-450 oxidoreductase (CPR) is essential for the catalytic activity of cytochrome P-450 (P-450), On a molar basis, the amount of P-450 exceeds that of CPR in human liver. In this study, we investigated whether drug-drug interactions can occur as a result of competition between P-450 isozymes for this ancillary protein. For this purpose, combinations of P-45 0 isozymes were coexpressed together with P-450 reductase in Escherichia co li. We show that testosterone inhibited the CYP2D6-mediated bufuralol 1'-hy droxylase activity in bacterial membranes containing both CYP2D6 and CYP3A4 but not in membranes containing CYP2D6 alone. Conversely, bufuralol inhibi ted the CYP3A4-medjated testosterone GP-hydroxylase activity in bacterial m embranes containing both CYP3A4 and CYP2D6 but not in membranes containing only CYP3A4. In each case, inhibition was seen even at a P-450 to P-450 red uctase ratio of 1.9:1, which is more favorable than the ratio of 4 reported for human liver. The physiological significance of this mechanism was demo nstrated by the observation that testosterone inhibited several prototypica l P-450 enzyme activities, such as bufuralol 1'-hydroxylase, coumarin 7-hyd roxylase, and 7-ethoxyresorufin O-dealkylase, in human liver microsomes, bu t not if tested against a panel of bacterial membranes containing the human P-450 isozymes that mainly catalyze these reactions.