Vol. 289, Issue 2, 661-667, May 1999

Competition Between Cytochrome P-450 Isozymes for NADPH-Cytochrome P-450 Oxidoreductase Affects Drug Metabolism¹

Dongtao N. Li, Michael P. Pritchard, Steven P. Hanlon, Brian Burchell, C. Roland Wolf and Thomas Friedberg

Biomedical Research Centre (D.N.L., M.P.P., S.P.H., C.R.W., T.F.) and Department of Molecular and Cellular Pathology (B.B.), University of Dundee, Ninewells Hospital and Medical School, Dundee, United Kingdom

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-450 isozymes were coexpressed together with P-450 reductase in Escherichia coli. We show that testosterone inhibited the CYP2D6-mediated bufuralol 1'-hydroxylase activity in bacterial membranes containing both CYP2D6 and CYP3A4 but not in membranes containing CYP2D6 alone. Conversely, bufuralol inhibited the CYP3A4-mediated testosterone 6-hydroxylase activity in bacterial membranes 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 reductase 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 demonstrated by the observation that testosterone inhibited several prototypical P-450 enzyme activities, such as bufuralol 1'-hydroxylase, coumarin 7-hydroxylase, and 7-ethoxyresorufin O-dealkylase, in human liver microsomes, but not if tested against a panel of bacterial membranes containing the human P-450 isozymes that mainly catalyze these reactions.