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

• Published in: The Journal of pharmacology and experimental therapeutics Vol. 289; no. 2; p. 661
• Main Authors: Li, D N, Pritchard, M P, Hanlon, S P, Burchell, B, Wolf, C R, Friedberg, T
• Format: Journal Article
• Language: English
• Published: United States American Society for Pharmacology and Experimental Therapeutics 01.05.1999
• Subjects: Aryl Hydrocarbon Hydroxylases; Cytochrome P-450 CYP1A1 - antagonists & inhibitors; Cytochrome P-450 CYP1A1 - metabolism; Cytochrome P-450 CYP2A6; Cytochrome P-450 CYP2D6 - biosynthesis; Cytochrome P-450 CYP2D6 - genetics; Cytochrome P-450 CYP2D6 - metabolism; Cytochrome P-450 CYP2D6 Inhibitors; Cytochrome P-450 CYP3A; Cytochrome P-450 Enzyme Inhibitors; Cytochrome P-450 Enzyme System - biosynthesis; Cytochrome P-450 Enzyme System - genetics; Cytochrome P-450 Enzyme System - metabolism; Drug Interactions; Enzyme Inhibitors - metabolism; Enzyme Inhibitors - pharmacology; Escherichia coli - enzymology; Escherichia coli - genetics; Ethanolamines - metabolism; Humans; In Vitro Techniques; Isoenzymes - antagonists & inhibitors; Isoenzymes - biosynthesis; Isoenzymes - genetics; Isoenzymes - metabolism; Membranes; Microsomes, Liver - drug effects; Microsomes, Liver - enzymology; Mixed Function Oxygenases - antagonists & inhibitors; Mixed Function Oxygenases - biosynthesis; Mixed Function Oxygenases - genetics; Mixed Function Oxygenases - metabolism; NADPH-Ferrihemoprotein Reductase - antagonists & inhibitors; NADPH-Ferrihemoprotein Reductase - biosynthesis; NADPH-Ferrihemoprotein Reductase - genetics; NADPH-Ferrihemoprotein Reductase - metabolism; Plasmids; Recombinant Proteins - antagonists & inhibitors; Recombinant Proteins - biosynthesis; Recombinant Proteins - genetics; Recombinant Proteins - metabolism; Steroid Hydroxylases - antagonists & inhibitors; Steroid Hydroxylases - metabolism; Testosterone - metabolism; Testosterone - pharmacology
• ISSN: 0022-3565; 1521-0103

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 6β-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.