Abstract
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.
Footnotes
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Send reprint requests to: T. Friedberg, Biomedical Research Centre, University of Dundee, Ninewells Hospital and Medical School, Dundee, DD1 9SY, United Kingdom. E-mail:t.h.friedburg{at}dundee.ac.uk
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↵1 This work was sponsored by the United Kingdom Biological Sciences Research Council, the United Kingdom Department of Trade and Industry and the LINK consortium of pharmaceutical companies: Astra, Glaxo-Wellcome, Janssen Pharmaceutica, Lilly, Novo-Nordisk, Parke-Davis, Pfizer, Roche Products, Sanofi, Servier, Smith-Kline Beecham, Wyeth-Ayerst and Zeneca.
- Abbreviations:
- CPR
- NADPH-cytochrome P-450 oxidoreductase
- P-450
- cytochrome P-450
- IPTG
- isopropyl β-d-thiogalactopyranoside
- EROD
- 7-ethoxyresorufinO-deethylase
- Received July 7, 1998.
- Accepted December 14, 1998.
- The American Society for Pharmacology and Experimental Therapeutics
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