title: Competition between cytochrome P-450 isozymes for NADPH-cytochrome P-450 oxidoreductase affects drug metabolism.
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 6beta-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.
authors.affiliation: Biomedical Research Centre, University of Dundee, Ninewells Hospital and Medical School, Dundee, United Kingdom.
authors.forename: D N
authors.initials: DN
authors.lastname: Li
authors.forename: M P
authors.initials: MP
authors.lastname: Pritchard
authors.forename: S P
authors.initials: SP
authors.lastname: Hanlon
authors.forename: B
authors.initials: B
authors.lastname: Burchell
authors.forename: C R
authors.initials: CR
authors.lastname: Wolf
authors.forename: T
authors.initials: T
authors.lastname: Friedberg
mesh_terms: D001189:Aryl Hydrocarbon Hydroxylases; D019363:Cytochrome P-450 CYP1A1 / Q000037:antagonists & inhibitors / Q000378:metabolism; D065637:Cytochrome P-450 CYP2A6; D019389:Cytochrome P-450 CYP2D6 / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism; D065690:Cytochrome P-450 CYP2D6 Inhibitors; D051544:Cytochrome P-450 CYP3A; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism*; D004347:Drug Interactions; D004791:Enzyme Inhibitors / Q000378:metabolism / Q000494:pharmacology; D004926:Escherichia coli / Q000201:enzymology / Q000235:genetics; D004983:Ethanolamines / Q000378:metabolism*; D006801:Humans; D066298:In Vitro Techniques; D007527:Isoenzymes / Q000037:antagonists & inhibitors / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism; D008566:Membranes; D008862:Microsomes, Liver / Q000187:drug effects / Q000201:enzymology; D006899:Mixed Function Oxygenases / Q000037:antagonists & inhibitors / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism; D009251:NADPH-Ferrihemoprotein Reductase / Q000037:antagonists & inhibitors / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism*; D010957:Plasmids; D011994:Recombinant Proteins / Q000037:antagonists & inhibitors / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism; D013250:Steroid Hydroxylases / Q000037:antagonists & inhibitors / Q000378:metabolism; D013739:Testosterone / Q000378:metabolism* / Q000494:pharmacology
chemical_list: D065690:Cytochrome P-450 CYP2D6 Inhibitors; D065607:Cytochrome P-450 Enzyme Inhibitors; D004791:Enzyme Inhibitors; D004983:Ethanolamines; D007527:Isoenzymes; D011994:Recombinant Proteins; D013739:Testosterone; C010831:bufuralol; D003577:Cytochrome P-450 Enzyme System; D006899:Mixed Function Oxygenases; D013250:Steroid Hydroxylases; D001189:Aryl Hydrocarbon Hydroxylases; C104464:CYP3A protein, human; D019363:Cytochrome P-450 CYP1A1; D065637:Cytochrome P-450 CYP2A6; D019389:Cytochrome P-450 CYP2D6; D051544:Cytochrome P-450 CYP3A; C022657:steroid hormone 6-beta-hydroxylase; C510163:CYP3A4 protein, human; C043872:bufuralol 1'-hydroxylase; D009251:NADPH-Ferrihemoprotein Reductase