title: Differential mechanisms of cytochrome P450 inhibition and activation by alpha-naphthoflavone.
abstract: The anticarcinogenicity of some flavonoids has been attributed to modulation of the cytochrome P450 enzymes, which metabolize procarcinogens to their activated forms. However, the mechanism by which flavonoids inhibit some P450-mediated activities while activating others is a longstanding, intriguing question. We employed flash photolysis to measure carbon monoxide binding to P450 as a rapid kinetic technique to probe the interaction of the prototype flavonoid alpha-naphthoflavone with human cytochrome P450s 1A1 and 3A4, whose benzo[a]pyrene hydroxylation activities are respectively inhibited and stimulated by this compound. This flavonoid inhibited P450 1A1 binding to benzo[a]pyrene via a classical competitive mechanism. In contrast, alpha-naphthoflavone stimulated P450 3A4 by selectively binding and activating an otherwise inactive subpopulation of this P450 and promoting benzo[a]pyrene binding to the latter. These data indicate that flavonoids enhance activity by increasing the pool of active P450 molecules within this P450 macrosystem. Activators in other biological systems may similarly exert their effect by expanding the population of active receptor molecules.
authors.affiliation: Laboratory of Molecular Carcinogenesis, NCI, National Institutes of Health, Bethesda, Maryland 20892, USA.
authors.forename: A P
authors.initials: AP
authors.lastname: Koley
authors.forename: J T
authors.initials: JT
authors.lastname: Buters
authors.forename: R C
authors.initials: RC
authors.lastname: Robinson
authors.forename: A
authors.initials: A
authors.lastname: Markowitz
authors.forename: F K
authors.initials: FK
authors.lastname: Friedman
mesh_terms: D001564:Benzo(a)pyrene / Q000378:metabolism; D001571:Benzoflavones / Q000494:pharmacology*; D002248:Carbon Monoxide / Q000378:metabolism; D051544:Cytochrome P-450 CYP3A; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D004789:Enzyme Activation; D006801:Humans; D007700:Kinetics; D006899:Mixed Function Oxygenases / Q000378:metabolism; D008956:Models, Chemical; D010782:Photolysis
chemical_list: D001571:Benzoflavones; D065607:Cytochrome P-450 Enzyme Inhibitors; D001564:Benzo(a)pyrene; C011512:alpha-naphthoflavone; D002248:Carbon Monoxide; D003577:Cytochrome P-450 Enzyme System; D006899:Mixed Function Oxygenases; C104464:CYP3A protein, human; D051544:Cytochrome P-450 CYP3A