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Differential mechanisms of cytochrome P450 inhibition and activation by alpha-naphthoflavone
A P Koley1, J T Buters, R C Robinson
1Laboratory of Molecular Carcinogenesis, NCI, National Institutes of Health, Bethesda, Maryland 20892, USA.
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.
Insights
Flavonoids differentially affect cytochrome P450 enzymes. Alpha-naphthoflavone inhibits P450 1A1 but activates P450 3A4 by increasing the pool of active enzyme molecules.
Area of Science:
- Biochemistry
- Pharmacology
- Enzymology
Background:
- Flavonoids exhibit anticarcinogenic properties, partly through modulating cytochrome P450 (P450) enzymes.
- The dual effect of flavonoids—inhibiting some P450 activities while activating others—remains an intriguing question in drug metabolism and toxicology.
Purpose of the Study:
- To elucidate the mechanism by which the flavonoid alpha-naphthoflavone interacts with human P450 1A1 and P450 3A4.
- To understand how alpha-naphthoflavone differentially modulates P450-mediated metabolism of benzo[a]pyrene.
Main Methods:
- Utilized flash photolysis to measure carbon monoxide binding kinetics to P450 enzymes.
- Assessed the interaction of alpha-naphthoflavone with human P450 1A1 and P450 3A4 in vitro.
Main Results:
- Alpha-naphthoflavone competitively inhibited P450 1A1's interaction with benzo[a]pyrene.
- Alpha-naphthoflavone selectively activated a subpopulation of P450 3A4, promoting benzo[a]pyrene binding and enhancing its hydroxylation activity.
Conclusions:
- Flavonoids can enhance P450 activity by increasing the number of active enzyme molecules.
- The findings suggest a novel mechanism for P450 activation by small molecules, potentially applicable to other biological systems.