Abstract
Caffeine and 7,8-benzoflavone activate CYP3A2 in rat liver microsomes. Both activators appear to enhance enzyme activity by an increase in Vmax and to a lesser extent a decrease in Km . Additive effect studies demonstrated that the two activators oppose one another’s effect. Electron transfer steps in the cytochrome P450 cycle are involved in the mechanism of cytochrome P450 activation, as indicated by the lack of effect of caffeine or 7,8-benzoflavone on cumene hydroperoxide-supported oxidation of acetaminophen by cytochrome P450. The involvement of cytochrome b5 in the formation of N-acetyl-p-benzoquinone imine (NAPQI) was implicated through a synergistic effect of NADH on the NADPH-supported reaction. Anti-cytochromeb5, but not anti-cytochrome P450 reductase IgG, diminished the activation effect of caffeine on NAPQI formation. Neither antibody altered the effect of 7,8-benzoflavone on NAPQI formation. The impairment of NAPQI formation by cytochromeb5 antibody suggests that cytochrome P450 activation by caffeine but not 7,8-benzoflavone is mediated in part through enhancement of the transfer of the second electron to cytochrome P450 from cytochrome b5.
Footnotes
-
Send reprint requests to: John T. Slattery, Ph.D., Department of Pharmaceutics, Box 357610, University of Washington, Seattle, WA 98195-7610. E-mail: jts{at}u.washington.edu.
-
This work was supported in part by GM 32165 from the National Institutes of Health.
-
↵2 The apparent decrease inKm2 with the addition of 1.5 mM caffeine is insignificant in comparison with the standard deviation.
- Abbreviations used are::
- APAP
- acetaminophen
- NAPQI
- N-acetyl-p-benzoquinone imine
- CHP
- cumene hydroperoxide
- Received March 24, 1997.
- Accepted June 10, 1997.
- The American Society for Pharmacology and Experimental Therapeutics
DMD articles become freely available 12 months after publication, and remain freely available for 5 years.Non-open access articles that fall outside this five year window are available only to institutional subscribers and current ASPET members, or through the article purchase feature at the bottom of the page.
|
