Mechanism of cytochrome P450 activation by caffeine and 7,8-benzoflavone in rat liver microsomes.

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-cytochrome b5, 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 cytochrome b5 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.