Inhibition of Mouse and Human CYP 1A- and 2E1-dependent Substrate Metabolism by the Isoflavonoids Genistein and Equol

The inhibitory effect of the isoflavonoids genistein and equol on cytochrome P450 activities has been investigated. Genistein and equol inhibited the high capacity component of p-nitrophenol (CYP2E1 substrate) metabolism in liver microsomes from acetone-induced mice with IC50 values of approximately 10 mm and 560 μm, respectively (cf. diethyldithiocarbamate, IC50, 69 μm). Using human CYP2E1 from a specific expression system (which overcame multienzyme involvement in the rodent system), non-competitive inhibition was also seen with both isoflavonoids. Genistein and equol also inhibited the high capacity component of ethoxyresorufin (CYP1A substrate) metabolism in liver microsomes from β-naphthoflavone-induced mice with IC50 values of 5.6 mm and 1.7 mm, respectively (cf. α-naphthoflavone, IC50 0.8μm). Using human CYP1A2 from a specific expression system, non-competitive inhibition was seen with both isoflavonoids. CYP1A1 inhibition offers a possible explanation for the chemopreventative effect of genistein against, for example, dimethylbenz[a]anthracene genotoxicity reported in animals but the IC50 values negate the relevance of this specific chemopreventative action at the levels likely to be achieved from the human diet.