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Inhibition of extrahepatic human cytochromes P450 1A1 and 1B1 by metabolism of isoflavones found in Trifolium pratense (red clover)

Roberts, D.W., Doerge, D.R., Churchwell, M.I., Gamboa da Costa, G., Marques, M.M., Tolleson, W.H.
Journal of agricultural and food chemistry 2004 v.52 no.21 pp. 6623-6632
metabolism, humans, medicinal properties, formononetin, cytochrome P-450, Trifolium pratense, genistein, daidzein, liver microsomes, enzyme inhibition
Biochanin A and formononetin are the predominant isoflavones in red clover. In a previous study (J. Agric. Food Chem. 2002, 50, 4783-4790), it was demonstrated that human liver microsomes converted biochanin A and formononetin to genistein and daidzein. This paper now shows CYP1B1-catalyzed O-demethylation of biochanin A and formononetin to produce genistein and daidzein, respectively, which inhibit CYP1B1. Recombinant human CYP1A1 or CYP1B1 was incubated with biochanin A or formononetin. CYP1A1 catalyzed isoflavone 4'-O-demethylation and hydroxylations with similar efficiency, whereas CYP1B1 favored 4'-O-demethylation over hydroxylations. Three of the biochanin A metabolites (5,7,3'-trihydroxy-4'-methoxyisoflavone, 5,7,8-trihydroxy-4'-methoxyisoflavone, and 5,6,7-trihydroxy-4'-methoxyisoflavone) were characterized by 1H NMR spectroscopy and mass spectrometry. Daidzein (K(i) = 3.7 micromolar) exhibited competitive inhibition of CYP1B1 7-ethoxyresorufin O-deethylase activity, and genistein (K(i) = 1.9 micromolar) exhibited mixed inhibition. Biochanin A and/or formononetin may exert anticarcinogenic effects directly by acting as competitive substrates for CYP1B1 or indirectly through their metabolites daidzein and genistein, which inhibit CYP1B1.