Received for publication November 11, 2005.
Revised April 4, 2006.
Accepted for publication April 5, 2006.

Identification of human hepatic cytochrome P450 enzymes involved in the metabolism of 8-prenylnaringenin and isoxanthohumol from hops (Humulus lupulus L.)

Abstract

The female flowers of hops (Humulus lupulus L.) are used in the brewing of beer and are under investigation for use in dietary supplements for the management of menopausal symptoms in women. Hop extracts contain the weakly estrogenic compound isoxanthohumol, pro-estrogenic xanthohumol, and the potent estrogen 8-prenylnaringenin. Since isoxanthohumol can be metabolized in the human liver to form 8-prenylnaringenin, the specific cytochrome P450 enzymes responsible for this O-demethylation reaction were identified. In addition, the enzymes that convert isoxanthohumol and 8-prenylnarigenin to their most abundant metabolites were identified, since these metabolic pathways might also affect the estrogenicity of hop preparations. Specifically, the CYP450 enzymes that catalyze the oxidation of the prenyl side chains of isoxanthohumol and 8-prenylnarigenin into trans or cis alcohols were investigated. Human liver microsomes and monoclonal antibodies that inhibit specific CYP450 enzymes were used in combination with liquid chromatography-mass spectrometry to identify the enzymes responsible for these transformations. CYP2C19 was found to catalyze the formation of both cis and trans alcohols of the prenyl side chain of 8 prenylnarigenin with K_m values of 14.8 ± 3.2 µM and 16.6 ± 4.6 µM, respectively. CYP2C8 converted 8-prenylnarigenin regioselectively to the trans alcohol of the prenyl group with a K_m of 3.7 ± 0.9 µM. Finally, CYP1A2 was found to catalyze the O-demethylation of isoxanthohumol to generate 8-prenlynarigenin, with a K_m value of 17.8 ± 3.7 µM. These results suggest that the estrogenicity of hop constituents in vivo will depend in part upon metabolic conversion that might show individual variation.

Key words: cytochrome P450 isoforms, human CYP enzymes, liver microsomes, mass spectrometry, metabolite kinetics

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