Receptor binding and transactivation activities of red clover isoflavones and their metabolites

Red clover extracts contain a variety of isoflavones, which have affinity toward estrogen receptor alpha (ERα), estrogen receptor beta (ERβ), androgen receptor (AR), and progesterone receptor (PR). Upon ingestion, they undergo various metabolic transformations. For a complete evaluation of red clover extracts and possible health benefits, the resulting metabolites should also be investigated. Biochanin A, formononetin, genistein, daidzein, dihydrobiochanin A, dihydroformononetin, dihydrogenistein, dihydrodaidzein, 3′-hydroxygenistein, 6-hydroxydaidzein, 6-hydroxydesmethylangolensin, equol, O-desmethylangolensin, angolensin, and p-ethylphenol were tested for their transactivation potential toward ERα, AR, and PR in yeast. Competitive binding assays with radiolabeled 17β-estradiol, 17α-methyltrienolone or progesterone assessed binding to the respective ERα and ERβ, AR, and PR. The compounds showed only weak binding affinity to AR and PR, with IC50 values being greater (i.e., lesser affinity) than 10−5 M for the respective receptor. So far, beneficial health effects have been attributed to the production of equol. We propose that other metabolites can also contribute to these effects. However, more detailed information for the formation of these metabolites in humans and for bioavailability data are required to confirm our assumptions.