The rat, mouse and human estrogen receptor (ER) exists as two subtypes, ERα and ERβ, which differ in the C-terminal ligand-binding domain and in the N-terminal transactivation domain. In this study, we investigated the estrogenic activity of environmental chemicals and phytoestrogens in competition binding assays with ERα or ERβ protein, and in a transient gene expression assay using cells in which an acute estrogenic response is created by cotransfecting cultures with recombinant human ERα or ERβ complementary DNA (cDNA) in the presence of an estrogen-dependent reporter plasmid.

Saturation ligand-binding analysis of human ERα and ERβ protein revealed a single binding component for[ ³H]-17β-estradiol (E₂) with high affinity[ dissociation constant (K_d) = 0.05 - 0.1 nm]. All environmental estrogenic chemicals [polychlorinated hydroxybiphenyls, dichlorodiphenyltrichloroethane (DDT) and derivatives, alkylphenols, bisphenol A, methoxychlor and chlordecone] compete with E₂ for binding to both ER subtypes with a similar preference and degree. In most instances the relative binding affinities (RBA) are at least 1000-fold lower than that of E₂. Some phytoestrogens such as coumestrol, genistein, apigenin, naringenin, and kaempferol compete stronger with E₂ for binding to ERβ than to ERα. Estrogenic chemicals, as for instance nonylphenol, bisphenol A, o, p′-DDT and 2′,4′,6′-trichloro-4-biphenylol stimulate the transcriptional activity of ERα and ERβ at concentrations of 100-1000 nm. Phytoestrogens, including genistein, coumestrol and zearalenone stimulate the transcriptional activity of both ER subtypes at concentrations of 1–10 nm. The ranking of the estrogenic potency of phytoestrogens for both ER subtypes in the transactivation assay is different; that is, E₂ ≫ zearalenone = coumestrol > genistein > daidzein > apigenin = phloretin > biochanin A = kaempferol = naringenin> formononetin = ipriflavone = quercetin = chrysin for ERα and E₂ ≫ genistein = coumestrol > zearalenone > daidzein > biochanin A = apigenin = kaempferol = naringenin > phloretin = quercetin = ipriflavone = formononetin = chrysin for ERβ. Antiestrogenic activity of the phytoestrogens could not be detected, except for zearalenone which is a full agonist for ERα and a mixed agonist-antagonist for ERβ. In summary, while the estrogenic potency of industrial-derived estrogenic chemicals is very limited, the estrogenic potency of phytoestrogens is significant, especially for ERβ, and they may trigger many of the biological responses that are evoked by the physiological estrogens.