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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 …