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The rat 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 messenger RNA expression of both ER subtypes in rat tissues by RT-PCR and compared the ligand binding specificity of the ER subtypes.

Saturation ligand binding analysis of in vitro synthesized human ERα and rat ERβ protein revealed a single binding component for 16α-iodo-17β-estradiol with high affinity[ dissociation constant (K_d) = 0.1 nm for ERα protein and 0.4 nm for ERβ protein]. Most estrogenic substances or estrogenic antagonists compete with 16α-[¹²⁵I]iodo-17β-estradiol for binding to both ER subtypes in a very similar preference and degree; that is, diethylstilbestrol > hexestrol > dienestrol > 4-OH-tamoxifen > 17β-estradiol > coumestrol, ICI-164384 > estrone, 17α-estradiol > nafoxidine …