Interaction of ring B unsaturated estrogens with estrogen receptors of human endometrium and rat uterus

The present investigation was undertaken to compare the binding affinities (Ka) of the ring B unsaturated equine estrogens (equilin [Eq], equilenin [Eqn], 17β-dihydroequilin [17β-Eq], 17β-dihydroequilenin [I7β-Eqn], 17α-dihydroequilin[17α-Eq], and 17α-dihydroequilenin[17α-Eqn]) and the classic estrogens (estrone [E 1], 17β-estradiol [17β-E 2], and 17α-estradiol [I7α-E 2]) for estrogen receptors in human endometrium and rat uterus. In both species, the ring B unsaturated estrogens bind with cytosol and nuclear receptors with high affinity (Ka × 10 9 M −1). The relative binding affinities of these estrogens were measured by determining the amount of unlabeled estrogen required to reduce by 50% the specific binding of [ 3H]17β-Eq to endometrial cytosol receptors. The order of activity found was 17β-Eq>17β-E 2>17β-Eqn>E 1>Eq >17α-Eq>17α-E 2>17α-Eqn>Eqn. Essentially the same order of activity was observed when the apparent affinity constants of these estrogens for human and rat cytosol and nuclear receptors were determined by a competitive (inhibition) binding assay. Sucrose density gradient analysis indicated that these estrogens form protein complexes with cytosol and nuclear preparation that sediment at approximately 8S and4S, respectively. The affinity constants for 17β-Eq were approximately two- to six-fold higher than E 2 in both species. In a rat uterotropic assay, all nine estrogens were uterotropic. These data indicate that all ring B unsaturated estrogens present in conjugated equine estrogen preparations are biologically active and they express their biologic effects in the human endometrium by mechanisms similar to those described for the classic estrogens.