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], 17 beta-dihydroequilin 17 beta-Eq], 17 beta-dihydroequilenin 17 beta-Eqn], 17 alpha-dihydroequilin 17 alpha-Eq], and17 alpha-dihydroequilenin 17 alpha-Eqn]) and the classic estrogens (estrone E1], 17 beta-estradiol 17 beta-E2], and 17 alpha-estradiol 17 alpha-E2]) 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 x 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]17 beta-Eq to endometrial cytosol receptors. The order of activity found was 17 beta-Eq greater than 17 beta-E2 greater than 17 beta-Eqn greater than E1 greater than Eq greater than 17 alpha-Eq greater than 17 alpha-E2 greater than 17 alpha-Eqn greater than 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 and 4S, respectively. The affinity constants for 17 beta-Eq were approximately two- to six-fold higher than E2 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.