Estriol and estrone interaction with the estrogen receptor. II. Estriol and estrone-induced inhibition of the cooperative binding of [³H]estradiol to the estrogen receptor

Kinetic analysis of the estrogen receptor's cooperative equilibrium [³H]estradiol binding provides a sensitive method for probing the binding of partial agonists to the estrogen receptor. We studied the effects of estriol and estrone on the positive cooperativity of [³H]estradiol binding to the partially purified, calf uterine estrogen receptor. The receptor was titrated with variable concentrations of [³H]estradiol in combination with estriol or estrone, while maintaining a constant molar ratio of the estriol or estrone to the [³H]estradiol. With either a 4-fold molar excess of estriol or a 25-fold molar excess of estrone above the [³H]estradiol concentrations, the receptor's positive cooperative [³H]estradiol binding was inhibited. The Scatchard plot showed a transition from a convex to a linear curve and a decrease in the Hill coefficient value from 1.61 ± 0.02 (n = 7) in the absence of estriol or estrone to 1.04 ± 0.04 (n = 4) in the presence of estriol and 0.99 ± 0.03 (n = 4) in the presence of estrone. The inhibition of the positive cooperativity of [³H]estradiol binding by estriol or estrone was shown not to be due to isotope dilution of the specifically bound [³H]estradiol by the unlabeled estriol or estrone. These kinetic analyses demonstrate that the positively cooperative equilibrium binding of [³H]estradiol by the receptor, which is characteristic of the receptor's activation process, is eliminated by estriol and estrone and consistent with their partial agonist-antagonist activities observed in vivo.