Estrogen Receptor Activation and the Dissociation Kinetics of Estradiol, Estriol, and Estrone*

Abstract

The relationship among estrogen binding to the receptor of hormones of differing biological activities was investigated using estradiol, estriol, estrone, and the calf uterine estrogen receptor. For the three estrogens with different biological activities several kinetic parameters were similar. The association rate constants (k₊₁) at 25 C were 0.9–1.1 × 10⁸m⁻¹ min⁻¹. The rate constants of receptor activation (k_act) were 2.1-3.1 × 10⁷m⁻¹ min⁻¹. The equilibrium association constants (K_a) at 0 C were: estradiol, 5.9 × 10⁹m⁻¹; estriol, 3.8 × 10⁹m⁻¹; and estrone, 1.3 × 10⁹m⁻¹ (less than a 5-fold difference exists between estradiol and estrone). The dissociation rate constants were the only parameters distinguishing the three estrogens. The dissociation of estradiol and estriol from the estrogen receptor occurs in two exponential phases: the faster phase (k₋₁) from the nonactivated state of the receptor and the slower phase (k₋₂) from the activated state. Estrone dissociation from the receptor occurs as a single exponential process; the dissociation rate decreased with receptor activation. The dissociation rate constants (k₋₁) from the nonactivated state of the receptor at 25 C were: estradiol, 0.12 min⁻¹; estriol, 0.16 min⁻¹; and estrone, 0.57 min⁻¹. The dissociation rate of an estrogen from the nonactivated state of the receptor determines the magnitude of the fast exponential. The more rapid this dissociation, the larger the magnitude of the k₋₁ exponential and the less receptor is transformed to the activated state. The dissociation rate constants (k₋₂) from the activated state of the receptor at 25 C were: estradiol, 4 × 10⁻² min⁻¹; estriol, 1.7 × 10⁻² min⁻¹; and estrone, 0.24 min⁻¹. The slower an estrogen dissociates from the activated state of the receptor, the longer the lifetime of the activated receptor state, the longer the duration of nuclear retention, and the greater the biological activity of the estrogen. We find that the kinetics of the dissociation of an estrogen from the receptor can be used as an index for biological activity, since it reflects the estrogen’s capacity to activate and maintain the receptor in the activated state. (Endocrinology106: 434, 1980)