Estrogen Receptor in Avian Embryo and Adult Liver: Estrogen Receptor Activation and Dissociation Kinetics of Estradiol, Ethynylestradiol, and Moxestrol*

Abstract

Estrogen receptor-enriched fractions from cockerel and embryo liver cytosol were obtained by precipitation at 0--35% ammonium sulfate saturation. At low ionic strength conditions, the embryo receptor sediments in sucrose density gradients mainly as an 8S entity. The cockerel receptor, however, sediments at 4.9S, unless an inhibitor of thiol proteases, iodoacetate, is included in the isolation medium. Moreover, the affinity of the cockerel receptor for moxestrol is doubled in the presence of iodoacetate and equals that of the embryo receptor (Kd = 1.4--1.8 x 10(-10) M). We conclude that thiol cathepsins from cockerel liver cause proteolytic degradation of the estrogen receptor. At temperatures between 0--28 C, the dissociations of moxestrol, ethynylestradiol, and estradiol follow monophasic kinetics. At 28 C, the half-times for the moxestrol-, ethynylestradiol-, and estradiol-receptor complexes are 65, 11, and 6 min, respectively. The chaotropic salt, NaSCN, reduces 6-fold the half-time of the moxestrol-receptor complex at 28 C (t 1/2 = 10 min). In 0.15 M KCl, the estrogen-receptor complex from embryo liver sediments at 4S. Incubation at 28 C, before the sucrose gradient analysis at 0 C, does not change the sedimentation coefficient. Sucrose gradient centrifugation carried out at 20 C results in the transformation to a 5S form of the moxestrol- as well as the estradiol-receptor complex. Our findings suggest that the estrogen receptors in the cytosol of avian liver and mammalian reproductive tissues are very similar. From both tissues, the native 8--10S form is extracted, and the receptors are transformed into the 5S form upon incubation with hormone at 20--30 C.