Similarities and differences of the binding of estradiol and 4-hydroxytamoxifen (an antiestrogen) in the chick oviduct cytosol.

The binding characteristics of [3H]estradiol and 4-[3H] hydroxytamoxifen (a powerful estradiol antagonist) in the chick oviduct cytosol was analyzed by sucrose gradient centrifugation and dissociation kinetics experiments at 28 degrees C. Heating the cytoplasmic estradiol-estrogen receptor complexes led to the 'transformation' of the receptor; as with the estrogen receptor in other target tissues and species, the transformed receptor sedimented in the 5 S region of sucrose gradients containing 0.4 M KCl and had a slower rate of dissociation of bound estradiol. Upon heating, the cytoplasmic 4-hydroxytamoxifen complexes also appeared to undergo similar changes in their physical states as analyzed by sedimentation rates and dissociation kinetics, and we conclude that antiestrogen can transform the receptor. Sodium molybdate inhibited the temperature mediated changes with both estrogen and antiestrogen complexes. Slight but consistent differences in the sedimentation coefficient and rate of ligand dissociation were observed between the complexes formed by estradiol and 4-hydroxytamoxifen but the relevance to opposite biological activities remains unknown.