The heterogeneity of soluble estrogen receptors from rat uteri and their modification by temperature, imidazole compounds and estradiol

The nature of the soluble receptor-estrogen complexes and the changes that they undergo during the binding of estradiol have been investigated. Soluble receptors from mature rat uteri exist in multiple forms some of which require the presence of imidazole compounds or ribonuclease in order to bind the hormone in vitro . Raising the temperature to 23 or 37° during the binding process activates another fraction of the receptors. Pretreatment of the soluble extract of uteri at 37° sensitizes the native 9-S receptors so that they dissociate to 4-S units upon binding the hormone at 0°. In contrast to the 4-S unit obtained in the salt dissociation of the native 9-S receptor, this 4-S form does not reassociate or form hybrids with fresh receptor units when centrifuged through a low-salt gradient. A relationship of this 4-S unit to the 5-S receptor in the nucleus is discussed with respect to a possible biological action of the receptor.