Non-Interaction Between in vivo and Cell Free Nuclear Binding of Estrogen Receptor

Abstract

It is well established that rat uterine cytoplasm contains a receptor protein for 17β-estradiol and that the receptor-steroid complex is concentrated in the nucleus, but the nature of the nuclear binding sites is unclear. After injecting increasing doses of estradiol into mature ovariectomized rats, we find increasing levels of receptor bound estradiol (R-E) in the uterine nuclei. When purified nuclei from these uteri are incubated with a fixed amount of cytosol receptor labeled with ³H-estradiol (R-E*), the binding of R-E* is not affected by any amount of R-E bound previously in vivo. Thus competition by in vivo bound R-E for cell free R-E* binding sites does not occur. Since the available evidence supports the assumption that the receptor is bound to the same type of sites both in vivo and under cell free conditions, we conclude that there are a large number of nuclear acceptor sites for R-E, so that these sites are far from saturated in vivo even at hyperphysiological estradiol concentrations.