Heterogeneity of estrogen receptors in the cytosol and nuclear fractions of the rat uterus

Multiple species of estrogen binding sites have been demonstrated in cytosol and nuclear fractions of uteri from immature and adult ovariectomized female rats. Equilibrium binding analyses of uterine cytosol yielded two binding sites, I and II, with dissociation constants of 0.8 and 33 n M respectively. The high affinity cytosol site (0.8 n M - Type I) translocated to the nuclear compartment following estrogen treatment in vivo and appears to represent the classical estrogen receptor which can be measured by 3H-estradiol exchange. Type II sites remain in the cytosol after estrogen injection. A third binding component was observed in the nuclear compartment (nuclear Type II) which binds 3H-estradiol at 4° and displayed cooperative binding characteristics. The presence of these sites in both cytosol and nuclear compartments complicates the accurate measurement and differentiation of these sites. Valid estimates of binding parameters for cytosol Type I and II sites may be obtained by saturation analyses over a wide range of 3H-estradiol concentrations (0.05–40 n M ). Nuclear Type I can be differentiated from nuclear Type II by performing saturation analysis under exchange conditions which measure both Type I and II sites and comparing the values obtained when the assay is performed at 4°C which measures only Type II sites.