Specific binding of [3H]estradiol by the cytoplasmic fraction of rat adrenal gland was studied at 0 C. The [3H]estradiol-binding component sedimented at 7 S on 5 to 20% sucrose gradients. Scatchard analysis of the cytosol binding reaction, measured by a charcoal-dextran assay, indicated a single class of estradiol binding sites of high affinity (Kd = 2.2 - 4.5 × 10-10 M) and limited number (5-22 fmol/mg cytosol protein) (n = 7). Competition studies indicated specificity of the binding reaction for the biologically active estrogens, estradiol, estrone, estriol, and diethylstilbestrol; binding of [3H]estradiol was unaffected by a 1000- fold excess of androgens, glucocorticoids, mineralocorticoids, or progesterone. The binding activity was temperature dependent (no binding was observed when incubations were performed at 25 C or above) and appeared to be by a protein since binding was destroyed by trypsin and sulfhydryl reagents, but not by ribonuclease or deoxyribonuclease. Kinetic studies indicated an apparent second order association rate constant of 3.3 × 10-5 M-1 sec-1 and a first order dissociation rate constant of 3.1 × 10-5 sec-1 at 0 C. Specific binding of [3H]estradiol by rat adrenal nuclei was also studied utilizing a nuclear exchange assay. One hour after estrogen injection in vivo, specific binding of [3H]estradiol by isolated adrenal nuclei in vitro increased more than 8-fold, suggesting nuclear translocation of the cytoplasmic adrenal estrogen receptor. Scatchard analysis of the specific nuclear binding indicated a single class of saturable high affinity sites with a dissociation equilibrium constant of 5 × 10-10 M. Since the specific [3H]estradiol-binding component of rat adrenal gland has the properties of a physiological estrogen receptor, the possibility of a direct effect of estrogens on rat adrenal cortical function is suggested.
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