RAT THYMIC ESTROGEN-RECEPTOR .1. PREPARATION, LOCATION AND PHYSIOCHEMICAL PROPERTIES

Specific, high affinity estrogen receptor is present in the thymus tissue from both rat and bovine sources. The receptor is located in the reticuloepithelial portion of the organ in the rat and is not present in the T-lymphocyte fraction. The equilibrium association constant and concentration of specific estrogen receptor was determined in thymic cytosol by Scatchard plot analysis. The equilibrium association constant (KA) for bovine was 1.72 .+-. 0.12 .times. 109 M-1 and for rat it was 7.14 .+-. 0.52 .times. 109 M-1. The receptor concentration for bovine thymus was 0.298 .+-. 0.03 pmol/g tissue and for rat it was 0.362 .+-. 0.017 pmol/g tissue, or for bovine 6.57 .+-. 0.13 f[femto]mol/mg protein and for rat 7.18 .+-. 0.61 fmol/mg protein. From competition assays, the receptor possessed specificity for estradiol and the estrogen-like compound diethylstilbestrol, but not for progesterone, testosterone, triamcinolone or cortisol. Dihydrotestosterone bound slightly (0.04% with respect to estradiol) and only at high concentrations (4 .times. 10-6 M). By sucrose gradient centrifugation studies the thymic estrogen receptor has a sedimentation value of 7-8 s in low salt or 3-4 s in high salt buffer. A possible mechanism for estradiol suppression of the cell-mediated immune system is discussed.