Evidence for a specific estradiol binding site on rat pituitary membranes

Using adenohypophyses from normal female rats, we demonstrate that estradiol binds pituitary membranes to one homogeneous population of sites with high affinity [dissociation constant (Kd) = 0.041 +/- 0.014 nM; n = 6] and low capacity [maximum binding (Bmax) = 13.6 +/- 5.6 fmol/mg protein]. The binding is thermolabile. Association experiments show that the best experimental conditions are an overnight incubation at 0 C. When the amount of proteins is increased more than 0.3 mg/ml of membrane suspension, binding is rapidly nonlinear. The presence of 0.5 M leupeptin does not improve the binding. Extensive washing of the membranes does not decrease the amount of sites, indicating that the binding is not loosely attached to the membranes. Parenthetically, it should be noted that the membrane fraction was devoid of the cytosolic enzyme marker, lactate dehydrogenase. Binding is specific for estrogenic compounds. When 100% specific binding was determined in the presence of 10(-6) M diethylstilbestrol, 17 beta-estradiol, estrone, and estriol displaced total binding by 110, 80, and 75%, respectively. Neither 4-OH-tamoxifen nor dihydrotestosterone, progesterone, or cortisol displaced the binding. Taken together, these data argue in favor of the presence of specific membrane recognition sites for estradiol in the rat pituitary.