Estradiol and progesterone binding in human term placental cytosol

Human term placentas were studied with regard to their ability to bind estrogens and progestins. A proteinaceous molecule was demonstrated in the high-speed supernatant fraction of disrupted cells that bound estradiol and sedimented in a sucrose density gradient with a coefficient of variation of 4 to 4.5 S with respect to bovine serum albumin. Estradiol was also bound by a single class of binding sites with a dissociation constant (Kd) of 7.35 +/- 1.10 X 10(-9) M and a concentration of binding sites of 54.67 +/- 17.62 fmol/mg protein (N = 7). A synthetic progestin tracer, R5020, was used in an exchange assay to detect any progestin binding and a Kd of 8.13 +/- 1.07 X 10(-9) M, and number of binding sites of 35 +/- 10 fmol/mg protein (N = 6) were obtained at 15 degrees C for 2 hours. No binding was observed in sucrose density gradients. The apparent specificity of the receptor protein for estrogens and progestins was demonstrated by means of competition studies. The possible role of these macromolecules in the human placenta is suggested.