The Journal of Biological Chemistry

Plasma membranes isolated from bovine corpora lutea showed specific binding with 1251-human chorionic gonadotropin and 1251-human luteinizing hormone; unlabeled hormones competitively inhibited the binding. The binding of the 1251-human chorionic gonadotropin to the receptor was a saturable phenomenon and the half-saturation was attained at a concentration of 4 X lo-r0 M. Maximum hormone receptor binding was obtained within 15 min at pH 7.2 and 37”. The rate constants of association and dissociation of human chorionic gonadotropin to the receptor determined at 37” were 2.8 X lo6 M-' s-r and 2.1 X lop5 s-r, respectively. The dissociation constant calculated from these rates was 8.0 X lo-r0 M. At 4’ the rate of association was extremely slow and the hormone receptor complex was stable up to 48 hours. The dissociation constants obtained from equilibrium data were calculated to be 1.5 X lo-r0 M for human chorionic gonadotropin and the number of binding sites were approximately 6.3 x lo+ per mg of tissue. The binding of 1251-human chorionic gonadotropin to the plasma membranes was optimum at pH 7.2 and was reduced at acidic and basic pH. The binding was also inhibited at high concentrations of CaC12, Mg&, and NaCI, and in the presence of guanidine HCl and urea. The inhibition of binding at pH 5 and 9 and up to 1 M concentration of various ions was reversible. Treatment of plasma membrane with phospholipase C, neuraminidase, glucosidase, trypsin, and cy-chymotrypsin did not decrease the binding of 1251-human chorionic gonadotropin. Pepsin and phospholipase A inhibited the binding of lZ51-human chorionic gonadotropin to plasma membranes. The plasma membranes were solubilized in detergents. The proteins reprecipitated from the solution by ethanolammonium acetate retained the ability to bind 1251-human chorionic gonadotropin. Solubilization of the plasma membranes in 6 M guanidine HCI resulted in the separation of protein and lipid components. Fractionation of the protein component was achieved by gel filtration on a column of Sepharose 4B in 6 M guanidine HCl and ion exchange chromatography on DEAE-cellulose in 3 M urea. The receptor