Studies of thyroid-stimulating hormone binding to bovine thyroid plasma membranes.

131I-TSH prepared by the lactoperoxidase method was used to study the binding of hormone to bovine thyroid plasma membrane. Specific binding was obtained using as little as 0.12 mU/ml 131I-TSH. Half-maximal binding occurred with 17.1 plus or minus 3.5 mU/ml and saturation at approximately 40 mU/ml. Scatchard plot analysis revealed two classes of binding sites, with association constants of 1.1 plus or minus 0.06 x 10(8) M(-1) and 1.4 x 10(7) M(-1) for the high- and low-affinity sites, respectively. Binding of 131I-TSH was linearly related to the amount of thyroid plasma membrane protein. Other polypeptide hormones and prostaglandin E1 did not inhibit specific TSH binding. Identical results were obtained using two TSH preparations of different biologic specific activity. 12.5 mU/ml unlabeled TSH decreased 131I-TSH binding 50%, and 156 mU/ml caused complete inhibition. After equilibrium of 131I-TSH binding was established, maximal displacement was achieved by 120 min using about 300 mU/ml TSH. However, only about one-half of the 131I-TSH was displaced. Although GTP potentiated the stimulation of adenylate cyclase by TSH, it inhibited binding of 131I-TSH. Binding of TSH correlated very well with activation of adenylate cyclase.