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

Abstract

131I-TSH prepared by the lactoperoxidase method was used to study the binding of hormone to bovine thyroid plasma membranes. Specific binding was obtained using as little as 0.12 mU/ml 131I-TSH. Half-maximal binding occurred with 17.1 ± 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 ± 0.06 × 10 8 M −1 and 1.4 × 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 E 1 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.