INTERACTION OF RADIOIODINATED THYROTROPIN WITH HUMAN-PLASMA MEMBRANES FROM NORMAL AND DISEASED THYROID-GLANDS - RELATION OF THYROTROPIN BINDING TO ADENYLATE-CYCLASE ACTIVITY

  • 1 January 1979
    • journal article
    • research article
    • Vol. 40  (3) , 211-227
Abstract
Plasma membranes were purified from homogenates of normal human thyroid glands and multinodular euthyroid and Graves'' goiters by discontinuous sucrose gradient centrifugation. Preparations of reasonable purity were obtained containing specific binding sites for thyrotropin [TSH] and TSH-sensitive adenylate cyclase. Optimum conditions for 125I-labeled TSH binding were pH 7.8 and 37.degree. C. Na ions and concentration of Tris above 20 mM reduced thyrotropin binding. Human plasma membranes showed no species specificity toward thyrotropins from 3 different species (ox, hog and man). Displacement curves of 125I-labeled bovine TSH by unlabeled hormones was in the order of increasing concentrations of bovine, porcine and human highly purified TSH and was inversely related to the specific biological activity of these preparations as determined by the bioassay in the mouse. Analysis of the interaction between membranes and 125I-labeled TSH resulted in curvilinear Scatchard plots which can indicate the presence of 2 types of sites with high affinity-low capacity (KD = 5 nM) and low affinity-high capacity (KD = 500 nM) or site-site interaction of the negative cooperativity type. No significant difference in binding site characteristics was found in normal and diseased glands (multinodular and Graves'' goiters). A good correlation was found at equilibrium and in the conditions of adenylate cyclase assay between receptor occupancy and cyclase activation by b-TSH.