Interaction between Thyroid-Stimulating Immunoglobulins and Thyrotropin Receptors in Fat Cell Membranes*

Abstract

In the TSH radioreceptor assay to study the interaction between Graves' immunoglobulins (Ig) and TSH receptors in guinea pig fat cell membranes, Graves' Ig were found to inhibit [125I]TSH binding to fat cell membranes in a dose-dependent manner. Scatchard analysis of [125I]TSH displacement curves by Graves' Ig indicated a single population of the binding sites in fat cell membranes, in contrast to two populations of TSH-binding sites in the membranes. Displacement of [125I]TSH bound to fat cell membranes by both Graves' Ig and unlabeled TSH were time and temperature dependent, with similar dissociation curves, suggesting a specific binding of Graves' Ig to the membrane sites related to the TSH receptor in the fat cells. Such Ig are referred to as fat cell-binding Ig, to be distinguished from the thyroid-stimulating Ig (TSI) detected by TSH radioreceptor assay using human thyroid membranes. Both fat cell-binding Ig and TSI were detected in the sera of a great majority of untreated patients with Graves' disease. A significant correlation was found between both values (r = 0.80; n = 19; P less than 0.001). According to these results, TSI might represent an autoantibody to the membranes associated with the TSH receptor of the target tissues without a strict organ specificity.