The Binding of Thyrotropin to Isolated Bovine Thyroid Plasma Membranes

Plasma membranes have been prepared from bovine thyroid cells and have been characterized as being reasonably free of other subcellular structures by electron microscopy and enzyme marker analysis. Bovine thyrotropin specifically binds to these membranes, and this binding can be correlated with adenylate cyclase activation. The binding of thyrotropin to the membranes is immediate, is inversely dependent on temperature, is inhibited by increasing salt concentrations, and is not affected by a range of mono-, di-, and trisaccharides. The thyroid-stimulating activity of the thyrotropin preparations is unaffected by incubation with membranes under conditions of the binding assay. Both the α and the β subunits of bovine thyrotropin bind to thyroid membranes, β better than α on a molar basis; only the β subunit, however, inhibits thyrotropin binding at the concentrations tested. Mouse tumor thyrotropin is a potent inhibitor of bovine thyrotropin binding; human thyrotropin is not as effective. Since γ-globulin preparations with or without long acting thyroid-stimulating activity inhibit thyrotropin binding only slightly at high γ-globulin concentrations, the long acting thyroid stimulator itself appears to have no effect on thyrotropin binding to thyroid membranes.