Binding of thyrotropin to receptors in fat tissue

Binding of thyrotropin to receptors in membrane preparations from fat tissue has been examined using receptor-purified [¹²⁵I]thyrotropin labelled by a solid phase lactoperoxidase method. Using a range of membrane concentrations, binding to receptors in fat tissue from six species was compared. Specific binding to membranes from guinea pig fat tissue was found to exceed binding to preparations from other species. Thyrotropin binding to membranes from various fat deposits in the guinea pig appeared to be identical and binding to receptors in guinea pig epididymal fat tissue has been characterized in detail. Binding appeared to be hormonally specific, staturable, and of an apparent high affinity (K_a of 1.1 × 10¹⁰ M^-1) with no evidence of significant low affinity binding. Binding was rapid and stable at 37°C though the rate of the association reaction was temperature-dependent. Binding activity was retained by unoccupied receptors after preincubations at temperatures up to 41°C. The dissociation reaction, though initially rapid, appeared to be governed by more than one rate constant. Specific binding was optimal at pH 7, was inhibited by all concentrations of Na⁺ and K⁺ ions, but was enhanced by Mg²⁺ and Ca²⁺ ions at concentrations between 1 and 10 mM, higher concentrations proving inhibitory.