Characterization of thyrotropin binding to specific receptors in human fat tissue

Specific receptors for thyrotropin were found to exist in membranes from whole human subcutaneous fat tissue. The characteristics of the interaction of ¹²⁵I-labelled thyrotropin with such receptors were determined and compared with the stable, high-affinity thyrotropin receptor shown previously to exist in guinea pig fat membranes. Specific binding was readily detectable using low concentrations of membranes (up to 80 μg/ml), though specific binding was reduced at higher membrane concentrations. Increasing concentrations of unlabelled thyrotropin reduced fractional binding, revealing saturation of a population of mixed affinity sites (highest K_a of the order of 0.3 × 10⁹ M^-1). Little cross-reactivity was observed with other lipolytic or structurally related hormones, though some cross-reactivity was observed in the presence of human chorionic gonadotropin. Association was temperature-dependent and rapid at 37°C, though prolonged incubation revealed some instability of binding at this temperature. Binding was reversible with a high dissociation rate constant, and was particularly sensitive to the presence of low concentrations of sodium or calcium ions. Using membranes prepared from isolated human fat cells, binding of thyrotropin was equally sensitive to the addition of cations.