Thyroid hormone binding to plasma membrane preparations: studies in different thyroid states and tissues

Abstract

Two orders of high-affinity saturable binding sites for L-T₄ and L-T₃ were evidenced in purified plasma membrane preparations from rat liver (apparent equilibrium dissociation constant K_D for T₄ ≃ 0.6 and 23 nM, for T₃ ≃ 9 and 237 nM) and kidney (K_D for T₄ ≃ 4 and 127 nM; for T₃ ≃ 15 and 270 nM). Differences of statistical significance were only found for the higher affinity T₄ binding site. In contrast, no saturable T₄ or T₃ binding could be detected in spleen plasma membranes. Testis plasma membranes exhibited 2 sets of T₄ binding sites but with a lower affinity than in liver and kidney (K_D ≃ 28 and 286 nM), and only one set of T₃ binding sites (K_D ≃ 266 nM). A good correlation was found between the plasmalemma T₄ and T₃ binding properties of a tissue and its ability to respond to and/or metabolize thyroid hormones. T₄ and T₃ binding was also examined in liver plasma membranes of rats under various thyroid status; no difference could be detected in either K_D or total capacity for both sets of T₄ and T₃ binding sites when comparing normal with hyper- or hypothyroid rats. The distribution of plasmalemma high-affinity specific T₄ and T₃ binding sites in different tissues suggests that these sites are involved in hormone action, or in the transport of these hormones within the cell.