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

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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 KD for T4 ≃ 0.6 and 23 nM, for T3 ≃ 9 and 237 nM) and kidney (KD 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 (KD ≃ 28 and 286 nM), and only one set of T3 binding sites (KD ≃ 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 KD 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.

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