Binding of Thyroid Hormones in Serum and Liver Cytosol of Rana catesbeiana Tadpoles*

Abstract

Binding of thyroid hormones to proteins in tadpole serum and liver cytosol was studied using both equilibrium dialysis and dextran-coated charcoal techniques to separate bound and free hormone. Measured directly by equilibrium dialysis, approximately 1% of the total T₃ or T₄ in serum and less than 0.5% of the total T₃ and T₄ in cytosol (diluted 1:5 or 1: 10) were in the free form. Free T₃ and T₄ fractions were independent of hormone concentration between 10^-10 and 10^-6 M. Values for free hormone fractions determined by the charcoalbinding technique were much higher than those determined directly, and it was evident that this technique could not be used to measure free hormone fractions if any of the protein-hormone complexes dissociated very rapidly. Nevertheless, important information was obtained using the charcoal technique. It was found that both cytosol and serum contain a number of sets of sites that bind thyroid hormone, some of which are saturable. However, the affinity of the saturable sites for both T₄ and T₃ is relatively low. The presence of alternative binding sites for excess hormone prevented any increase in the free fraction at high hormone concentrations. No evidence was obtained in either cytosol or serum of any high affinity, low capacity binding sites similar to those detected in tadpole liver nuclei. It is suggested that the binding of thyroid hormones in serum and liver cytosol in tadpoles serves a function other than the initiation of thyroid hormone action.