Saturable binding of thyroid hormone to isolated rat hepatocytes

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Abstract
The binding of [125I]triiodothyronine (T3) to freshly prepared rat hepatocytes was studied at 0°C. The abundant non-saturable binding could be suppressed by washing the cells with alkaline buffer, pH 10.5 at 0° C, without loss of cell viability, thus allowing detection of saturable binding. Three classes of binding sites were indentified from analysis of the sautrable T3 binding in the presence and absense of bromosulfophthalein (BSP). One of these classes was inhibited by BSP. The T3 dissociation constants were 3.5, 35 and 115 nM and the number of sites was respectively 0.9, 20 and 36 × 106 sites/cell. L-T3 had a 10-times higher affinity than D-T3 and a 50-times higher affinity than triiodothyroacetic acid. Saturable T3 binding was associated with plasma membrane-containing subcellular fractions. These binding sites may be related to those previously described in isolated plasma membranes from rat liver and could be involved in the entry of T3 into the hepatocyte.