Saturable binding of thyroid hormone to isolated rat hepatocytes

The binding of [ 125I]triiodothyronine (T 3) 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 T 3 binding in the presence and absense of bromosulfophthalein (BSP). One of these classes was inhibited by BSP. The T 3 dissociation constants were 3.5, 35 and 115 nM and the number of sites was respectively 0.9, 20 and 36 × 10 6 sites/cell. L-T 3 had a 10-times higher affinity than D-T 3 and a 50-times higher affinity than triiodothyroacetic acid. Saturable T 3 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 T 3 into the hepatocyte.