Rat thymus: demonstration of specific thyroxine receptors in nuclear extract

Nuclear protein fraction from the nuclei of rat thymus was prepared by extraction in 0.4 mol l-1 NaCl and the presence of saturable thyroxine binding sites was demonstrated. Scatchard analysis revealed one class of high affinity binding sites with Ka = 4.15 X 10(9) 1 mol-1, the maximum binding capacity (expressed as number of ligand molecules per mg of protein) of 5.56 +/- 1.37 X 10(10), was found. Amino acid composition (expressed as mumol per mg of protein) of acid hydrolyzates of rat thymus and liver crude "receptors" for thyroid hormones is presented. The results show that the nuclear protein fraction of thymus, responsible for thyroid hormone binding shows some basic characteristics of non-histone proteins and differs in concentration of lysine, histidine, aspartic acid, proline, methionine and slightly in concentration of glycine, isoleucine and phenylalanine from that obtained from the liver. The data suggest that the rat thymus might be considered as a target tissue for thyroxine.