THE SPECIFICITY OF THYROXINE BINDING BY SERUM ALPHA GLOBULIN1

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Abstract
The specificity of the binding of thyroxine by thyroxine binding serum globulin was examined. The technique used was the measurement of the extent of displacement of a small quantity of labeled l-thyroxine from this protein by equal and excess quantities of thyroxine analogues. All of the analogues were found to have less affinity than thyroxine for this protein. Among the analogues triiodothyronine and d-thyroxine showed the greatest affinity, the others in decreasing order of affinity were thyroxamine, methoxythyroxine, "ortho"-thyroxine, "meta"-thyroxine, and the propionic acid analogue. Diiodotyrosine, diiodothyronine and iodide had no significant binding effect. The amino, carboxyl and phenolic groups are essential for the binding of thyroxine, with the amino group being the most significant. The influence of iodine atoms at the 3[image] and 5[image] positions on the dissociation of the phenolic group is consistent with the decreasing affinity of thyroxine, triiodothyronine and diiodothyronine for binding globulin. A critical spatial distribution of the binding sites on the serum protein is suggested by the absence of binding by diiodotyrosine and reduced binding of the "meta"- and "ortho"- thyroxine, and d-thyroxine isomers. The binding of thyroxine by serum thyroxine binding globulin is highly specific.