A new aspect of the interaction between thyroxine and proteins

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Abstract
The transient instability of thyroxine and related iodophenols described previously (Tata, 1959) was inhibited by the presence of human serum in the aqueous medium, even when the serum was diluted fivefold. If the serum was added after the initial loss of thyroxine was noted, it caused the instantaneous reappearance of the original amount of thyroxine. Thus serum behaved as an agent masking the ionization of the phenolic hydroxyl group of the iodophenol. The following human serum protein fractions were tested for the thyroxine "stabilization" effect: albumin, Y -globulin, B-globulin, pre-albumin, Cohn fraction IV-6, and B1-globulin. A rabbit skeletal muscle extract with thyroxine-binding properties was also tested. The effects were observed in decreasing order, with human serum pre-albumin, Cohn fraction IV-6, albumin and rabbit muscle extract; B-, Y - and B1-globulins had very little, if any, influence on the stability of thyroxine. By simultaneous electrophoretic analysis, it was found that the "stabilization" effect of any protein was a direct function of the well-known thyroxine-binding properties. By studying the interaction with binding proteins at different dilutions, the fraction of thyroxine "stabilized" was found to be equal to the protein-bound fraction. Human serum pre-albumin exhibited more intense binding and "stabilization" properties than any other protein fraction. Although the mechanism of this new aspect of interaction between proteins and thyroxine is unknown, the "stabilization" effect has been applied in studying some problems of the biochemistry of thyroid hormones. The superiority of the "stabilization" method over the conventional electrophoretic method for studying thyroxine-binding have been discussed. A new method has been proposed for distinguishing between a true and apparent deiodination of thyroid hormones, when studied by chromatographic analysis. These studies also point out the possible importance of the ionization of the phenolic hydroxyl group in some biochemical problems related to thyroid hormones.