Effect of Various Analogues on the Binding of Labeled Thyroxine to Thyroxine-Binding Globulin and Prealbumin

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Abstract
The ability of a number of analogues of thyroxine to inhibit the binding of 131I-labeled thyroxine to thyroxine-binding globulin (TBG) and thyroxine-binding prealbumin (TBPA) has been studied. Striking differences in the ability of a number of these compounds to inhibit the binding of thyroxine to TBG and TBPA have been demonstrated. For optimal binding to TBG, the alanine side chain appears to be essential, and the amino group appears to be the essential constituent. The substituted diphenyl ether structure, with a free or methylated phenolic hydroxyl, is necessary for normal binding. For optimal binding to TBPA, on the other hand, the alanine side chain appears not to be essential; the amino group in the side chain may, in fact, inhibit binding. The diiodo-substituted phenolic ring with the hydroxyl group in the 4''-position appears to be necessary for optimal binding. The diphenyl ether structure is not essential.

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