Nov 20, 1978

Thyroxine-protein interactions. Binding constants for interaction of thyroxine analogues with the thyroxine binding site on human thyroxine-binding globulin

Biochimica Et Biophysica Acta
M Tabachnick, L Korcek


The binding constants for interaction of various thryoxine analogues with the thyroxine binding site on human thyroxine-binding globulin have been determined. Equilibrium dialysis, at pH 7.4 and 37 degrees C, was used to measure the competitive effects of different iodothyronine compounds on the binding of 125I-labeled thyroxine to highly purified thyroxine-binding globulin. Relative to L-thyroxine, K = 6 . 10(9) M-1, the association constants of some important analogues were D-thyroxine, 1.04 . 10(9) M-1, 3,5-diiodo-3'-isopropyl-L-thyronine, 4.9 . 10(8) M-1; L-triiodothyronine, 3.3 . 10(8) M-1, 3,3',5'-DL-triiodothyronine (reverse triiodothyronine), 3.1. 10(8) M-1; tetraiodothyropropionic acid, 2.7 . 10(8) M-1; tetraiodothyroacetic acid, 2.6 . 10(8) M-1; 3', 5'- diiodo-DL-thyronine, 8.3 . 10(7) M-1; and 3,5-diiodo-DL-thyronine, 7.1 . 10(7) M-1. Calculation of the deltaG0 values for binding of the analogues indicates that a major contribution to the free energy favoring binding is made by the alanine side chain of thyroxine. A change in configuration of the alpha-amino group from the L to D form causes an unfavorable change of 1 kcal/mol in the free energy of binding. Removal of the alpha-amino group as in tetraiodothyropropion...Continue Reading

Mentioned in this Paper

Structure-Activity Relationship
Plasma Protein Binding Capacity
Thyroxine-Binding Proteins

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