A proton and fluorine-19 nuclear magnetic resonance and fluorescence study of the binding of some natural and synthetic thyromimetics to prealbumin (transthyretin)

Interactions between prealbumin and several thyromimetic compounds have been examined by proton magnetic resonance spectroscopy. One equivalent of thyroxine (T4) or reverse triiodothyronine (rT3) selectively broadens a number of protein signals, while addition of a second equivalent induces much less widespread changes. One equivalent of triiodothyronine (T3), however, produces much less dramatic changes, and effects comparable with T4 and rT3 are only apparent when a second equivalent binds. The broadening is ascribed to immobilization of flexible residues. The non-halogenated analogue 3,5-dimethyl-3'-isopropylthyronine induces qualitatively different changes suggesting incomplete entry into the thyromimetic binding channel. The fluorinated analogue SK&F 95049 (3,5-bis-thiotrifluoromethyl-3'-isopropylthyronine) induces very similar changes to T3. A fluorine-19 NMR signal with a half-height line width of approximately 150 Hz can be observed from the bound ligand. Finally, a spin-labeled T4 analogue, with a nitroxyl on the alanyl moiety, induces changes identical to those induced by T4 itself, and additionally broadens some signals corresponding to residues at the opening of the ligand binding channel. The natural tryptophan fluorescence of the protein is shown to be a sensitive indicator of binding. The possible influence of the dynamic restrictions induced by binding the first molecule of T4 or rT3 on the protein's affinity for a second hormone is discussed. It is suggested that the first interaction confers rigidity on the second site and reduces its ability to flex open and accommodate a second thyromimetic, which results in the marked negative co-operativity associated with the occupancy of this site.