The interaction of thyroxine with human plasma prealbumin and with the prealbumin-retinol-binding protein complex.

Prealbumin was isolated from human plasma by chromatography on columns of diethylaminoethyl Sephadex and Sephadex G-200, followed by preparative polyacrylamide gel electrophoresis. The prealbumin was homogeneous in the analytical ultracentrifuge with an s=z~,~ of 3.7 S and with a molecular weight of about 50,000. Prealbumin formed a protein-protein complex with plasma retinol-binding protein in a molar ratio of 1: 1. The interaction of thyroxine with prealbumin was quantitatively studied by the method of equilibrium dialysis with rJII-labeled thyroxine. Studies were conducted at 24” and at 37’ in phosphate buffer, pH 7.4, and in phosphate buffer containing 0.14 M NaCl. Prealbumin was found to possess a single binding site for 1 molecule of thyroxine. The association constant for the thyroxine-prealbumin interaction was approximately 1.6 X lo’, and was only slightly affected by temperature change or by NaCl. The binding capacity and affinity of prealbumin for thyroxine were similar in the presence and absence of retinol-binding protein. Moreover, the binding of thyroxine to prealbumin did not interfere with the interaction of prealbumin with retinol-binding protein. The interaction of prealbumin with thyroxine appears to be independent of the prealbumin-retinol-binding protein interaction.