Thyroxine-Protein Interactions

The effect of temperature on the binding of l-thyroxine to human serum albumin and to albumin defatted by the method of Goodman has been studied by equilibrium dialysis. The albumin was first freed of other contaminating thyroxine-binding proteins by zone electrophoresis on a preparative Porath cellulose column. Albumin, both native and defatted, shows only a small temperature dependence of binding. At 30° and 37° (pH 7.4), the constants that best fit the binding data for native albumin are n1 = 0.85 ± 0.15, k'1 = 1.41 x 106 m-1 ± 0.2, n2 = 5 or 6, k'2 = 0.095 x 106 m-1 or 0.067 x 106 m-1. At 6° the constants for native albumin are n1 = 1, k'1 = 1.6 x 106 m-1, n2 = 6, k'2 = 0.05 x 106 m-1. For defatted albumin the primary binding constants at 30° are n1 = 1, k'1 = 1.4 x106m-1. At 6° the primary binding constants for defatted albumin are n1 = 1, k'1 = 1.6 x 106 m-1. The thermodynamic parameters for association at the primary binding site are (a) for native albumin, ΔF° = -8.40 kcal per mole (30°, n1 = 1), ΔH° = -2.02 kcal per mole, ΔS° = +21 e.u.; (b) for defatted albumin, ΔF° = -8.51 kcal per mole (30°), ΔH° = -0.94 kcal per mole, ΔS° = +25 e.u. Chloride ions inhibit thyroxine binding to albumin. At 30° and pH 7.4, thyroxine binding to albumin is reduced by 50% in the presence of 0.1 m NaCl.