Separation of Thyroxine-Binding Proteins in Human Serum at pH 7.4. II. Effect of pH and Temperature on the Binding Capacities of Thyroxine-Binding Globulin (TBG) and Thyroxine-Binding Prealbumin (TBPA)

The binding capacities of human serum TBG and TBPA have been shown in this study to be pH-dependent. In polyacrylamide gel electrophoresis at 5 C the mean TBG maximum binding capacity (MBC) for 6 individual sera was 18.5± 3.5(S.D.) μg/100 ml at pH 7.4 and 12.3 ± 1.9 μg/100 ml at pH 9.0. The increased binding capacity at physiologic pH parallels previously demonstrated tracer T4 binding by TBG at pH 7.4. The MBC of TBPA was 207 ± 31 μg/100 ml at pH 7.4 and 308 ± 32 μg/100 ml at pH 9.0, in accord with earlier tracer T4 studies which showed enhanced TBPA binding at alkaline pH. Albumin binding in the present experiments was unaffected by pH. The fact that TBG and TBPA MBCs are pH-dependent suggests that the number of available binding sites for T4 on these proteins changes with pH. Studies of the effect of temperature on TBG and TBPA binding capacities showed TBG MBC at pH 7.4 to be about 20% lower at 37 C compared with 5 C. However, at pH 9.0 the binding capacity for TBG at 37 C was 35% to 70% below that obtained at 5 C. The TBPA MBC at pH 7.4 was not significantly altered at 37 C compared with 5 C.