Binding of Thyroxine by Human Serum Albumin

In previous work from this laboratory (1) studies on equilibrium dialysis were carried out to determine the binding constants for the interaction of human serum albumin with thyroxine and its analogs. The classical technique of equilibrium dialysis as described by Scatchard and others (2-6) was employed. At present it seems clear that thyroxine in human serum migrates to at least three different sites during electrophoresis at pH 8.6 (7-10). The three sites most frequently reported have been the thyroxine-binding a-globulin (TBG), albumin, and the "prealbumin" region. The thyroxinebinding a-globulin has generally been considered of major physiological importance, the others being regarded as secondary carriers. Albumin is the only one of the thyroxine carriers of serum readily available in pure form and with a known molecular weight. The findings of the previous work (1) appeared compatible with the hypothesis that the anionic phenolate groups of thyroxine and triiodothyronine interact with free cationic groups on the protein molecule. In the present report the observations have been extended to studies with variations in temperature and pH, as well as to experiments with mercaptalbumin, isooctane-extracted albumin, and acetylated albumin preparations. The present findings provide further support for the concept of an interaction of the anionic phenolate groups of thyroxine and four cationic groups on the protein molecule. More-