A systematic study of factors affecting the binding of thyroxine and related substances to serum proteins

Abstract

The influence of the following factors on the interaction between thyroxine and human serum proteins has been studied in serum from the same individual: electrophoretic buffer, supporting medium for electrophoresis, binding of substances related to thyroxine, binding capacity and mutual displacement by structurally related derivatives. In the absence of veronal buffer, thyroxine at low concentrations was bound to the α-globulin TBG (thyroxine-binding globulin) and to TBPA (thyroxinebinding prealbumin). Veronal ions eliminated the interaction between TBPA and thyroxine, irrespective of the electrophoretic medium. In starch gel electrophoresis (borate buffer), at relatively higher concentrations of thyroxine, four protein fractions corresponding to TBG, TBPA, albumin and an unknown component, bound the hormone. Four binding zones were similarly observed when human serum was fractionated by ion-exchange chromatography on DEAE-cellulose. TBG and TBPA exhibited marked differences in their selectivity in binding substances related to thyroxine. Alterations in the alanine side-chain of T 4 and T 3 had opposite effects on their interaction with the two proteins. TBG interacted most intensely with thyroxine but hardly at all with tetraiodothyroacetic acid; TBPA has over a 100-fold higher affinity for the acetic acid analogue. From mutual displacement studies with different derivatives, it is concluded that TBG has a single type of binding site which is not sensitive to veronal ions. TBPA has two types of binding site only one of which interacts with thyroxine and is veronal-sensitive ; the other site which is not affected by veronal does not interact with thyroxine. Both sites of TBPA which exhibit the highest affinity for tetraiodothyroacetic acid fail to bind triiodothyronine. The physiological significance of these findings has been discussed.