The Plasma Protein Binding of Triiodothyronine and Its Relation to the Uptake of I¹³¹-Labelled Triiodothyronine by Human Erythrocytes In Vitro

Abstract

In the present investigation the binding of triiodothyronine (TIT) in serum and its relation to the red blood cell uptake of I¹³¹ labelled TIT (RCU)have been studied. The binding was studied with starch gel electrophoresis in borate buffer at pH 8.6.

The TIT in this system is bound to a serum fraction migrating in the post-albumin area. Some of the added TIT migrates behind the post-albumin area. Experimental data are presented supporting the view that this slower portion consists of free TIT and is thus an expression for the relative amount of tree TIT. It is shown that the relative amount of trailing TIT is significantly positively correlated to the RCU, i. e. that the RCU ought to be an expression for the re lative amount of free TIT in the plasma.

It is also shown that the TIT-binding post-albumin fraction is not stable at +56°C. Heating serum to this temperature increases the RCU and increases also the trailing portion in electrophoresis to the same degree.

Addition of non-radioactive thyroxine or TIT in amounts sufficient to increase the RCU will also increase the trailing portion in electrophoresis.

The significance of these findings is discussed.