Effect of Various Compounds on the Binding of Thyroxine to Serum Proteins in the Rat1

Abstract

The binding of L-thyroxine to serum proteins in the rat was determined by measuring the uptake of radiothyroxine from serum by the anion exchange resin IRA-400 contained within a polyurethane sponge. Sera from rats injected with various compounds were tested, and it was found that a number of compounds which inhibited deiodination of thyroxine in vivo did not alter the binding to serum proteins. Salicylate and dinitrophenol inhibited protein binding, as did estradiol to a lesser extent, and diphenylhydantoin did not alter binding. Studies performed on pooled rat serum to which various compounds were added in vitro showed that structural analogs of thyroxine which contained a 3,5-diiodo-4-phenolate group, as well as L-3,5,3′- triiodothyronine and 3,5,3′-triiodothyroacetic acid, were effective inhibitors of serum protein binding. The binding of thyroxine to serum proteins was also inhibited by high concentrations of many compounds which had in common the presence of one or more polar anionic groups on a 6-membered ring.