Effect of ionic strength and ionic composition of assay buffers on the interaction of thyroxine with plasma proteins

When plasma proteins are diluted with buffer the ionic strength and ionic composition of that buffer affects the interactions between thyroxine (T4) and its plasma protein-binding sites. Increases in phosphate, chloride or barbiturate ion concentration from 50 to 200 mmol/l caused a significant decrease in the affinity of plasma proteins for T4, and a concurrent increase in the concentration of unbound T4. These results cannot be completely accounted for by changes in ionic strength since at the same ionic strength different anions caused quantitatively different effects on unbound T4 concentration. The degree of depression of T4 binding by the three anions studied was in the order barbiturate greater than chloride greater than phosphate. The results of a systematic study on the composition of diluent buffer systems indicated that when a 50 mM-sodium phosphate-100 mM-NaCl buffer (pH 7-4) was used as a plasma diluent, there were unlikely to be gross changes in the T4-binding properties of plasma proteins with dilution.