Effect of thiol blocking agents on the binding of T3 and T4 in rat liver nuclear extract

Liver nuclei bind thyroid hormones (T3 and T4) with different binding affinities. In out studies it was estimated that the apparent association constant (Ka) in the liver nuclei for T3 was 8.8 X 10(10) l mol-1 and for T4 was 4.1 X 10(9) l mol-1. We have found that the binding sites for T4 are also saturated by an excess of unlabelled T4 and that saturation was dependent on the purification step of liver binding proteins. In liver nuclear extracts with 0.4 mol l-1 KCl the specific binding of T3 and T4 was blocked with a typical -SH blocking agent N-ethylmaleinimide (NEM) and by new types of -SH blocking agents such as p-bromphenylisothiocyanate (p-BPI) and 2,3 dicyano-1,4-dithio-9,10 antrachinone (Delan). NEM and p-BPI increased the non-specific binding of T4 and completely abolished specific binding. All these agents blocked the specific binding of T3. These results demonstrate that T3 and T4 binding sites in the liver nuclei may not be altogether identical and that the different effects of -SH blocking agents on the binding of T3 and T4 is probably associated with the structural configuration of these drugs.