Comparison of binding of thyroid hormone analogues to hepatic organic anion binding proteins

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D v protein and ligandin are two hepatic cytosolic proteins which bind organic anions, including endogenous thyroid hormones. Binding studies were performed using the ANS displacement technique to compare the binding of a variety of thyroid hormone analogues to purified organic anion binder and ligandin. Inhibition of ANS binding by these compounds was competitive. Both proteins bound L- and D-thyroxine with comparable affinity ( K d 30–45 μM), whereas ligandin boudn 3′,3′,5-triiodo- L-thyronine, 3′,3′,5-triiodo- L-thyronine and most analogues with greater affinity. Nevertheless, the order of ligand affinities for both binders was highly correlated, suggesting that the nature of the binding site on both proteins is similar. The binding affinities of these organic anion binders are 2–3 orders of magnitude lower than an hepatic cytosolic thyroid binder reported by others, suggesting that ligandin and organic anion binder may not be important in intracellular thyroid hormone transfer.