Preferential Binding of Tri-Substituted Thyronine Analogs By Bullfrog Tadpole Tail Fin Cytosol11

Abstract

The relative strength of binding of several triiodothyronine (T3) analogs by cytosol prepared from the tail fin of the bullfrog tadpole was tested and compared with their thyromimetic activity in tail tissue. In competitive binding experiments, 4 tri-substituted analogs were bound much more strongly than 2 tetra-derivatives. Correlation between binding and relative thyromimetic activity was observed only for 3 triiodo-substituted compounds, T3, its methylene bridge analog and the acetic acid analog. The affinity for T3 was 250 times that of T4 [L-thyroxine], making it unlikely that the T3 binding sites in tadpole tail fin cytosol bind T4 under physiological conditions. The difference in binding by tadpole tail fin cytosol constituted one of the largest differences observed between T3 and T4 in a potentially significant biological system.