Specific binding sites for L-triiodothyronine in rat liver and kidney cytosol

Binding of 125I-labelled triiodothyronine (T3) to rat liver and kidney cytosol was examined. In both organs a very similar binding protein was detected. The apparent equilibrium association constants amounted to 4.8 X 10(7) M-1 for the liver protein and to 1.8 X 10(7) M-1 for the kidney protein. In the two organs a high capacity for T3 binding was observed: maximal binding capacities of respectively 4.3 and 33.2 pmol per mg total cytosol protein. Displacement studies using thyroid hormone analogues showed that the binding exhibited similar specificity.