In vitro demonstration of specific triiodothyronine binding sites in rat liver nuclei

Limited-capacity, high-affinity binding of triiodothyronine (T₃) by liver nuclei in vitro was demonstrated by incubating isolated nuclei with increasing amounts of T₃. The mean apparent association constant was 6.1 × 10⁸ M^-1 Both the mean nuclear binding capacity, 1.7 pmoles T₃ for nuclei derived from 1 g liver, and the stereochemical specificity of the nuclear sites as assessed by competition studies with T3 analogues were virtually identical to analogous measures previously reported using in vivo techniques. The data indicate that specific nuclear T₃ binding in vitro occurs at the same site as in vivo.