Subcellular localization and binding of 125I-triiodothyronine in calf thyroid

The subcellular distribution of 125I-T3 was studied in calf thyroid slices, under the same experimental conditions where T3 inhibits protein and RNA synthesis, labelled hormone was found mainly in the 20,000 X g supernatant. The specificity of each subcellular localization was determined by incubating the slices with 10(-5)M T3. Only in the purified nuclei a significant decrease was found, indicating a specific localization of the labelled hormone. When slices were incubated with 125I both labelled T3 and T4 were found in purified nuclei, indicating that endogenously synthesized hormones can reach thyroid nuclei. Purified thyroid nuclei were incubated with labelled T3 and increasing amounts of cold hormone. Specific binding reached a plateau after 90 min of incubation at 20 degrees C. When the displacement curves were analysed by a Scatchard plot a binding site with a Ka of 5.2 X 10(7) M-1 and a capacity of 3.0 X 10(-15) moles/microgram DNA was observed. Digestion of nuclei with trypsin and protease abolished completely the binding of 125I-T3 thus indicating the protein nature of the receptor. The hormone-receptor complex could be extracted with 0.4M KCI and eluted in the void volume after Sephadex G-25 column chromatography, similar to peripheral tissues nuclear T3 receptors. The present studies provide the first evidence for the existence of nuclear receptors for T3 in the thyroid, an event probably related to the autoregulatory mechanism.