Preferential formation of triiodothyronine residues in newly synthesized [14C]tyrosine-labeled thyroglobulin molecules in follicles reconstructed in a suspension culture of hog thyroid cells.

Early processes of thyroid hormone (T4 and T3) synthesis in thyroglobulin molecules were studied using follicles reconstructed in a primary culture of hog thyroid cells under the influence of TSH. When the reconstructed follicles were incubated with 14C-tyrosine, thyroglobulin containing the labeled tyrosine was newly synthesized and in the presence of iodide, some of the labeled tyrosine residues were iodinated and coupled to produce labeled iodothyronines, T4 and T3. Coupling efficiency, especially the efficiency of T3 production, was much higher than that obtained from the average iodoamino acid composition of mature thyroglobulin from the gland, indicating a preferential iodination of hormonogenic tyrosines and synthesis of T3. The total production of T3 was higher than T4 under the present conditions. However, free labeled T4 released into the medium was more than T3 after 16 h incubation of the labeled follicles with non-labeled tyrosine, suggesting the preferential liberation of T4 from the labeled peptide and/or release from the cells.