Iodination of newly synthesized thyroglobulin by FRTL-5 cells is selective and thyrotropin dependent.

This study shows that the Fisher rat thyroidal cell line (FRTL-5) can iodinate newly synthesized thyroglobulin. Iodinated thyroglobulin was found intra- and extracellularly. Both the synthesis of thyroglobulin and its subsequent iodination were found to be thyrotropin (TSH) dependent, with optimal activity at 10-100 microU TSH/ml. Thyroglobulin was the only protein in the culture medium, that was iodinated with high specificity and in a TSH-dependent fashion. Albumin, which was abundantly present in the culture medium, was only weakly iodinated. Various proteins, including thyroglobulin, were found to be iodinated intracellularly. Of these iodoproteins only thyroglobulin appeared in the medium suggesting selective secretion of iodinated thyroglobulin. It was shown that the other intracellular iodoproteins were no thyroglobulin breakdown products. Their function is as yet unknown.