Localization of iodine binding in the thyroid gland in vitro

Protein-bound radioiodine was localized by electron microscopic autoradiography in follicles and cells isolated from pig and rat thyroid tissue after incubation with 125I- for 1-10 min. Labeled proteins were analyzed by gel electrophoresis and immunoprecipitation. In closed follicles, with colloid-filled follicle lumina, the autoradiographic labeling was concentrated over the lumina and no labeling occurred over the cells. In open follicles, without colloid content, autoradiographic grains were invariably found along the apical cell surface and in some cells over intracellular lumina. Isolated cells with preserved polarity showed some labeling associated with remaining microvilli whereas isolated cells with lost polarity showed no grains at the cell surface. In isolated cells, particularly those with lost polarity, most grains were located over intracellular lumina (which were common in such cells) and some grains over vesicular elements associated with these lumina. About 80% of the labeled soluble proteins and 40% of the labeled proteins solubilized by sodium dodecyl sulfate were thyroglobulin. It is concluded that the site of iodination in follicles is the same in vitro as in vivo, viz. the apical surface of the follicle cells. When thyroid cells are removed from the follicle and lose their polarity, intracellular lumina become the major site of iodination. This shift in iodination sites is thought to be due to retention of Golgi-derived secretory vesicles in nonpolarized cells, leading to coalescence of vesicles with the formation of intracellular lumina and activation of membrane-bound enzymes catalyzing iodination.