Thyroidal iodine metabolism during the development of the chick embryo

Stable iodine was measured in the thyroid gland of the chick embryo from day 9 to day 20 of incubation in order to evaluate quantitatively the functional development of the gland. Total iodine content increased progressively during incubation. From day 9 to day 17 of incubation, this increase resulted from the increases of pellet-bound iodine and of soluble iodine. Afterwards, it essentially paralleled the increase of the soluble thyroglobulin-bound iodine which reflected the increase in both thyroglobulin content and the degree of iodination of the thyroglobulin. The total iodine, thyroglobulin-bound iodine and thyroglobulin (TG) content, increased as power functions of time during incubation, with critical times on days 11 and 15. Their concentrations also increased during the whole incubation period, while the iodide concentration remained roughly constant (25 ng/mg) from day 13 to day 19. Only one iodoprotein, 19.5 S TG, was found, and its heterogeneity of iodination was demonstrated during the whole period of incubation studied (from day 11 to day 20). The degree of dissociation with sodium dodecyl sulfate (SDS) of the TG into 12 S subunits decreased as the degree of iodination of the TG increased. Throughout embryonic development, iodine was bound more and more to TG molecules, which were resistant to dissociation with SDS. While the average iodine content of the TG increased, no appreciable changes were found in iodotyrosine and iodothyronine percentages of TG-bound iodine: monoiodotyrosine, 26%; diiodotyrosine, 43%; thyroxine 12%; 3,5,3'-triiodothyronine, 2.5%. As a consequence, a linear relationship existed for each iodoamino acid between the number of its residues per mole of TG and the iodine content of TG (127I atoms per mole)-- about 30 atoms of iodine was required to form 1 mole of T4. The low efficiency of the TG of the chick embryo as a thyroidal hormone-forming protein was compensated for by its high degree of iodination.