Thyroidal Iodine Metabolism During the Development of the Chick Embryo

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Abstract
Stable I was measured in the thyroid gland of the chick embryo from day 9-20 of incubation to evaluate quantitatively the functional development of the gland. Total I content increased progressively during incubation. From day 9-17 of incubation, this increase resulted from the increases of pellet-bound I and of soluble I. Afterwards, it essentially paralleled the increase of the soluble thyroglobulin-bound I which reflected the increase in both thyroglobulin content and the degree of iodination of the thyroglobulin. The total I, thyroglobulin-bound I 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 I concentration remained roughly constant (25 ng/mg) from days 13-19. Only 1 iodoprotein, 19.5 S TG, was found; its heterogeneity of iodination was demonstrated during the whole period of incubation studied (from days 11-20). 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, I was bound more and more to TG molecules, which were resistant to dissociation with SDS. While the average I content of the TG increased, no appreciable changes were found in iodotyrosine and iodothyronine percentages of TG-bound I: monoiodotyrosine 26%, diiodotyrosine 43%, thyroxine 12%, and 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 I content of TG (127I atoms per mole); about 30 atoms of I 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.