In vivo sequential model for thyroglobulin iodination

Abstract

Abstract. In order to study sequential events at the follicular level, thyroglobulin (Tg) iodination kinetics were followed by double labelling experiments. The experiments were performed on two groups of rats receiving 50 μg (group 50) and 5 μg (group 5) of iodine daily. The rats were isotopically equilibrated with 125I. Each subgroup received a single dose of 131I at various periods of time between 2 and 48 h. The renewal kinetic curves, displayed by the iodine specific radioactivity (SRA = 0/00 of injected dose, μg 127I) show that, in both groups, Tg SRA presents two maxima, the first at about 6 h, the second at about 48 h, when, as shown for group 50, the SRA of the precursor iodide decreases monotically. This is interpreted as the result of two iodination steps: 1) a direct iodination of newly exocytosed Tg molecules, 2) a delayed iodination on Tg molecules previously stored within the colloid. The Tg SRA ratio between the two maxima indicates that the renewal assured by the delayed iodination is 2.5 times greater than that assured by direct iodination in group 50, while resting equal in group 5. For both groups, the hormonal renewal, in comparison to the iodotyrosine renewal, increases during the direct iodination, but stays constant after the delayed maximum value is reached: it is concluded that the higher hormogenesis efficiency could only be reached after the delayed iodination, probably because of higher iodine content of Tg molecules. These results emphasize the importance of the role of the Tg molecules stored in the colloid in the process of iodide organification and indicate that iodination, for the greater part, does not take place on de novo synthesized Tg molecules.