LOW MOLECULAR WEIGHT INTRACELLULAR IODOCOMPOUNDS WITH LONG INTRATHYROIDAL HALF-LIFE: REMNANTS OF THYROGLOBULIN HYDROLYSIS?

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Abstract
These compounds were previously localized autoradiographically within the follicular cells. Radioiodide was administered to rats on a normal I intake (6-7 .mu.g/day) for 80 days to approach isotopic equilibration of the intrathyroidal I with the dietary radioiodide. When the isotope was omitted from the diet, the intrathyroidal radioiodine was released with an apparent half-lie of approximately 12 days. When the individual soluble components carrying radioiodine were analyzed after separation on Sephadex G-200, different apparent half-lives were found, the half-life of thyroglobulin (TGB) being roughtly 10 days and that of the low MW iodocomounds being in the order of 60 to 100 days or more. In addition to the soluble low MW iodocompounds, the radioactivity in the particulate fraction increased by 100% during the tracter washout when compared to TGB and the total soluble fraction. The soluble slow turnover iodocompounds contained a higher percentage of carbohydrate and total I than TGB, while the relative amounts of each sugar analyzed (hexoses, fucose, hexosamine and sialic acid) were close to those in TGB. Sephadex G-25 chromatography of the low MW iodocompounds obtained after G-200 separation resulted in the separation of 4 peaks. Two peaks identified as iodopeptides could be further analyzed. The carbohydrate composition of these peptides was similar to that of 2 glycopeptides obtained after in vitro enzymatic hydrolysis of purified TGB with pronase. Slow equilibration with radioiodine, long apparent intrathyroidal half-life and carbohydrate content similar to that of TGB, taken together with previously published data on intracellular localization of soluble intrathyroidal iodocompounds, suggest that the low MW iodocompounds are products of in vivo hydrolysis of engulfed TGB droplets.