Incorporation of labelled carbohydrates into the soluble thyroid proteins of rat and man

This thesis examines the patterns of incorporation of labelled carbohydrates into the soluble proteins of the rat thyroid and of human pathological glands in vitro. It has been previously postulated by Herscovics (1969) that 14C mannose is incorporated like amino acids, that is into presumed thyroglobulin subunits of 3 - 8s and 125 size before there is labelling of the 19S protein. In contrast to this it was claimed that 14C galactose and 14C fucose are both incorporated directly into a protein just short of thyroglobulin running at approximately the 17 - 18s position on ultracentrifugation. The present studies using the rat thyroid are in agreement with Herscovics with respect to the incorporation of 14C mannose. However in the present studies C galactose was also incorporated in a pattern consistent with subunit incorporation and not direct lii incorporation into a 17 - 18S protein. 14C fucose, however, did seem to be incorporated directly into thyroglobulin-like protein and not to be incorporated into presumed thyroglobulin subunits. Various factors affecting the incorporation of these labelled sugars into the rat thyroid protein were examined. The process of incorporation was stimulated by the administration of an antithyroid drug or by TSH. All goitrogenic drugs gave the same effect suggesting that they might act through a TSH controlled mechanism. Furthest support for this possibility came from the study of the effect of sodium thyroxine and potassium iodide on the incorporation of labelled sugars when it could be shown that both substances interfered with the incorporation of all sugars studied. These effects were probably mediated via a TSH inhibiting mechanism. In vitro studies of human thyroid glands showed results in `normal` human, thyroids and non-toxic goitres similar to those anticipated from the rat study, that is that the pattern of incorporation of 14C mannose and 14C galactose was consistent with that of subunit incorporation before labelling of thyroglobulin-like proteins. In certain human pathological conditions, however, discrepancies from this were found. In thyrotoxicosis there was very poor incorporation of mannose as compared to galactose; in thyroid adenomas there was very little labelling of 12S protein compared to labelling of proteins in the thyroglobulin region using either mannose or galactose and in Hashimoto's thyroiditis there was in general very poor labelling of proteins in the thyroglobulin region with either sugar in contrast to the good labelling obtained with iodine, These discrepancies suggest that perhaps the differences in these pathological conditions might be due to deficiencies of specific glycosyltransferase enzymes