Inhibitory Effects of Tunicamycin and 2‐Deoxyglucose on Thyroglobulin Synthesis

The kinetic incorporation of labelled sugars and amino acids by rat thyroid hemilobes was measured in the presence of 2‐deoxyglucose and tunicamycin, inhibitors of the glycosylation of glycoproteins. With either inhibitor the carbohydrate content of exocytosed thyroglobulin was only slightly decreased (< 20% of control) whereas the rate of exocytosis was strongly inhibited (by 60–80%). As no intracellular accumulation or proteolysis of non‐glycosylated molecules was detected, the reduced rate of thyroglobulin release seems essentially due to a decrease in protein synthesis. In a whole cell system (hemilobes), it is impossible to uncouple glycosylation and protein synthesis by incubation with tunicamycin; 50 μg/ml tunicamycin for 270 min inhibited total [ 3 H]‐glucosamine and 14 C‐labelled amino acid incorporation by 65% and 33% respectively. This can be contrasted with cell‐free incubation of thyroid rough microsomes where glycosylation was blocked by the same tunicamycin concentration (90% inhibition of N −[ 3 H]acetylglucosamine transfer from UDP‐ N −[ 3 H]acetylglucosamine) whilst ongoing protein synthesis was not significantly modified (< 4% inhibition). This clearly suggests that, in thyroid follicular cells, a regulatory link exists between the synthesis of the peptide moiety of a glycoprotein and its glycosylation.