Glycosylation of thyroglobulin--its role in secretion, iodination, and stability

Tunicamycin, an inhibitor of glycosylation, was incubated with ovine thyroid cells in culture to determine the role of glycosylation in the subsequent processing of thyroglobulin to form thyroid hormone. After a 6-h preincubation with tunicamycin (1 microgram/ml), mannose incorporation into glycoproteins in the cell layer was inhibited effectively (greater than 90%), whereas leucine incorporation into proteins was inhibited by less than 30%. Conversely, the quantity of radioactively labeled proteins secreted into the medium by the thyroid cells was markedly inhibited. Thyroglobulin secretion into the medium and iodination were negligible. A low mol wt fragment of thyroglobulin was found after tunicamycin treatment, indicating increased susceptibility to or contact with proteases. These data suggest that glycosylation is obligatory for the processing of thyroglobulin, including iodination and subsequent thyroid hormone production.