title: The site of sialic acid incorporation into thyroglobulin in the thyroid gland.
abstract: Iodine incorporation into thyroglobulin is known to occur within the lumen of the thyroid follicle. Since incorporation of sialic acid, which occupies a terminal position in the oligosaccharide chains, is also a later event in thyroglobulin synthesis, the possibility that sialic acid might be incorporated after thyroglobulin secretion was investigated. In one experimental approach normal rat thyroid hemilobes were incubated with radioactive precursors. Thyroglobulin, analyzed by equilibrium centrifugation in RbCl, had a median density which varied according to the moiety labeled in the following increasing order: leucine smaller than galactose smaller than sialic acid smaller than iodine. The molecules having the highest density were labeled only with iodine. In the second approach, thyroid hemilobes were taken from rats treated with cycloheximide for 16 hours to stop protein synthesis and allow nascent molecules to be secreted, and incorporation of precursors into thyroglobulin was analyzed by sucrose gradient centrifugation. Leucine incorporation was 6% of control but the amino acid was found in the NH2-terminal position. N-Acetylmannosamine (sialic acid precursor) and galactose incorporation were also completely inhibited whereas iodine incorporation was 10% of control. Incorporation was not restored by thyrotropin treatment, and the sialyltransferase and iodination systems were reduced only to 50 to 70% of control. The results indicate that sialic acid is incorporated only in nascent thyroglobulin and not in thyroglobulin molecules already secreted into the follicular lumen. A large fraction of the iodine incorporation also seems to occur in newly synthesized thyroglobulin.
mesh_terms: D000818:Animals; D002499:Centrifugation, Density Gradient; D003513:Cycloheximide / Q000494:pharmacology; D003597:Cytosine Nucleotides / Q000378:metabolism; D006595:Hexosamines / Q000378:metabolism; D006801:Humans; D007454:Iodides / Q000378:metabolism; D007930:Leucine / Q000378:metabolism; D007931:Leucyl Aminopeptidase; D008358:Mannose; D051381:Rats; D012709:Serum Albumin; D012794:Sialic Acids / Q000378:metabolism*; D013954:Thyroglobulin / Q000096:biosynthesis*; D013961:Thyroid Gland / Q000187:drug effects / Q000378:metabolism*; D013972:Thyrotropin / Q000494:pharmacology; D013997:Time Factors
chemical_list: D003597:Cytosine Nucleotides; D006595:Hexosamines; D007454:Iodides; D012709:Serum Albumin; D012794:Sialic Acids; D013972:Thyrotropin; D013954:Thyroglobulin; D003513:Cycloheximide; D007931:Leucyl Aminopeptidase; D007930:Leucine; D008358:Mannose