Analyses of the molecular forms of serum thyroglobulin from patients with Graves' disease, subacute thyroiditis or differentiated thyroid cancer by velocity sedimentation on sucrose gradient and Western blot
Serum thyroglobulin (Tg) concentration increases in diverse thyroid pathophysiological situations. We examined whether Tg molecules appearing in the serum of patients with Graves' disease (GD), subacute thyroiditis (ST) or differentiated thyroid cancer (DTC) have distinctive biochemical properties. We used ultracentrifugation on sucrose gradient and Western blot to analyze structural parameters of immunoreactive Tg in complete serum from 40 patients with GD, ST or DTC. Purified human Tg was used as reference. Immunoreactive Tg from ST or DTC sera sedimented in a single, rather symmetrical peak as purified 19S Tg. In GD sera without detectable anti-Tg autoantibodies (TgaAb), about 80% of immunoreactive Tg was recovered in a Tg dimer peak that often split into two components; the remaining Tg immunoreactivity (10-30%) sedimented faster and was polydispersed. In GD sera containing TgaAb, immunoreactive Tg was recovered in a peak sedimenting faster than 19S Tg corresponding to immune complexes identified by protein A adsorption. Using a Western blot procedure, optimized to detect 0.1 ng Tg in serum. a single band of Tg, migrating as the intact Tg subunit, was always found in non-reducing conditions; the intensity of the band correlated with the immunoassayable Tg concentration. In reducing conditions, the Tg band obtained with GD or ST sera was decreased by up to 70% compared with that of purified Tg or serum Tg from patients with DTC. In conclusion, serum Tg from DTC is remarkably homogeneous and in the form of dimers dissociable into uncleaved monomers. In contrast, serum Tg from GD or ST is heterogeneous with respect to its sedimentation properties and/or the structural integrity of its polypeptide chains. These data provide information on the processes whereby Tg is released into the circulation.
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