Characterization of microheterogeneity of human thyroglobulin from different thyroid disorders

Human thyroglobulin (Tg) was isolated to apparent purity from various thyroid tissue samples obtained after surgery. Iodine and iodothyronine content of Tg was determined. Isoelectric focussing (IEF) was performed on thin layer agarose gels. Tg revealed a microheterogeneity of 6 bands in the range between pH 4.2 and 4.6. The intensity of the single bands depended on the iodothyronine content of Tg. With increasing degree of iodination, the bands with a lower pI (pI 4.35, 4.40) became more prominent, whereas the bands with higher pI (pI 4.55, 4.60) diminished. This typical change in microheterogeneity pattern could be confirmed by kinetic in vitro iodination and consecutive iodothyronine formation of low iodinated Tg (0.05% iodine). After in vitro desialylation, the bands shifted to a higher pH range (pH 4.60 to pH 4.90), but no reduction of the number of bands occurred. Even in desialylated Tg microheterogeneity is still dependent on iodine content. These results suggest, that the microheterogeneity of Tg is influenced, but not caused by different iodine and NANA content. Different polypeptide composition may be responsible for the microheterogeneity of Tg. In thyroid diseases without disturbance in Tg synthesis (endemic, diffuse and nodular goitre, Graves' disease) variations in relative intensity of single bands could be related to differences in iodine content. In thyroid cyst fluid and cold nodules in addition to low iodinated Tg, further bands were found with pI-values comparable to desialylated Tg.