Multimerization of thyroglobulin (TG) during extracellular storage: Isolation of highly cross-linked TG from human thyroids

Thyroglobulin (TG) is the major soluble protein of the thyroid and is known to be extracellularly stored for future liberation of thyroid hormones. We have developed techniques for the isolation of an insoluble storage form of human TG present in the follicle lumen. The application of these techniques yielded insoluble and translucent colloid globules varying in size (50-500 mu m) and shape and consisting primarily of densely packed TG. Intact colloid globules exhibited the imprints of the apical cell surfaces of thyrocytes that had surrounded the colloid globules in situ. Hence, in size and surface morphology, isolated colloid globules represent authentic lumenal content. Based on the total protein of single colloid globules and their volume, an average protein concentration of 590 mg/mL was calculated. The presence of protein disulfide isomerase in colloid globules and in the secretory product of cultured thyrocytes suggests its involvement in the extracellular multimerization of human TG. Native colloid globules increased their volume considerably upon reduction of disulfide bonds; they were completely dissolved by treatment with dithiothreitol and SDS. The results show that part of extracellular human TG undergoes multimerization, primarily by the formation of intermolecular disulfide bonds, thus allowing the storage of TG at excessively high, previously unknown, concentrations.