Printed in U.S.A. Multimerization of Thyroglobulin (TG) during Extracellular Storage: Isolation of Highly Cross-Linked

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 in-soluble 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 pm) 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 i