The thyroid lysosomal system: dynamic state of the organelles in relation to iodine release.

Endocytosis and hydrolysis of thyroglobulin (Tg) by the thyroid lysosomal system were studied in vivo in normal rats. By double labelling experiments, with 125I (old label) and 131I (new label), it was found that the well known preferential utilisation of newly labelled iodine by the thyroid follicles did not take place during endocytosis: the specific radioactivity (SRA = 131I/125I) was the same for the soluble Tg, luminal in origin, and the particulate Tg, lysosomal in origin. By contrast, the SRA of the total iodine content of the lysosomes was 2-15 times lower than the SRA of the soluble Tg. When the total lysosomal fraction (LT) was fractionated by size into subpopulations, with L1 less than LII less than LIII (Miquelis et al. [14]), it was found, after kinetics experiments, that SRA(LI) greater than SRA(LII) greater than SRA(LIII) for their iodine content. As observed after ultracentrifugation analysis on a discontinuous Ficoll gradient, only a small portion of the lysosomes is responsible for the difference of the SRA between LI and LT. Moreover, the LI subpopulation, significantly enriched in organelles of 0.03-0.25 micron diameter (Simon et al. [25], exhibits a higher acid phosphatase activity/beta-galactosidase activity ration than LT. It is suggested that 1) endocytosis is not preferential for newly iodinated Tg, 2) the thyroid lysosomes hydrolyse preferentially recently iodinated Tg, 3) Tg enters the cell by micropinocytosis, 4) the micropinocytotic vesicles fuse rapidly with primary lysosomes, 5) progressive release of iodinated compounds occur during the maturation of secondary lysosomes which fuse amongst themselves.