Purification of thyroid lysosomes by colloidal silica density gradient centrifugation

A procedure was devised for fractionating crude thyroid lysosomal particles (P750-15,000) by self-forming density gradient centrifugation with colloidal silica. Two discrete particle-containing peaks were observed, based on 131I-labeling and acid phosphatase activity: a heavy peak (density, 1.11-1.12) and a light peak (density, 1.05). Ultrastructural analysis revealed that the heavy peak consisted almost entirely of lysosomes, whereas the light peak represented a heterogeneous mixture of small vesicles and fragments of other intracellular organelles. In thyroids removed from rats 30 min after 131I injection, almost all of the 131I was present in the low density peak. This 131I appeared on sucrose density gradient centrifugation as a 19S peak, and it was almost completely insoluble in trichloroacetic acid. This was interpreted as indicating that the low density peak contained pinocytotic vesicles. In thyroids removed 4 days after 131I injection, the radioactivity appeared largely in the high density peak. Both the trichloroacetic acid solubility and the pattern on sucrose density gradient centrifugation indicated that the [131I] thyroglobulin had undergone extensive proteolysis. Thyroglobulin proteolytic activity was found primarily in the high density particles and to only a small extent in the low density particles. Studies performed at intervals after 131I injection combined with double labeling (131I and 125I) experiments provided evidence that radioactivity was transferred from the low density to the high density particles. Heterogeneity existed within the dense peak, related to the degree of thyroglobulin degradation, as it was observed that thyroid lysosomes become denser with increasing proteolysis of thyroglobulin. The acid phosphatase in the low density particles could be distinguished from that in the high density (lysosomal) particles by its elution pattern on Sephadex G-200 column chromatography, its response to freezing and thawing, and its reactivity with p-nitrophenylphosphate. It was concluded, therefore, that the acid phosphatase in the low density fraction was derived from prolysosomal structures such as vesiculated Golgi-endoplasmic reticulum-lysosomes. The prolysosomal acid phosphatase associated with the low density fraction appeared to be a large membrane-bound molecule which could be transformed into lysosomal acid phosphatase by incubation at pH 5.0.