Endocytosis of superoxide dismutase by rat liver

We have investigated the endocytosis by rat liver of superoxide dismutase (SOD) labelled with 125I. (125I) SOD is quickly taken up by the liver where it remains in significant amounts for at least 150 min. Adsorptive endocytosis is probably involved. Distribution of radioactivity was established after differential and isopycnic centrifugation and compared with that of cathepsin C, a lysosomal enzyme. Results show that the behavior of radioactivity is similar to that of the hydrolase. SOD activity is only marginally affected by incubation in the presence of a purified lysosome extract; moreover, when (125I) SOD is treated in the same conditions, only a few percent of radioactivity becomes acidosoluble. These observations indicate that SOD taken up by the liver accumulates in lysosomes where it can stay for a relatively long time owing to its relative resistance to lysosomal hydrolases.