Appearance of autolysosomes in rat liver after leupeptin treatment

The administration to rats of leupeptin produced prominent numbers of enlarged and irregularly shaped autolysosomes in hepatocytes. Percoll density equilibration of crude lysosomal fractions from rat livers showed that most lysosomal enzyme migrated form normal lysosomal density fractions toward higher density fractions within 30 min after leupeptin injection. The denser particles were ultrastructurally identified with the autolysosomes. In analysis by differential centrifugation of the liver homogenates, lysosomal enzymes became sedimentable with particles of greater sedimentation rate within 30 min after leupeptin injection. These changes in physical properties of lysosomes reverted to normal after 24 h, and concomitant disappearance of autolysosomes in hepatocytes was observed by electron microscopy. When the time course of distribution of leupeptin in subcellular fractions was analyzed, particle-bound leupeptin shifted with time to larger particle fractions in parallel with the shift of lysosomal enzyme distribution. Upon injection with leupeptin, cathepsin B activity was inhibited by more than 80% for about 3 h and was gradually restored to a normal level after 24 h. Leupeptin treatment caused marked delay of the degradation of endocytosed FITC-labeled asialofetuin in hepatic lysosomes, and its inhibitory effect lasted for over 9 h after the injection. It was suggested that autophagy is probably a normal process of protein degradation in hepatocytes, and because of a retarded digestion of sequestered materials, autolysosomes persisted for a long period and made up the majority of the lysosome population in the leupeptin-treated cells.