SEPARATION OF LYSOSOMES AND AUTOPHAGOSOMES BY MEANS OF GLYCYL-PHENYLALANINE-NAPHTHYLAMIDE, A LYSOSOME-DISRUPTING CATHEPSIN-C SUBSTRATE

In density-gradient analyses of autophagic vacuoles from isolated rat hepatocytes, autophagosomes could be recognized by the presence of an autophagically sequestered cytosolic enzyme, lactate dehydrogenase (LDH). Lysosomes were identified by marker enzymes such as acid phosphatase, or by degradation products from I-125-tyramine-cellobiose-asialoorosomucoid (I-125-TC-AOM) loaded into the lysosomes by an intravenous injection in vivo 18 h prior to cell isolation. Autophagosomes and lysosomes showed similar, largely overlapping, density distributions both in hypertonic sucrose gradients and in isotonic Nycodenz gradients. As a step towards the purification of autophagosomes, we investigated the possibility of using lysosomal enzyme substrates to achieve selective destruction of lysosomes by swelling. Hepatocytes were first incubated for 2h at 37 degrees C with vinblastine (50 mu M) to obtain an accumulation of autophagosomes (to 3-5-times above the control level). The cells were then electrodisrupted and the disruptates incubated with a variety of substrates for lysosomal enzymes. Among these, glycyl-phenylalanine-2-naphthylamide (GPN), a cathepsin-C substrate, and methionine-O-methylester (MetOMe), an esterase substrate, turned out to induce extensive rupture of lysosomes, as measured by a strongly reduced sedimentability of acid phosphatase and a nearly complete loss of I-125-TC-AOM sedimentability in substrate-treated preparations from control or vinblastine-treated cells. The lysosomes of cells treated with leupeptin or asparagine were largely resistant to the action of GPN, probably as a result of interference with cathepsin-C activity or lysosomal function in general. Autophagosomes were partially destroyed by MetOMe, as indicated by a reduction in sedimentable LDH, but GPN had no effect on either autophagosomes or mitochondria. The ability of GPN to selectively destroy lysosomes without affecting the autophagosomes of vinblastine-treated cells should make GPN treatment a useful aid in the purification of rat liver autophagosomes.