Inhibition of glycoprotein catabolism in vivo and in the perfused rat liver.
Leupeptin is a peptide which inhibits several of the lysosomal proteases. When this compound was added in low concentrations to a perfused liver, the degradation of 125I-asialo-fetuin by the liver was dramatically slowed. When 5 mg leupeptin were added to the perfusate 1 h prior to the radioactive glycoprotein, the liver retained from 70 to 90% or the radioisotope 60 min after infusing 125I-asialo-fetuin. However, untreated livers contained less than 20% of the radioactivity at that time. Subcellular fractionation experiments showed that the radioactivity accumulated in the heavy and light mitochondrial fractions (ML) of the homogenate. At 80 min after the glycoprotein was added, almost 40% of the radioactivity was still located with these fractions. Very similar inhibitory effects were seen upon treating rats intravenously with 5 mg of leupeptin 60 min prior to injection of 125I-labelled asialo-fetuin. A 7 fold increase in liver radioactivity was observed 6 hrs after the glycoprotein had been given to the treated animals. Purified human liver cathepsin B digested fetuin to about 3% of total hydrolysis and the major peptide fragment produced had an SDS-electrophoretic mobility equivalent to that of ovalbumin.