THE SELECTIVE ROLE OF CATHEPSIN-B AND CATHEPSIN-D IN THE LYSOSOMAL DEGRADATION OF ENDOGENOUS AND EXOGENOUS PROTEINS

A selective inhibitor of cathepsin B, a derivative of E-64 (compound CA-074), and pepstatin-asialofetuin, a potent inhibitor of cathepsin D, were used for an in vivo study of the selective role of these proteinases in lysosomal proteolysis. Administration of compound CA-074 or pepstatin-asialofetuin to rats caused only a slight shift of the lysosmal density and no increase in sequestered enzymes in the autolysosomal fraction, although cathepsin B or D activity in the liver was markedly inhibited. These treatments also had little effect on the inhibition of the degradation of endocytosed FITC-labeled asialofetuin. In contrast, leupeptin treatment caused marked inhibition of lysosomal degradation of endogenous and exogenous proteins. These results suggest a small contribution of cathepsins B and D to the initiation of lysosomal proteolysis.