[Intracellular protein breakdown. VI. Isolation, properties and biological significance of cathepsin D from rat liver].

The preparation and properties of cathepsin D from rat liver are reported. The enzyme is an endopeptidase of lysosomal origin. The molecular weight was estimated to be 49000 by sodium-dodecylsulfate electrophoresis. We did not find any dissociation into subunits under reducing conditions, in contrast to some other authors. We found the enzyme to occur in at least 4 forms with the isoelectric points 5.87, 5.65, 5.41 and 5.13. Strong -SH-blocking reagents inhibit the activity, but the most powerful and specific inhibitor was pepstatin (Ki=38 nM). The substrate specificity is discussed. There was no proof for any zymogen activation in a great number of experiments. Since the cathepsins B1, B3 and L obviously seem to play the major role in the intracellular protein breakdown within the rat liver, the main task of cathepsin D is the degradation of extracellular proteins in this organ.