Specific Catalytic Activity of Cathepsin S in Comparison to Cathepsins B and L along the Rat Nephron^*

Assay conditions were elaborated to determine the catalytic activity of cathepsin S fluorometrically for direct comparison with the activities of cathepsins B+L(+S) and B along the nephron of the normal rat. These conditions include the use of 0.5 mM Z-Phe-Arg-AMC as substrate, which is saturating for the three enzymes. The stability of cathepsin S at pH 7.5 and the resistance of cathepsin B against inactivation by 0.5 μM Z-Phe-Phe-CHN₂ permitted differentiation of these enzyme activities. The catalytic activity of cathepsin S in rat kidney homogenate (1.11 μmol/min × g protein) amounted to 2.1% of that of cathepsins B+L(+S) and to 3.2% of that of cathepsin B. It was ten-fold higher in the cortex (1.54 μmol/min × g protein) than in the medulla resembling the activity ratio of cathepsins B+L(+S) and B. In suspensions of isolated glomeruli and isolated proximal tubules the activities of cathepsin S were 0.76 and 3.21 μmol/min × g protein, respectively. The corresponding activities of cathepsins B+L(+S) amounted to 80.0 and 211.7 μmol/min × g protein consisting of 71% cathepsin B activity. In nephron segments microdissected from lyophilized renal sections, highest cathepsin S activity was found in the proximal convoluted tubules (4.21 μmol/min × g dry weight) followed by 0.83 μmol/min × g dry weight in proximal straight tubules of the superficial cortex. In the remaining segments cathepsin S activity was hardly detectable. Unlike cathepsin S activity, the activity of cathepsin B was distributed in parallel to that of cathepsins B+L(+S). The presence of relatively high cathepsin S activity in proximal convoluted tubules in co-localization with the activities of cathepsins B+L(+S) and B suggests a primary role of these enzymes in heterophagocytosis of proteins from the ultrafiltrate.