SPECIFIC CATALYTIC ACTIVITY OF CATHEPSIN-S IN COMPARISON TO CATHEPSIN-B AND CATHEPSIN-L ALONG THE RAT NEPHRON

Assay conditions were elaborated to determine the catalytic activity o f cathepsin S fluorometrically for direct comparison with the activiti es of cathepsins B+L(+S) and B along the nephron of the normal rat. Th ese conditions include the use of 0.5 mM Z-Phe-Arg-AMC as substrate, w hich 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 mu M Z-Phe-Phe-CHN2 permitted differentiation of these enzyme activ ities. The catalytic activity of cathepsin S in rat kidney homogenate (1.11 mu mol/min x 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 t he cortex (1.54 mu mol/min x g protein) than in the medulla resembling the activity ratio of cathepsins B+L(+S) and B. In suspensions of iso lated glomeruli and isolated proximal tubules the activities of cathep sin S were 0.76 and 3.21 mu mol/min x g protein, respectively. The cor responding activities of cathepsins B+L(+S) amounted to 80.0 and 211.7 mu mol/min x g protein consisting of 71% cathepsin B activity. In nep hron segments microdissected from lyophilized renal sections, highest cathepsin S activity was found in the proximal convoluted tubules (4.2 1 mu mol/min x g dry weight) followed by 0.83 mu mol/min x g dry weigh t in proximal straight tubules of the superficial cortex. In the remai ning segments cathepsin S activity was hardly detectable. Unlike cathe psin S activity, the activity of cathepsin B was distributed in parall el to that of cathepsins B+L(+S). The presence of relatively high cath epsin S activity in proximal convoluted tubules in co-localization wit h the activities of cathepsins B+L(+S) and B suggests a primary role o f these enzymes in heterophagocytosis of proteins from the ultrafiltra te.