Alterations in lysosomal enzymes of the proximal tubule in gentamicin nephrotoxicity

Gentamicin accumulates in proximal tubule lysosomes, increases their number, and changes their structure. An important lysosomal function is degradation of intracellular proteins. To evaluate the effect of gentamicin on this lysosomal function, we measured the activity of the key lysosomal proteinases, cathepsin B and L, in microdissected S1, S2, and S3 segments of rat proximal tubules by means of a fluorometric microassay. The cathepsin activities were decreased in S1 and S2 following one and four gentamicin injections of 100 mg/kg body weight. The lysosomal enzyme, acid phosphatase, was also measured and was not decreased by gentamicin. The urine excretion of cathepsins B and L was decreased after gentamicin. This excludes an increase in urinary loss of cathepsins as the cause of decreased tubule activity. Structural changes of the lysosomes per se were excluded as the factor responsible for the reduced cathepsin activity by demonstrating increased cathepsin B and L activity in proximal tubule segments from rats injected with dextran, since dextran induces an increase in number and size of proximal tubule lysosomes. In vitro incubation of urine and tubule segments with gentamicin demonstrated a concentration-dependent reversible inhibition of cathepsin B and L. We conclude that gentamicin per se decreased cathepsin B and L activities in proximal tubule segments as early as 24 hours following one injection due to either enzyme inhibition or reduced generation of active intralysosomal cathepsin B and L. Gentamicin may, therefore, reduce renal protein catabolism by decreasing the activity of the key proteolytic enzymes, cathepsin B and L. Since cathepsin B and L are proteolytic activators of other lysosomal enzymes, their reduced activity may also decrease the activities of other lysosomal enzymes.