Increased cathepsin D-like activity in cortex, tubules, and glomeruli isolated from rats with experimental nephrotic syndrome

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Abstract
The activity and distribution of cathepsin D, a major renal lysosomal endoproteinase, was examined in the various anatomical and functional areas of normal rat kidney. Cathepsin D-like activities (.DELTA.A280/h per mg of protein) in normal rat tissues were: cortex, 0.78 .+-. 0.05, n = 37; medulla, 0.62 .+-. 0.03, n = 12; papilla, 0.63 .+-. 0.04, n = 12; tubules, 0.74 .+-. 0.04, n = 28; glomeruli, 0.59 .+-. 0.03, n = 28; and liver, 0.41 .+-. 0.02, n = 28. Enzyme activity was maximal at pH 3.0-3.5 and inhibited more than 90% by pepstatin (6.7 .mu.g/ml), suggesting that the enzyme is cathepsin D. In subsequent experiments, cathepsin D-like activity was measured in cortex, tubules and glomeruli isolated from rats with puromycin aminonucleoside(PAN)-induced nephrotic syndrome. Treated animals (15 mg of PAN/100 g body wt., i.p.) developed proteinuria beginning 4 days after injection and exceeding 900 mg/24 h on day 9. In 2 separate experiments involving 52 animals, a significant increase in cathepsin D-like activity was observed in cortex (+82.7%), tubules (+109.6%) and glomeruli (+54.7%) isolated from PAN-treated rats killed during marked proteinuria (day 9, mean total urinary protein excretion: 937 .+-. 94 mg/24 h). This increase was observed whether the activity was expressed per mg of DNA or per mg of protein. Increased cathepsin D-like activity was first observed in cortex and tubules coincident with the onset of proteinurea (day 4, mean total urinary protein excretion: 112 .+-. 23 mg/24 h). In contrast with the significant elevation of renal cathepsin D-like activity, the activity (nmol/h per mg of protein) of .alpha.-L-fucosidase (EC 3.2.1.51), a non-proteolytic enzyme, was markedly decreased in the identical samples used for the measurement of cathepsin D-like activity: cortex (-46.4%), tubules (-46.1%) and glomeruli (-38.5%). In addition to changes in renal enzyme activities, PAN-treated rats excreted large amounts of cathepsin D-like activity in their urine (beginning on day 3) compared with nearly undetectable cathepsin D-like activity in the urine from control rats. The significant increases in glomerular and tubular cathepsin D activity may reflect an important role for this enzyme in the pathophysiology associated with PAN-induced nephrotic syndrome.