Lysosomal enzymes are decreased in the kidney of diabetic rats

The objective of the present study was to investigate the expression and activities of lysosomal enzymes that act upon proteins and sulfated polysaccharides in diabetic rat kidney. Cathepsins, glycosidases and sulfatases were studied on the 10th (DM-10) and on the 30th (DM-30) day of streptozotocin-induced diabetes mellitus (DM). The activity of cathepsin B, the main kidney cysteine protease, was decreased both in DM-10 and DM-30. Gel filtration chromatography of urinary proteins has shown the prevalence of low molecular weight peptides in normal and DM-10 urine, in contrast to the prevalence of high molecular weight peptides and intact proteins in DM-30. These results show that the decrease in lysosomal proteases could explain, at least in part, the increased albuminuria detected by radial immunodiffusion (RID), due to the excretion of less degraded or intact albumin. Concerning sulfated polysaccharides, the activities of β-glucuronidase, N-acetyl-β-d-glucosaminidase, and N-acetyl-β-d-galactosaminidase were also decreased in DM-30, while aryl sulfatases did not vary. Increased toluidine blue metachromatic staining of the tissue suggests that the lower activities of glycosidases could lead to intracellular deposition of partially digested molecules, and this could explain the decreased urinary excretion and increased tissue buildup of these molecules. The main morphological changes observed in kidney were proximal convoluted tubules with thinner walls and thinner brush border. Immunohistochemistry revealed that most of cathepsin B was located in the brush border of proximal tubular cells, highlighting the involvement of proximal convoluted tubules in diabetic nephropathy.