Effect of low molecular weight proteins and dextran on renal cathepsin B and L activity

Abstract

Renal extraction of low molecular weight proteins (LMWP) accounts for 30% to 80% of their total metabolic clearance. Extraction includes glomerular filtration, proximal tubular uptake, and intralysosomal proteolysis. To characterize the anatomic sites and enzymes involved in digestion of reabsorbed LMWP, the lysosomal proteases, cathepsin B and L, were measured by ultramicroassay in isolated S1, S2 and S3 segments of the proximal tubule of proteinuric rats. Increased glomerular filtration and tubular uptake of LMWP were induced by i.v. and i.p. injections of myoglobin and cationic and anionic lysozyme. Both cationic lysozyme and myoglobin increased cathepsin B and L activities in the proximal tubule, while anionic lysozyme had no effect. Morphologic examination of kidney tissue suggested that proximal tubular uptake of anionic lysozyme was negligible in comparison with the cationic form. Hence, only LMWP absorbed by the proximal tubule cells stimulated cathepsin B and L activities. Proximal tubular uptake of cationic lysozyme was determined by measurement of lysozyme activities in S1, S2, and S3. S1 segments contained the highest lysozyme activity, while S2 and S3 had much lower activities, and cathepsin B and L activity following cationic lysozyme injection was stimulated only in S1 segments. These results suggest that cathepsin B and L participate in lysosomal digestion of certain LMWP. Furthermore, the activities of cathepsin B and L adapt to increased uptake of LMWP. To gain additional insight into the mechanism of cathepsin adaptation, the cathepsin B and L activities were measured following injection of dextran with a similar low molecular weight.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of myoglobin degradation in nephrotoxicity after rhabdomyolysis

“…Rhabdomyolysis has been shown to associate with accumulation of myoglobin in lysosomes and its accumulation has proved to be remarkably reduced in megalin-deficient mice [6]. In addition, data show that injections of low-molecular weight proteins (myoglobin and lysozyme) increase the activity of the lysosomal proteases, cathepsins B and L in rats' proximal tubules thus promoting intralysosomal degradation of myoglobin [7].…”

“…1). Obviously, after entering renal tubule cells, most likely by endosomes, myoglobin is transported into lysosomes with subsequent cleavage by lysosomal enzymes [7]. This hypothesis is underscored by the fact that a large amount of myoglobin is detected in kidneys only on the first day after rhabdomyolysis and later immune staining cannot reveal myoglobin [4].…”

“…It has been hypothesized that endocytosis receptors megalin and cubilin may be involved in myoglobin absorption by tubule cells [6] and indeed, knocking-out of the megaline gene in mice has been shown to significantly reduce accumulation of myoglobin in tubule cells and its nephrotoxicity [6]. Obviously, after being absorbed by a tubule epithelium cell, myoglobin may undergo modifications, for example, through proteolysis by lysosomal enzymes, which has been supported by the fact that lysosomal cathepsins B and L activity increases after myoglobin injection [7].…”

Renal uptake of myoglobin is mediated by the endocytic receptors megalin and cubilin

“…The precise mechanism, whether due to free iron, heme, or heme protein, and which free radical mechanism is involved, as well as which cellular organelles are affected, are still debated (11,26). It has been convincingly shown that myoglobin undergoes reabsorption from the glomerular filtrate and is catabolized within proximal tubule cells (2,14,21). Thus it is conceivable that particular components of myoglobin can act on different intracellular levels with the involvement of diverse radical species during reabsorption of holoprotein and its subsequent decomposition.…”

Protein reabsorption in renal proximal tubule?function and dysfunction in kidney pathophysiology

“…Electron microscopic analyses of proteinuric kidneys reveal fully loaded lysosomes, cytoplasmic protein droplets, and/or crystalline protein inclusions, indicating that the capacity of the lysosomal digestive function has been substantially exceeded [1]. In proteinuric rats, cathepsin B and L synthesis rates increase, possibly as an adoptive mechanism, which may in part compromise the overloaded state [78]. It is possible, however, that such activation may have an adverse effect due to uncontrolled digestion of molecules important for cell phenotype and homeostasis.…”