Role of N-glycosylation in cathepsin E. A comparative study of cathepsin E with distinct N-linked oligosaccharides and its nonglycosylated mutant.

Cathepsin E (CE), a nonlysosomal, intracellular aspartic proteinase, exists in several molecular forms that are N-glycosylated with high-mannose and/or complex-type oligosaccharides. To investigate the role of N-glycosylation on the catalytic properties and molecular stability of CE, both natural and recombinant enzymes with distinct oligosaccharides were purified from different sources. An N-glycosylation minus mutant, that was constructed by site-directed mutagenesis (by changing asparagine residues to glutamine and aspartic acid residues at positions 73 and 305 in potential N-glycosylation sites of rat CE) and expressed in normal rat kidney cells, was also purified to homogeneity from the cell extracts. The kinetic parameters of the nonglycosylated mutant were found to be essentially equivalent to those of natural enzymes N-glycosylated with either high-mannose or complex-type oligosaccharides. In contrast, the nonglycosylated mutant showed lower pH and thermal stabilities than the glycosylated enzymes. The nonglycosylated mutant exhibited particular sensitivity to conversion to a monomeric form by 2-mercaptoethanol, as compared with those of the glycosylated enzymes. Further, the high-mannose-type enzymes were more sensitive to this agent than the complex-type proteins. A striking difference was found between the high-mannose and complex-type enzymes in terms of activation by ATP at a weakly acidic pH. At pH 5.5, the complex-type enzymes were stabilized by ATP to be restored to the virtual activity, whereas the high-mannose-type enzymes as well as the nonglycosylated mutant were not affected by ATP. These results suggest that N-glycosylation in CE is important for the maintenance of its proper folding upon changes in temperature, pH and redox state, and that the complex-type oligosaccharides contribute to the completion of the tertiary structure to maintain its active conformation in the weakly acidic pH environments.

Identification of cellular compartments involved in processing of cathepsin E in primary cultures of rat microglia.

Cathepsin E is a major nonlysosomal, intracellular aspartic proteinase that localizes in various cellular compartments such as the plasma membrane, endosome-like organelles, and the endoplasmic reticulum (ER). To learn the segregation mechanisms of cathepsin E into its appropriate cellular destinations, the present studies were initiated to define the biosynthesis, processing, and intracellular localization as well as the site of proteolytic maturation of the enzyme in primary cultures of rat brain microglia. Immunohistochemical and immunoblot analyses revealed that cathepsin E was the most abundant in microglia among various brain cell types, where the enzyme existed predominantly as the mature enzyme. Immunoelectron microscopy studies showed the presence of the enzyme predominantly in the endosome-like vacuoles and partly in the vesicles located in the trans-Golgi area and the lumen of ER. In the primary cultured microglial cells labeled with [35S]methionine, >95% of labeled cathepsin E were represented by a 46-kDa polypeptide (reduced form) after a 30-min pulse. Most of it was proteolytically processed via a 44-kDa intermediate to a 42-kDa mature form within 4 h of chase. This processing was completely inhibited by bafilomycin A1, a specific inhibitor of vacuolar-type H+-ATPase. Brefeldin A, a blocker for the traffic of secretory proteins from the ER to the Golgi complex, also inhibited the processing of procathepsin E and enhanced its degradation. Procathepsin E, after pulse-labeling, showed complete susceptibility to endoglycosidase H, whereas the mature enzyme almost acquired resistance to endoglycosidases H as well as F. The present studies provide the first evidence that cathepsin E in microglia is first synthesized as the inactive precursor bearing high-mannose oligosaccharides and processed to the active mature enzyme with complex-type oligosaccharides via the intermediate form and that the final proteolytic maturation step occurs in endosome-like acidic compartments.

Age-related and dexamethasone-induced changes in cathepsins E and D in rat thymic and splenic cells.

Age-related and dexamethasone (DEX)-induced changes in the cellular levels, distributions, and molecular forms of two distinct intracellular aspartic proteinases, cathepsin E (CE) and cathepsin D (CD), were investigated in rat thymus and spleen by immunohistochemical and quantitative analyses. In the thymus, CE was predominantly restricted to thymocytes and macrophage-like cells, whereas CD was associated mainly with the stromal cells. The increased thymic CE level observed in young rats up to 8 weeks of age was markedly decreased in aged rats (78-80 weeks of age), in accordance with the involution of the thymus, while there was little difference in the thymic CD level between young and aged rats. Subcutaneous administration of DEX also caused a marked decrease of the thymic CE level in response to the depletion of thymocytes. In contrast, a great accumulation of CD occurred in the thymic stromal cells after DEX treatment. Immunoblotting analyses revealed that CE in thymocytes isolated from young rats consisted predominantly of a 46-kDa proform which was greatly converted into a 42-kDa mature form in DEX-treated thymocytes. This conversion, however, was scarcely observed during the normal aging process. In the spleen, CE was also abundant in macrophage-like cells and lymphocytes and its level was not significantly changed between young and aged rats. However, DEX treatment caused a marked decrease of the splenic CE and CD levels in accordance with the depletion of the white pulp. Among the lymphoid cell types examined, splenic B cells were the most abundant in CE. The CE level in thymocytes and splenic T-cells was more than twice that in circulating lymphocytes. We concluded that CE is related to the process of activation-induced lymphocyte depletion.

Hypocalcemic action of the several types of salicylic acid analogues.

The present study was performed to see the structure-activity relationships on the aspirin-induced hypocalcemia. Several kinds of salicylic acid (SA) analogues administered orally with a stomach tube. In general, the drugs were suspended in the 2% CMC solution. At the scheduled times after the treatment, 60 microliters of the blood was collected to determine the level of calcium. Aspirin, sodium salt of o-hydroxybenzoic acid (Na-salicylate), sodium salt of m- and p-hydroxybenzoic acid (HBA), 2,5-dihydroxybenzoic acid (DHBA), PAS sodium dihydrate (PAS-Na), salicylamide (SAM) and 2% CMC control were used. Hypocalcemia was induced by aspirin and Na-salicylate but not by m- and p-HBA-Na. In addition, DHBA and PAS caused hypocalcemia when they were administered intravenously but not orally. These results suggest that the carboxyl group must be adjacent to the hydroxyl group on the benzene ring to induce this type of hypocalcemia and that the SA structure would be able to induce hypocalcemia, even in the presence of the additional third substituent on the same ring. On the comparison between aspirin-DL lysine (water soluble aspirin) and SA-DL lysine, SA-DL lysine, which is not an inhibitor of PG synthetase, was more effective on the hypocalcemic action than ASP-DL lysine. The phenomenon was observed at the stage especially immediately after intravenous injection, when the acetyl group may be more responsible to acetylate the PG synthetase in the aspirin-DL lysine group. The present results seems to be consistent with the previous hypothesis that PGs are not involved in the process of aspirin-induced hypocalcemia in the rat.