Age-related and phenylhydrazine-induced activation of the membrane-associated cathepsin E in human erythrocytes

We examined the effects of cell aging and phenylhydrazine-induced oxidant damage on erythrocyte cathepsin E, which is present as a latent, membrane-associated enzyme in normal human erythrocytes. When young erythrocytes isolated from human mature erythrocytes by Percoll density gradient centrifugation were aged in vitro, the membrane-associated cathepsin E was progressively released from the membrane as an active enzyme. During the cell aging up to 100 h, about 40% of the membrane-associated enzyme was activated and solubilized. When phenylhydrazine was incubated with the erythrocytes, it also caused the activation and solubilization of cathepsin E in a dose-dependent and time-dependent manner. Exposure of erythrocytes to 2.5 mM phenylhydrazine for up to 2 h led to about 40% activation of the membrane-associated enzyme. Both aging and phenylhydrazine-treatment were accompanied with an increase in the association of the cytosolic proteins, primarily hemoglobin, with the membrane, which occurred prior to the release of cathepsin E from the membrane. A similar activation for the membrane-associated enzyme was observed with in vitro-aged hemoglobin-free membrane ghosts. Thus, the primary mechanism for activation of cathepsin E in the intact cells seems to be through lesion of the membrane framework that results from increased binding of hemoglobin to the membrane. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotting employing polyclonal IgG antibodies for human spectrin and band 3 revealed that breakdown of the membrane proteins was enhanced in both aged and phenylhydrazine-treated cells. The relation between the cathepsin E activation and the membrane protein breakdown is discussed.     

Differential perturbation of erythrocyte membrane-associated transport and enzyme activities by structurally related lipophilic compounds

The alteration of two erythrocyte plasma membrane functions, acetylcholine hydrolysis and glucose exchange, by a series of structurally related small lipophilic compounds which exhibit similar antihemolytic behavior was studied. 2-Methyldimethylaminoazobenzene is a more potent inhibitor of acetylcholinesterase than the 3′-methyl analogue, while the unsubstituted compound fails to inhibit. Esterase inhibition by the 2-methyl compound is noncompetitive and dependent on the anion composition of the assay buffer. The temperature dependence of acetylcholinesterase activity in the presence of the 2-methyl compound suggests that interaction with inhibitor is influenced by the state of lipids tightly bound to the enzyme. Glucose exchange is inhibited to the same extent by both methyl derivatives but not by the unsubstituted dye, and the temperature dependence in the presence of inhibitor is not grossly altered. The lack of correlation between inhibition of membrane function and stabilization of erythrocytes against osmotic hemolysis is discussed.     

Mapping the triphosphatase active site of baculovirus mRNA capping enzyme LEF4 and evidence for a two-metal mechanism

The 464-amino acid baculovirus LEF4 protein is a bifunctional mRNA capping enzyme with triphosphatase and guanylyltransferase activities. The N-terminal half of LEF4 constitutes an autonomous triphosphatase catalytic domain. The LEF4 triphosphatase belongs to a family of metal-dependent phosphohydrolases, which includes the RNA triphosphatases of fungi, protozoa, Chlorella virus and poxviruses. The family is defined by two glutamate-containing motifs (A and C), which form a metal-binding site. Most of the family members resemble the fungal and Chlorella virus enzymes, which have a complex active site located within the hydrophilic interior of a topologically closed eight stranded β barrel (the so-called ‘triphosphate tunnel’). Here we probed whether baculovirus LEF4 is a member of the tunnel subfamily, via mutational mapping of amino acids required for triphosphatase activity. We identified four new essential side chains in LEF4 via alanine scanning and illuminated structure–activity relationships by conservative substitutions. Our results, together with previous mutational data, highlight five acidic and four basic amino acids that are likely to comprise the LEF4 triphosphatase active site (Glu9, Glu11, Arg51, Arg53, Glu97, Lys126, Arg179, Glu181 and Glu183). These nine essential residues are conserved in LEF4 orthologs from all strains of baculoviruses. We discerned no pattern of clustering of the catalytic residues of the baculovirus triphosphatase that would suggest structural similarity to the tunnel proteins (exclusive of motifs A and C). However, there is similarity to the active site of vaccinia RNA triphosphatase. We infer that the baculovirus and poxvirus triphosphatases are a distinct lineage within the metal-dependent RNA triphosphatase family. Synergistic activation of the LEF4 triphosphatase by manganese and magnesium suggests a two-metal mechanism of γ phosphate hydrolysis.     

Essential 110Cys in active site of membrane-associated prostaglandin E synthase-2

The amino acid sequence of membrane-associated prostaglandin (PG) E synthase-2 (mPGE synthase-2), which has a broad specificity in its thiol requirement for a catalytic activity, has the consensus region from 104Leu to 120Leu found in glutaredoxin and of thioredoxin. The sequence of Cys-x-x-Cys in the consensus region is the active site for thioredoxin and mPGE synthase-2 also has this amino acid sequence (110Cys-x-x-113Cys). The mutation from 110Cys to Ser or the double mutation from 110Cys and 113Cys to Ser caused loss of PGE synthase activity, whereas the single mutation from 113Cys to Ser did not affect the enzyme activity. These results indicate that 110Cys, but not 113Cys, is the essential amino acid in the active site of mPGE synthase-2. 110Cys is an important amino acid in PGE synthase activity and plays the critical role as Cys at the same position in redoxin. Moreover, we found that the reduced form of lipoic acid (dihydrolipoic acid) serves as one of the natural activators of mPGE synthase-2 in the cells.     

Computer-aided molecular modeling of cathepsin E,a possible endothelin-converting enzyme

A three-dimensional model of human cathepsin E, a possible endothelin-converting enzyme, is constructed using computer-aided molecular modeling techniques. The structure of porcine pepsin, another aspartic protease, was used as a template. The final structure, after all gaps and deletions were made, was optimized using the AMBER-4 package. A dipeptide (Trp-Val) representing the substrate was docked in the putative active site and the whole structure was optimized after several runs of minimization and dynamics calculations. The result of this modeling study showed that the structure of cathepsin E is similar to that of porcine pepsin and has three disulfide bonds that are conserved in both enzymes. There are two Asp-Thr-Gly sequences at the active site of enzyme. The active site cavity is large enough to accommodate its substrate.     

Activation of prothrombin Barcelona evidence for active high molecular weight intermediates

Prothrombin Barcelona is a varient of human prothrombin. Its activation by factor Xa is characterized by two abnormal features. The first one is the absence of one of the two Xa-catalyzed cleavages, and the second one the generation of a thrombin-like activity responsible for the thrombin-catalyzed cleavages on the prothrombin molecule and for the activity toward small substrates but without clotting activity.The molecular species exhibiting the thrombin-like activity have been characterized Diisopropyl phosphofluoridate incorporation experiments show that upon activation, two high molecular weight intermediates bearing an active site are formed: one has the molecular weight of prothrombin, the other of prethrombin 1. In the presence of hirudin, only the first intermediate is formed due to a single cleavage by factor Xa which is therefore responsible for the unmasking of the active site.     

Exocytosis of active cathepsin B enzyme activity at pH 7.0, inhibition and molecular mass.

Lysosomal cathepsin B has been implicated in parasitic, inflammatory and neoplastic diseases. Most of these pathologies suggest a role for cathepsin B outside the cells, although the origin of extracellular active enzyme is not well defined. The activity of extracellular cathepsin B is difficult to assess because of the presence of inhibitors and inactivation of the enzyme by oxidizing agents. Therefore, we have developed a continuous assay for measurement of cathepsin B activity produced pericellularly by living cells. The kinetic rate of Z-Arg-Arg-NHMec conversion was monitored and the assay optimized for enzyme stability, cell viability and sensitivity. To validate the assay, we determined that human liver cathepsin B was stable and active under the conditions of the assay and its activity could be inhibited by the selective epoxide derivative CA-074. Via this assay, we were able to demonstrate that active cathepsin B was secreted pericellularly by viable cells. Both preneoplastic and malignant cells secreted active cathepsin B. Pretreatment of cells with the membrane-permeant proinhibitor CA-074Me completely abolished pericellular and total cathepsin B activity whereas pretreatment with the active drug CA-074 had no effect. Immunoprecipitation and immunoblotting experiments suggested that the active enzyme species was 31-kDa single-chain cathepsin B. Exocytosis of cathepsin B was not related to secretion of proenzyme or secretion from mature lysosomes. Our results suggest an alternative pathway for exocytosis of active cathepsin B.     

Purification and characterization of putative endothelin converting enzyme in bovine adrenal medulla: evidence for a cathepsin D-like enzyme

A specific and sensitive assay has been established for measurement of endothelin converting activity in a tissue extract. This assay is based on measuring endothelin-1 generated from big endothelin-1 by endothelin converting enzyme (ECE) with radioimmunoassay using an endothelin C-terminal specific antibody. By using this assay, we purified and characterized ECE in bovine adrenomedullary chromaffin granules ECE was purified over 3,000 times by a combination of DEAE, hydrophobic and gel filtration chromatography. A molecular weight of ECE was estimated to be approximately 30,000 by gel filtration. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed that ECE had three major components with estimated molecular weights of 45,000, 30,000 and 15,000 like bovine spleen cathepsin D. ECE had a pH optimum at 3.5 and was inhibited by pepstatin. These results strongly suggest that ECE is a cathepsin D-like aspartic protease.     

Exploring the binding preferences/specificity in the active site of human cathepsin E

Aspartic proteinases are produced in the human body by a variety of cells. Some of these proteins, examples of which are pepsin, gastricsin, and renin, are secreted and exert their effects in the extracellular spaces. Cathepsin D and cathepsin E on the other hand are intracellular enzymes. The least characterized of the human aspartic proteinases is cathepsin E. Presented here are results of studies designed to characterize the binding specificities in the active site of human cathepsin E with comparison to othermechanistically similar enzymes. A peptide series based on Lys-Pro-Ala-Lys-Phe*Nph-Arg-Leu was generatedto elucidate the specificity in the individual binding pockets with systematic substitutions in the P_5? P₂ and P₂′-P₃′ based on charge, hydrophobicity, and hydrogen bonding. Also, to explore the S₂ binding preferences, asecond series of peptides based on Lys-Pro-Ile-Glu-Phe*Nph-Arg-Leu was generated with systematic replacements in the P₂ position. Kinetic parameters were determined forboth sets of peptides. The results were correlated to a rule-based structural model of human cathepsin E, constructed on the known three-dimensional structures of several highly homologous aspartic proteinases; porcine pepsin, bovine chymosin, yeast proteinase A, human cathepsin D, andmouse and human renin. Important specificity-determining interactions were found in the S₃ (Glu13) and S₂ (Thr-222, Gln-287, Leu-289, Ile-300)subsites. © 1995 Wiley-Liss, Inc.     

Structural evidence for two isozymic forms and the carbohydrate attachment site of human gastric cathepsin E

The amino acid sequences in the NH2-terminal region and some other parts of human gastric cathepsin E were investigated. The NH2-terminal sequencing revealed that the cathepsin E preparation which had been activated at pH 4.0 contained one major and one minor isozymes in an approximate molar ratio of 3:1. The NH2-terminal sequence of the former was very similar to but partly different from that predicted from cDNA sequencing by Azuma et al., whereas the latter had an NH2-terminal sequence identical with the predicted sequence. These results provide structural evidence for the presence of at least two isozymic forms in human gastric cathepsin E. In addition, the site of carbohydrate attachment was elucidated by isolation and analysis of a glycopeptide fraction from an enzymatic digest of cathepsin E. A single carbohydrate chain was deduced to be attached to the asparagine residue at position 34 in the major isozyme and to the corresponding asparagine residue in the minor isozyme.     

Peptide utilization in Pseudomonas aeruginosa: evidence for membrane-associated peptidase.

A methionine auxotroph of Pseudomonas aeruginosa grew on methionine-containing peptides as a source of the required amino acid. Amino-terminus-blocked peptides would not serve as growth substrates, despite the fact that peptidases active on these blocked peptides were readily detectable in cell extracts. No evidence was found for extracellular enzymes capable of degrading the oligopeptides investigated. The degradative enzymes were not found in the periplasmic space of the cellular envelope. A high proportion of cellular peptidase activity was associated with the particulate (membrane) fraction of the cell lysate.     

Specific immunocytochemical localization of cathepsin E at the ruffled border membrane of active osteoclasts

The immunocytochemical localization of cathepsin E, a non-lysosomal aspartic proteinase, was investigated in rat osteoclasts using the monospecific antibody to this protein. At the light-microscopic level, the preferential immunoreactivity for cathepsin E was found at high levels in active osteoclasts in the physiological bone modeling process. Neighboring osteoblastic cells were devoid of its immunoreactivity. At the electron-microscopic level, cathepsin E was exclusively confined to the apical plasma membrane at the ruffled border of active osteoclasts and the eroded bone surface. Cathepsin E was also concentrated in some endocytotic vacuoles of various sizes in the vicinity of the ruffled border membrane, some of which appeared to be secondary lysosomes containing the phagocytosed materials. These results strongly suggest that this enzyme is involved both in the extracellular degradation of the bone organic matrix and in the intracellular breakdown of the ingested substances in osteoclasts.     

Activation of cathepsin D by glycine ethyl ester.

Cathepsin D purified from bovine spleen is activated by glycine ethyl ester. A maximum of 70% activation was observed at a glycine ethyl ester concentration of 0.1 M and at pH 4.5. The activation effect appears to be reversible.     

Characterization of new fluorogenic substrates for the rapid and sensitive assay of cathepsin E and cathepsin D.

Cathepsin E and cathepsin D are two major intracellular aspartic proteinases implicated in the physiological and pathological degradation of intra- and extracellular proteins. In this study, we designed and constructed highly sensitive synthetic decapeptide substrates for assays of cathepsins E and D based on the known sequence specificities of their cleavage sites. These substrates contain a highly fluorescent (7-methoxycoumarin-4-yl)acetyl (MOCAc) moiety and a quenching 2,4-dinitrophenyl (Dnp) group. When the Phe-Phe bond is cleaved, the fluorescence at an excitation wavelength of 328 nm and emission wavelength of 393 increases due to diminished quenching resulting from the separation of the fluorescent and quenching moieties. The first substrate, MOCAc-Gly-Lys-Pro-Ile-Leu-Phe-Phe-Arg-Le u-Lys(Dnp)gamma-NH2, in which the Lys-Pro combination at positions P5 and P4 was designed for specific interaction with cathepsin E, is hydrolyzed equally well by cathepsins E and D (kcat/Km = 10.9 microM(-1) x s(-1) for cathepsin E and 15.6 microM(-1) x s(-1) for cathepsin D). A very acidic pH optimum o was obtained for both enzymes. The second substrate, MOCAc-Gly-Lys-Pro-Ile-Ile-Phe-Phe-Arg-Le u-Lys(Dnp)gamma-NH2, in which the isoleucine residue at position P2 was meant to increase the specificity for cathepsin E, is also hydrolyzed equally by both enzymes (kcat/Km = 12.2 microM(-1) x s(-1) for cathepsin E and 16.3 microM(-1) x s(-1) for cathepsin D). The kcat/Km values for both substrates are greater than those for the best substrates for cathepsins E and D described so far. Unfortunately, each substrate shows little discrimination between cathepsin E and cathepsin D, suggesting that amino acids at positions far from the cleavage site are important for discrimination between the two enzymes. However, in combination with aspartic proteinase inhibitors, such as pepstatin A and Ascaris pepsin inhibitor, these substrates enable a rapid and sensitive determination of the precise levels of cathepsins E and D in crude cell extracts of various tissues and cells. Thus these substrates represent a potentially valuable tool for routine assays and for mechanistic studies on cathepsins E and D.     

Mutational analysis of residues in two consensus motifs in the active sites of cathepsin E

Cathepsin E, an intracellular aspartic proteinase of the pepsin family, is composed of two homologous domains, each containing the catalytic Asp residue in a consensus DTG motif. Here we examine the significance of residues in the motifs of rat cathepsin E by substitution of Asp98, Asp283, and Thr284 with other residues using site-directed mutagenesis. Each of the mutant proenzymes, as well as the wild-type protein, was found in culture media and cell extracts when heterologously expressed in human embryonic kidney 293T cells. The single mutants D98A, D283A, and D283E, and the double mutants D98A/D283A and D98E/D283E showed neither autocatalytic processing nor enzymatic activities under acidic conditions. However, the D98E and T284S mutants retained the ability to transform into the mature forms, although they exhibited only about 13 and 40% of the activity of the wild-type enzyme, respectively. The K(m) values of these two mutants were similar to those of the wild-type enzyme, but their k(cat) values were greatly decreased. The K(i) values for pepstatin and the Ascaris pepsin inhibitor of the mutants and the wild-type enzyme were almost the same. The circular dichroism spectra of the two mutants were essentially the same as those of the wild-type enzyme at various pH values. These results indicate that (i) Asp98, Asp283, and Thr284 are indeed critical for catalysis, and (ii) the decrease in the catalytic activity of D98E and T284S mutants is brought about by an effect on the kinetic process from the enzyme-substrate complex to the release of the product.     

Processing of the human heparanase precursor and evidence that the active enzyme is a heterodimer.

Human platelet heparanase has been purified to homogeneity and shown to consist of two, non-covalently associated polypeptide chains of molecular masses 50 and 8 kDa. Protein sequencing provided the basis for determination of the full-length cDNA for this novel protein. Based upon this information and results from protein analysis and mass spectrometry, we propose a scheme to define the structural organization of heparanase in relation to its precursor forms, proheparanase and pre-proheparanase. The 8- and 50-kDa chains which make up the active enzyme reside, respectively, at the NH(2)- and COOH-terminal regions of the inactive precursor, proheparanase. The heparanase heterodimer is produced by excision and loss of an internal linking segment. This paper is the first to suggest that human heparanase is a two-chain enzyme.