A high-molecular-mass neutral endopeptidase-24.5 from human lung

Physiological Overview of the Potential Link between the UPS and Ca2+ Signaling

The ubiquitin–proteasome system (UPS) is the main proteolytic pathway by which damaged target proteins are degraded after ubiquitination and the recruit of ubiquitinated proteins, thus regulating diverse physiological functions and the maintenance in various tissues and cells. Ca²⁺ signaling is raised by oxidative or ER stress. Although the basic function of the UPS has been extensively elucidated and has been continued to define its mechanism, the precise relationship between the UPS and Ca²⁺ signaling remains unclear. In the present review, we describe the relationship between the UPS and Ca²⁺ signaling, including Ca²⁺-associated proteins, to understand the end point of oxidative stress. The UPS modulates Ca²⁺ signaling via the degradation of Ca²⁺-related proteins, including Ca²⁺ channels and transporters. Conversely, the modulation of UPS is driven by increases in the intracellular Ca²⁺ concentration. The multifaceted relationship between the UPS and Ca²⁺ plays critical roles in different tissue systems. Thus, we highlight the potential crosstalk between the UPS and Ca²⁺ signaling by providing an overview of the UPS in different organ systems and illuminating the relationship between the UPS and autophagy.

Combinatorial inhibition of Angiotensin converting enzyme, Neutral endopeptidase and Aminopeptidase N by N-methylated peptides alleviates blood pressure and fibrosis in rat model of dexamethasone-induced hypertension

Angiotensin converting enzyme (ACE), neutral endopeptidase (NEP) and aminopeptidase N (APN) are responsible for generation of vasoactive peptides that regulates vasoconstriction, vasodilation and natriuresis, which altogether regulate blood pressure. Cumulative inhibition of ACE, NEP and APN effectively blocks the progression of respective pathways. In this study, N-methylated peptide inhibitors F-N(Me)H-L, V-N(Me)F-R and R-N(Me)V-Y were synthesized against ACE, NEP and APN respectively, using their respective physiological substrates. F-N(Me)H-L inhibited ACE activity with an IC₅₀ of 83 nmol/L, V-N(Me)F-R inhibited NEP activity with an IC₅₀ of 1.173 μmol/L and R-N(Me)V-Y inhibited APN activity with an IC₅₀ of 3.94 nmol/L respectively. Further, the anti-hypertensive effect of N-methylated peptides was evaluated using rat model of dexamethasone-induced hypertension. Individual peptides and their cocktail treatment were started from day 6 of the study period and blood pressure was measured on every alternate day during 15 day study. Administration of F-N(Me)H-L (138 ± 3 mmHg) and cocktail of all the three peptides at a dose of 100 mg/kg significantly reduced systolic blood pressure (SBP) compared to dexamethasone group (SBP of Groups-dexamethasone; (167 ± 5 mmHg), F-N(Me)H-L (138 ± 3 mmHg), and Cocktail (122 ± 3 mmHg). Anti-hypertensive, anti-hypertrophic and anti-fibrotic effects of N-methylated peptides and cocktail was further reflected by the decreased levels of circulating Ang II and increased ANP levels in sera of hypertensive rats along with decrease in collagen deposition in heart and kidney. Though, ACE inhibition is adequate to reduce SBP, targeting NEP and APN along with ACE is beneficial in tackling hypertension and associated fibrosis of heart.

Differential effects of detergents, fatty acids, cations and heating on ostrich skeletal muscle 20S proteasome

The 20S proteasome, the catalytic core of the 26S proteasome, has previously been isolated, purified and partially characterised from ostrich skeletal muscle (Thomas, A.R., Oosthuizen, V., Naude, R.J., Muramoto, K. 2002. Biol. Chem. 383, 1267-1270). Due to the apparent latency of the 20S proteasome purified from various sources, this study focuses on further characterising the ostrich enzyme in terms of the effects of selected detergents, fatty acids and cations, as well as heating at 60 degrees C, on four of its activities. Results showed that ostrich skeletal muscle 20S proteasome was affected in a non-concentration-dependent manner by the selected detergents and fatty acids. Monounsaturated fatty acids, unlike unsaturated fatty acids, showed no major effects on the activities of the ostrich enzyme. The enzyme did not show sensitivity towards monovalent cations and the only divalent cations that showed a relevant effect were Ca2+ and Mg2+. Heating at 60 degrees C for 1-2 min had a substantial activating effect only on the peptidylglutamylpeptide-hydrolase (PGPH) and caseinolytic activities. In conclusion, many of the effects by the abovementioned reagents and conditions were noticeably different to those shown on different sources of the enzyme, further demonstrating the unique kinetic characteristics of the ostrich skeletal muscle 20S proteasome.

Proteasome inhibitors: complex tools for a complex enzyme

As the dominant protease dedicated to protein turnover, the proteasome shapes the cellular protein repertoire. Our knowledge of proteasome regulation and activity has improved considerably over the past decade. Novel inhibitors, in particular, have helped to advance our understanding of proteasome biology. They range from small peptide-based structures that can be modified to vary target specificity, to large macromolecular inhibitors that include proteins. While these reagents have played an important role in establishing our current knowledge of the proteasome's catalytic mechanism, many questions remain. Rapid advances in the synthesis and identification of new classes of proteasome inhibitors over the last 10 years serve as a positive indicator that many of these questions will soon be resolved. The future lies in designing compounds that can function as drugs to target processes involved in disease progression. It may only be a short while before the products of such research have safe application in a practical setting. Structural and combinatorial chemistry approaches are powerful techniques that will bring us closer to these goals.

Purification and characterization of a latent form of multicatalytic proteinase from fish muscle

1. A latent form of multicatalytic proteinase (MCP) was purified to apparent homogeneity from white croaker muscle by DEAE-Sephacel, Mono-Q, Sephacryl S-300 and second Mono-Q chromatographies. 2. The enzyme preparation was electrophoretically and immunologically similar to MCP purified from the same source by a different method (Folco et al., 1988b, Archs Biochem. Biophys. 267, 599-605) but showed much lower chymotrypsin- and trypsin-like activities. 3. These activities responded to sodium dodecyl sulphate (SDS), urea and heat treatments in different ways: SDS stimulated both activities, urea stimulated the former and inhibited the latter and heating stimulated the former and did not affect the latter. 4. The stimulation of chymotrypsin-like activity by the three treatments was irreversible. 5. Exposure of MCP to SDS or urea in the absence of substrate rapidly inactivated it, whereas heat activation took place irrespective of the presence of substrate. 6. The stimulating effect of SDS on chymotrypsin-like activity was lost in the presence of urea. 7. These results suggest that the enzyme may be activated by different mechanisms.

Sodium dodecyl sulfate and heat induce two distinct forms of lobster muscle multicatalytic proteinase: the heat-activated form degrades myofibrillar proteins

A multicatalytic proteinase (MCP) purified from lobster claw and abdominal muscles degrades a variety of peptide and protein substrates. The enzyme is activated by low concentrations (0.03%) of sodium dodecyl sulfate (SDS) and brief (1 min) heating at 60 degrees C. The lobster MCP can assume three stable and functionally distinct states in vitro; these are classified as the basal, heat-activated, and SDS-activated forms. The basal MCP possessed high trypsin-like peptidase activity and low chymotrypsin-like peptidase, peptidylglutamyl-peptide hydrolase, and caseinolytic activities; incubation of the basal form with SDS stimulated the peptidylglutamyl-hydrolase activity about 30-fold and inhibited the other three activities 80% to 100%. Heating the basal form stimulated caseinolytic activity about 6-fold with little effect on the peptidase activities. The heat-activated enzyme also degraded myosin, tropomyosin, troponin, and actin depolymerizing factor; alpha-actinin was resistant to proteolysis. Incubation of the heat-activated MCP with SDS inhibited the trypsin-like, chymotrypsin-like, and proteinase activities 95 to 100% and stimulated the peptidylglutamyl-hydrolase activity about 16-fold. Incubation of myosin with either the basal or the heat-activated forms in the presence of SDS generated identical proteolytic fragments of the myosin heavy chain, suggesting that SDS induced a third form that can be produced from either the basal or the heat-activated forms. The heat-activated form produced proteolytic fragments of myosin heavy chain different from those generated by either basal or heat-activated enzymes in the presence of SDS. Furthermore, 100 mM KCl stimulated the caseinolytic activity of the heat-activated form 24% and inhibited the trypsin-like and peptidylglutamyl-hydrolase activities 56 and 20%, respectively. These results, though indirect, suggest that heating induced a proteinase activity that was distinct from the three peptidase activities. Activation of the basal form with SDS was reversible, since precipitation of dodecyl sulfate with 100 mM KCl restored trypsin-like activity and inhibited peptidylglutamyl-hydrolase activity. In contrast, removal of dodecyl sulfate from the SDS-activated form that was derived from the heat-activated MCP induced its conversion to the basal form. Thus, although heat-activation was irreversible, the heat-activated form was converted back to the basal form via the SDS-activated form.

Covalent labelling of bovine lens multicatalytic proteinase complex with [3H]di-isopropyl fluorophosphate

Enzymatically active lens multicatalytic proteinase complex bound [3H]iPr2P-F after incubation for 3 hours at ambient temperature. Label was associated with the lowest molecular weight band (Mr 22,000) on sodium dodecyl sulfate polyacrylamide gels. This binding was inhibited by preincubation of the enzyme with the cysteine-directed reagent, p-chloromercuribenzoate, which inhibits all three hydrolytic activities of the enzyme. Leupeptin, which inhibits the arginyl-hydrolyzing component, but not the iPr2P-F-inhibitable leucyl-hydrolyzing component of the enzyme, does not inhibit [3H]iPr2P-F binding. These data suggest that the leucy-hydrolyzing component of the lens multicatalytic proteinase complex is localized to the 22,000 Mr subunit and is a member of the thiol-dependent subclass of serine proteinases.

Isolation of two high-molecular-mass proteinases from human erythrocytes

Two forms of a neutral--alkaline high-molecular-mass proteinase (termed A1 and A2) have been purified from human erythrocytes by a procedure including a DEAE-cellulose batchwise treatment of erythrocyte cytosol, gel filtration and DEAE-cellulose chromatography. Both enzymes show distinctive properties of multicatalytic proteinases. They have an apparent molecular mass of 700 kDa and are composed by eight major subunits (23-32 kDa). Both enzymes show a proteinase activity towards casein and hydrolyze synthetic peptides with tyrosine, arginine or lysine at the P1 position. Among the synthetic peptides tested, the tetrapeptide succinyl-leucyl-leucyl-valyl-tyrosyl-7-amido-4-methylcoumarin and tripeptides with arginine in the P1 position (benzyloxycarbonyl-valyl-leucyl-arginyl-4-methoxy-2-naphthylamide and benzyloxycarbonyl-alanyl-arginyl-arginyl-4-methoxy-2-naphthylamide) are the most effective substrates. The proteinases are devoid of amino and diaminopeptidase activity. Both enzymes are completely inhibited by hemin, chymostatin and thiol-group reagents. However, the enzymes can be distinguished by the isoelectric point, the different effect of nucleotides, glutathione disulphide, sodium dodecyl sulfate and cations on the catalytic activity.

Characterization of the active site of human multicatalytic proteinase

The activity of multicatalytic proteinase against synthetic substrates and the kinetics of its inhibition by a range of class-specific inhibitors have been investigated. The enzyme was found to have a broader pH activity profile than previously noted, being active against succinyl-Ala-Ala-Phe-7-amino-4-methylcoumarin optimally at pH 4.5 and against benzyloxycarbonyl-Gly-Gly-Arg-7-amino-4-methylcoumarin optimally at pH 10.5. Neither activity was inhibited by the class-specific inhibitors 1,10-phenanthroline, EDTA, pepstatin, di-isopropyl fluorophosphate, peptidyl chloromethanes, peptidyl diazomethanes or L-3-carboxy-2,3-trans-epoxypropionyl-leucylamido-(4-guanidin o)butane (E-64), indicating that the enzyme is not a typical metallo-, aspartic, serine or cysteine proteinase. Inhibition by HgCl2, iodoacetamide and N-ethylmaleimide suggests that free thiols are necessary for the enzyme to maintain activity, but that these thiols are not particularly reactive as is the case for cysteine proteinases of the papain superfamily. The peptidyl aldehydes chymostatin and leupeptin were found to be reversible inhibitors of multicatalytic proteinase. Chymostatin inhibited activity against succinyl-Ala-Ala-Phe-7-amino-4-methylcoumarin at pH 4.5 (Ki 160 +/- 22 microM) whereas leupeptin (200 microM) was not inhibitory. Inhibition of activity against benzyloxycarbonyl-Gly-Gly-Arg-7-amino-4-methylcoumarin by these compounds was more complex, in that they behaved as slow tight-binding inhibitors. kon values were determined to be 12 +/- 2 M-1.s-1 and 1290 +/- 125 M-1.s-1 for chymostatin and leupeptin, respectively. The upper limit for Ki values for these two inhibitors was estimated as 5 +/- 1.5 microM and 25 +/- 5 nM, respectively. The different inhibition characteristics for each substrate were also apparent at an intermediate pH of 8.5, showing that the two activities are distinct. Dichloroisocoumarin, a mechanism-based inhibitor of serine proteinases, did inhibit activity against succinyl-Ala-Ala-Phe-7-amino-4-methylcoumarin with a rate constant of 250 M-1.s-1, suggesting that multicatalytic proteinase is an atypical serine proteinase.

Common epitopes of bovine lens multicatalytic-proteinase-complex subunits

Component polypeptides of both the bovine lens and pituitary multicatalytic proteinase complexes demonstrate different immunoreactivities with a polyclonal antiserum raised against the purified pituitary enzyme. Four (Mr 24000, 26000, 34000 and 38000) of eight bands that have been resolved by SDS/polyacrylamide-gel electrophoresis are stained in immunoblot experiments. Monospecific antibodies obtained from this antiserum by affinity purification from the 38000- and 34000-Mr bands of the lens enzyme bound equally well to either band, but showed little or no binding to the 26000- and 24000-Mr bands upon immunoblotting. Antibody affinity-purified from the 24000-Mr band showed comparable binding to the 24000-, 34000- or 38000-Mr band. One explanation of these results is that the 24000-Mr polypeptide is derived from the higher-Mr polypeptide(s) and has lost some of the common immunodeterminants.

The multicatalytic proteinase of mammalian cells

A high-molecular-weight nonlysosomal proteinase has recently been discovered in mammalian cells. It is a widely distributed and abundant enzyme which has attracted attention because of its complex multisubunit structure and its unusual catalytic properties. The 700-kDa proteinase is composed of many different types of low-molecular-weight subunits (Mr 21,000-34,000) arranged in a hollow cylindrical structure. This 20 S complex is very similar, if not identical, to the 19-20 S cylindrical particles, ring-type particles, or prosomes which have been isolated from several different types of eukaryotic cells. The proteinase appears to have at least two distinct catalytic sites and can cleave bonds on the carboxyl side of basic, hydrophobic, or acidic amino acid residues. Inhibition of proteinase activity by thiol reagents supports the suggestion that the enzyme is a cysteine proteinase but there is some evidence that it may be a serine proteinase and the catalytic mechanism is at present unknown. ATP has little effect on proteinase activity in most purified preparations but recently the proteinase has been implicated in ATP-dependent pathways of protein degradation. Ther is a second type of high-molecular-weight complex multisubunit proteinase, a 26 S particle, which catalyzes the ATP-dependent degradation of ubiquitin-protein conjugates. The precise function of these two complex proteinases in intracellular proteolysis remains to be elucidated.

Proteinase yscE of yeast shows homology with the 20 S cylinder particles of Xenopus laevis

Proteinase yscE of the yeast Saccharomyces cerevisiae has been compared with the 20 S cylinder particles of Xenopus laevis. Both proteins are characterized by a similar group of 10-12 polypeptides with molecular masses ranging between 21 and 38 kDa. Antibodies generated against the 20 S Xenopus cylinder particles show cross-reactivity with yeast proteinase yscE subunits. The Xenopus particles and yeast proteinase yscE exhibit an identical image in electron microscopy. Both proteins appear as hollow cylinders mostly composed of four stacked annuli. The Xenopus 20 S particles exhibit proteolytic activity against the three peptide derivatives known to be substrates of proteinase yscE. The pH optimum for activity and the inhibition spectrum of the proteolytic activities of Xenopus 20 S particles and of yeast proteinase yscE are identical. The RNA content of the cylinder particles and of proteinase yscE is below 0.1 RNA chain per molecule. Our data suggest that proteinase yscE from yeast and the 20 S cylinder particles of X. laevis are homologous, highly conserved proteins carrying the catalytic character of a peptidase.

Drosophila small cytoplasmic 19S ribonucleoprotein is homologous to the rat multicatalytic proteinase

All eukaryotic cells so far analysed contain 19S particles which share a cylinder-like shape and are composed of a set of proteins of relative molecular mass ranging typically from 19,000 to 36,000 (refs 1-10). Proposed functions have included synthetase activity, transfer RNA processing or messenger RNA repression, but their biological importance remains obscure. A multicatalytic proteinase (MCP) of similar size and shape has been isolated from mammalian tissues. The apparent similarities of these high molecular weight complexes suggest a biochemical and functional homology between the small cytoplasmic 19S particle from Drosophila melanogaster (19S-scRNP) (ref. 7) and rat MCP (ref. 14). By means of electron microscopy, immunological techniques, RNA identification and proteinase activity assays, we were able to show that the two structurally similar complexes are immunologically related ribonucleoproteins (RNPs) with similar proteolytic activity.

Lens neutral endopeptidase occurs in other bovine and human tissues

Lens neutral endopeptidase (EC 3.4.24.5) was previously thought to be unique to the eye lens. We report here the finding of a neutral endopeptidase, in a variety of bovine and human tissues, which is very similar both biochemically and immunologically to the lens endopeptidase. SDS/polyacrylamide-gel electrophoresis of partially purified enzyme fractions from various bovine tissues shows the characteristic pattern of at least eight bands with Mr values ranging from 24,000 to 32,000 which was described for the bovine-lens neutral endopeptidase. The relative activity of the enzyme varies from tissue to tissue with lung having the highest activity. Partially purified enzyme fractions from these tissues cross-react with antiserum raised in rabbit against bovine lens endopeptidase showing apparent identity when examined side by side in Ouchterlony double-diffusion tests. The human enzyme also cross-reacts with the antiserum but when tested by double-diffusion against the bovine enzyme the precipitin lines show spurring at the joining edges indicating a structural difference between the human and the bovine enzymes. It was also found by Western blot experiments, after denaturing polyacrylamide-gel electrophoresis of the enzyme, that the polypeptide components of the human and bovine enzymes show somewhat different banding patterns.

A multicatalytic high-molecular-weight neutral endopeptidase from human kidney

A multicatalytic endopeptidase (ME) with three distinct activities, chymotrypsin-like, cucumisin-like, and trypsin-like, occurred in all rat tissues examined with highest activities in kidney, testes, liver, and spleen; they were assayed with benzyloxycarbonyl-Gly-Gly-Leu-p-nitroanilide (Z-Gly-Gly-Leu-pNA), benzyloxycarbonyl-Leu-Leu-Glu-2-naphthylamide (Z-Leu-Leu-Glu-2NA), and benzyloxycarbonyl-Gly-Gly-Arg-2-naphthylamide (Z-Gly-Gly-Arg-2NA), respectively. All three activities were recovered from a single protein band on a polyacrylamide gel after electrophoresis of purified human kidney ME. The native enzyme had a Mr of 650,000, and it consisted of about 5,135 amino acid residues. After denaturation and electrophoresis on sodium dodecyl sulfate (SDS)-polyacrylamide gels kidney ME dissociated into several low Mr components ranging from 23,000 to 33,000. Kidney ME had a pH optimum of 7.6-8.1 with Z-Gly-Gly-Leu-pNA, 7.3 with Z-Leu-Leu-Glu-2NA, and 9.8 with Z-Gly-Gly-Arg-2NA. SDS enhanced chymotrypsin- and cucumisin-like activities by two to three times whereas trypsin-like activity was not enhanced. The specificity constant (kappa cat/Km) of human kidney ME for Z-Gly-Gly-Leu-pNA was 6.7 X 10(3) M-1 S-1; Z-Gly-Gly-Leu-2NA was not hydrolyzed. The specificity constant for Z-Leu-Leu-Glu-2NA was similar to, and for Z-Gly-Gly-Arg-2NA was one half of that for Z-Gly-Gly-Leu-pNA. ME cleaves only the Phe5-Ser6 bond of bradykinin (BK); however, all three ME activities were inhibited by BK. Strong inhibition of ME by albumin suggests that ME is involved in cleavage of larger polypeptides. Antipain and leupeptin almost completely inactivated the trypsin-like activity whereas they had no significant effect on the other two activities. ME is not a metal-loenzyme nor is the serine residue essential for its activities; however, thiol groups are involved. Na+ and K+ inhibited all ME activities. Trypsin-like activity was more sensitive to divalent cations than the other two.