Investigation of the active center of rat pancreatic elastase

We have isolated rat pancreatic elastase I (EC 3.4.21.36) using a fast two-step procedure and we have investigated its active center with p-nitroanilide substrates and trifluoroacetylated inhibitors. These ligands were also used to probe porcine pancreatic elastase I whose amino acid sequence is 84% homologous to rat pancreatic elastase I as reported by MacDonald, et al. (Biochemistry 21, (1982) 1453-1463). Both proteinases exhibited non-Michaelian kinetics for substrates composed of three or four residues: substrate inhibition was observed for most enzyme substrate pairs, but with Ala3-p-nitroanilide, rat elastase showed substrate inhibition, whereas porcine elastase exhibited substrate activation. With most of the longer substrates, Michaelian kinetics were observed. The kcat/Km ratio was used to compare the catalytic efficiency of the two elastases on the different substrates. For both elastases, occupancy of subsite S4 was a prerequisite for efficient catalysis, occupancy of subsite S5 further increased the catalytic efficiency, P2 proline favored catalysis and P1 valine had an unfavorable effect. Rat elastase has probably one more subsite (S6) than its porcine counterpart. The rate-limiting step for the hydrolysis of N-succinyl-Ala3-p-nitroanilide by rat elastase was essentially acylation, whereas both acylation and deacylation rate constants participated in the turnover of this substrate by porcine elastase. For both enzymes, trifluoroacetylated peptides were much better inhibitors than acetylated peptides and trifluoroacetyldipeptide anilides were more potent than trifluoroacetyltripeptide anilides. A number of quantitative differences were found, however, and with one exception, trifluoroacetylated inhibitors were less efficient with rat elastase than with the porcine enzyme.     

Trifluoroacetyl tripeptides as potent inhibitors of human leukocyte elastase

Trifluoroacetylated peptides are much more potent inhibitors of human leukocyte elastase than the corresponding unblocked, acylated or benzyloxycarbonylated peptides. The most active compound was trifluoroacetyl-Val-Tyr-Val (Ki = 1.3 micron). A number of free and NH2-terminal-substituted peptides exhibited similar affinities for porcine pancreatic and human leukocyte elastase, indicating that these two enzymes must have similar specificity sites.     

Inhibition of human leukocyte elastase by acetyl and trifluoroacetyl oligopeptide chloromethyl ketones.

The action of human leukocyte elastase on a series of acetyl and trifluoroacetyl tri-, tetra-, and pentapeptide chloromethyl ketones has been investigated. Leukocyte and pancreatic elastases react quite differently with these irreversible inhibitors. For instance, leukocyte elastase has a much lower affinity for the compounds than pancreatic elastase. On the other hand, the inhibition rate constants of the two enzymes are not influenced in the same way by peptide chain elongation. The two elastases, however, share a common property: trifluoroacetyl tri- and tetraalanine chloromethyl ketones are more tightly bound but are less reactive than the corresponding acetylated inhibitors. This behavior is probably due to the formation of nonproductive complexes between the enzymes and the trifluoroacetylated inhibitors.     

Effect of plant Kunitz inhibitors from Bauhinia bauhinioides and Bauhinia rufa on pulmonary edema caused by activated neutrophils

Mediators released from polymorphonuclear neutrophils, in particular elastase, are known to induce acute edematous lung injury. In this study we show that the pulmonary edema in isolated perfused rabbit lungs caused by activated neutrophils via release of elastase is significantly decreased by the Kunitz-type Inhibitor BbCI (10(-5) M) from Bauhinia bauhinoides to the same degree as by eglin C (10(-5) M) from Hirudo medicinalis, which was used as a reference. The highly homologous proteinase inhibitor BrPI (10(-5) M) from Bauhinia rufa, however, did not reduce edema formation. The major difference between these inhibitors is the much higher Ki value of BrPI (Ki = 38 nM) for elastase compared to BbCI (Ki = 5.3 nM) and eglin C (Ki = 0.2 nM), respectively. Elastase liberation from activated PMNs was not influenced by the inhibitors. Our results indicate that BbCI can be a useful tool to study the role of neutrophil elastase in pathophysiological processes.     

Substrate inhibition of Pseudomonas aeruginosa elastase by 3-(2-furyl)acryloyl-glycyl-L-phenylalanyl-L-phenylalanine

The kinetics of hydrolysis by Pseudomonas aeruginosa elastase at 37 degrees C and pH 7.3 of 3-(2-furyl)acryloyl-glycyl-L-phenylalanyl-L-phenylalanine is compatible with nonproductive substrate inhibition, i.e., v = V.[S]/(Km + [S] + [S]2/K1), and the values of Km, Ki, and kappa cat are 1.4 mM, 5.0 mM, and 240 s-1, respectively. Product inhibition experiments are in agreement with an ordered release of product, with L-phenylalanyl-L-phenylalanine, the amino-containing product, being released first from the elastase.product complex. The values of Ki for L-phenylalanyl-L-phenylalanine and 3-(2-furyl)acryloyl-glycine are 1.5 and 4.0 mM, respectively. Kinetic experiments indicate that the second molecule of substrate combines with elastase.substrate to form a dead-end elastase . (substrate)2 complex.     

Inhibition of human granulocyte elastase and kininogenase

Using an inhibitory analysis, the different enzymatic nature of kininogenase and elastase activities in serine proteinase fractions (pI 8.3-10.75) isolated from human granulocyte lysates by isoelectrofocusing was demonstrated. The thermo- and acid-stable serine proteinase inhibitor from rabbit serum was shown to completely inhibit the kininogenase activity in these fractions but to have no inhibiting action on the elastase activity. On the contrast, the specific granulocyte elastase inhibitor, N-3-carbomethoxypropanoyl-L-alanyl-L-alanyl-L-prolyl-L-valyl-chloromethylketone , inhibits granulocyte elastase and does not inhibit the kininogenase activity in lysate fractions. The efficiency of granulocyte elastase inhibition by this chloromethylketone is evaluated by the kinetic parameters k3, Ki. The values of k3/Ki for granulocyte elastase forms with pI of 10.75, 8.9 and 8.0 are 1430, 670 and 360 M-1 S-1, respectively and show effective inhibition of the three forms by this inhibitor. Based on the different degree of inhibition of the three elastase forms by chloromethylketone inhibitor the existence of the family of elastaselike enzymes in human granulocytes is postulated.     

NMR and enzymatic investigation of the interaction between elastase and sodium trifluoroacetate.

At pH 5.5, sodium trifluoroacetate is a potent competitive inhibitor of porcine elastase (Ki = 2.6 mM) and human leukocyte elastase (Ki = 9.3 mM). For both enzymes the Ki increases strongly with pH. Sodium fluoride is inactive on pancreatic elastase and sodium acetate is a weak inhibitor of this enzyme. Trifluoroethanol inhibits both enzymes but is less active than trifluoroacetate in acidic pH conditions. Bovine trypsin and alpha-chymotrypsin are resistant to the action of sodium trifluoroacetate and trifluoroethanol. The interaction between sodium trifluoroacetate and pancreatic elastase is also demonstrated by 19F NMR spectroscopy. Trifluoroacetyltrialanine is able to displace trifluoroacetate from its complex with pancreatic elastase. In addition, a method using turkey ovomucoid for the active site titration of leukocyte and pancreatic elastase is described.     

Potent and selective inhibition of zinc aminopeptidase A (EC 3.4.11.7, APA) by glutamyl aminophosphinic peptides: importance of glutamyl aminophosphinic residue in the P1 position

Through the development of a new chemical strategy, aminophosphinic peptides containing a pseudoglutamyl residue (Glu Psi(PO2-CH2)Leu-Xaa) in the N-terminal position were synthesized and evaluated as inhibitors of aminopeptidase A (APA). The most potent inhibitor developed in this study, Glu Psi(PO2-CH2)Leu-Ala, displayed a Ki value of 0.8 nM for APA, but was much less effective in blocking aminopeptidase N (APN) (Ki = 31 microM). The critical role of the glutamyl residue in this phosphinic peptide, both in potency and selectivity, is exemplified by the P1 position analogue, Ala Psi(PO2-CH2)Leu-Ala, which exhibited a Ki value of 0.9 microM toward APA but behaved as a rather potent inhibitor of APN (Ki = 25 nM). Glu Psi(PO2-CH2)Leu-Xaa peptides are poor inhibitors of angiotensin converting enzyme (Ki values higher than 1 microM). Depending on the nature of the Xaa residue, the potency of these phosphinic peptides toward neutral endopeptidase 24-11 varied from 50 nM to 3 microM. In view of the in vivo role of APA in the formation of brain angiotensin III, one of the main effector peptides of the renin angiotensin system in the central nervous system, highly potent and selective inhibitors of APA may find important therapeutic applications soon.     

Inhibition of human skin fibroblast collagenase, thermolysin, and Pseudomonas aeruginosa elastase by peptide hydroxamic acids.

The hydroxamic acid HONHCOCH2CH(i-Bu)CO-L-Trp-NHMe, isomer 6A (GM 6001), inhibits human skin fibroblast collagenase with Ki of 0.4 nM using the synthetic thiol ester substrate Ac-Pro-Leu-Gly-SCH(i-Bu)CO-Leu-Gly-OEt at pH 6.5. The other isomer, 6B, which has the opposite configuration at the CH2CH(i-Bu)CO alpha-carbon atom, has a Ki of 200 nM for this enzyme. GM 6001 is one of the most potent inhibitors of human skin fibroblast collagenase yet reported. GM 6001 has a Ki of 20 nM against thermolysin and Pseudomonas aeruginosa elastase. Isomer 6B has a Ki of 7 nM against thermolysin and 2 nM against the elastase. 6A and 6B are the most potent hydroxamate inhibitors reported for these bacterial enzymes. The pattern of inhibition for all three enzymes suggests that isomer 6A is the (R,S) compound, stereochemically analogous to the L,L-dipeptide, and isomer 6B is the (S,S) compound, analogous to the DL-dipeptide. The tolerance of the D configuration by thermolysin and the elastase allows these inhibitors to discriminate between the human and bacterial enzymes simply by inversion of configuration at the CH2CH(i-Bu)CO alpha-carbon atom. Substitution of the potential metal liganding groups carboxylate and hydrazide for the hydroxamate group yields much weaker inhibitors for all three enzymes.     

Specific inhibition of human granulocyte elastase with peptide aldehydes

The kinetic features of human granulocyte elastase, chymotrypsin, porcine pancreatic elastase and elastomucoproteinase were compared. Amino acyl ester substrates were assayed and Km and kcat values were defined. Aldehyde analogues of the p-nitroanilide substrates designed for granulocyte elastase as optimal for Km appeared to be potent inhibitors. Suc-D-Phe-Pro-valinal (Ki = 40 microM) was found to inhibit granulocyte elastase competitively and specifically when measured with synthetic substrates, and the Ki was 3 microM with the natural protein substrate, elastin.     

Biochemical characterization of alpha-ketooxadiazole inhibitors of elastases.

A series of alpha-ketooxadiazole compounds was prepared and evaluated in vitro as potential inhibitors of human neutrophil elastase (HNE), proteinase-3 (PR-3), and porcine pancreatic elastase (PPE). Several compounds have been found to be very potent, fast, reversible, and selective inhibitors of HNE with Ki values below 100 pM. The highest kon value exceeded 10(7) M(-1) s(-1). Some alpha-ketooxadiazoles were also very effective against PR-3 and PPE with Ki values in the range of 5(-10) nM and 0.1(-2) nM, respectively. The two rings, 1,2,4- and 1,3,4-oxadiazole, are amenable to substitutions, extending the P' side of the inhibitor and allowing additional binding interactions at S' subsites of the enzyme. Nonpeptidic HNE inhibitors containing the oxadiazole heterocycle displayed promising oral bioavailability.     

The inhibition of human neutrophil elastase and cathepsin G by peptidyl 1,2-dicarbonyl derivatives

Neutrophil elastase and cathepsin G are serine proteases that can damage connective tissue and trigger other pathological reactions. Compounds containing a peptide sequence to impart specificity and bearing an alpha-dicarbonyl unit (alpha-diketone or alpha-keto ester) at the carboxy terminus are potent inhibitors of the neutrophil serine proteases (human neutrophil elastase: R-Val-COCH3, Ki = 0.017 microM; R-Val-COOCH3, Ki = 0.002 microM; human neutrophil cathepsin G: R-Phe-COCH3, Ki = 0.8 microM; R-Phe-COOCH3, Ki = 0.44 microM; R = N-(4-[(4-chlorophenyl)sulfonylaminocarbonyl]phenylcarbonyl)+ ++ValylProlyl).     

Inhibition of leucocyte elastase by heparin and its derivatives.

Leucocyte proteinases, e.g. leucocyte elastase and cathepsin G, are inhibited by heparin. The activities of pig pancreatic and Pseudomonas aeruginosa elastases are unaffected by this polysaccharide. Heparin derivatives of known Mr and degree of sulphation were isolated. The inhibition of leucocyte elastase by these oligosaccharides can be classified as tight-binding hyperbolic non-competitive. Ki values ranged from 40 nM to 100 microM and were found to be inversely correlated with the chain length of the oligosaccharides. Desulphated compounds lacked inhibitory potential towards leucocyte elastase. Over-O-sulphated di- and tetra-saccharides are more potent inhibitors than their over-N-sulphated counterparts. It is proposed that the therapeutic use of heparin and its derivatives could be extended to disease states such as emphysema and rheumatoid arthritis, where the role of leucocyte elastase has been clearly established.     

Comparison of inhibitory effects of prolinal-containing peptide derivatives on prolyl endopeptidases from bovine brain and Flavobacterium

The inhibitory effects of proline-containing peptides and their derivatives on prolyl endopeptidases from Flavobacterium meningosepticum and bovine brain were compared. Replacement of the carboxyl terminal proline in N-blocked peptides with prolinal resulted in remarkable decreases in Ki values for both prolyl endopeptidases. Further reduction of the prolinal to prolinol led to a decrease in their inhibitory effects. Z-Pro-, Z-Val-, and Suc-Pro-prolinals were similarly inhibitory for both the enzymes with Ki values of nM order. However, the inhibitory effects of Z-Pyr-prolinal and Boc-Pro-prolinal on these enzymes were significantly distinguished: they strongly inhibited the mammalian prolyl endopeptidase with Ki values of nM order, while the Ki values of these compounds for the microbial enzyme were only of microM order. These results suggest that there are some structural differences in the S2 and S3 subsites between the two enzymes, though their substrate specificities are apparently indistinguishable.     

Mechanism of inactivation of human leukocyte elastase by a chloromethyl ketone: kinetic and solvent isotope effect studies

The mechanism of inactivation of human leukocyte elastase (HLE) by the chloromethyl ketone MeOSuc-Ala-Ala-Pro-Val-CH2Cl was investigated. The dependence of the first-order rate constant for inactivation on concentration of chloromethyl ketone is hyperbolic and suggests formation of a reversible "Michaelis complex" prior to covalent interaction between the enzyme and inhibitor. However, the observed Ki value is 10 microM, at least 10-fold lower than dissociation constants for complexes formed from interaction of HLE with structurally related substrates or reversible inhibitors, and suggests that Ki is a complex kinetic constant, reflecting the formation and accumulation of both the Michaelis complex and a second complex. It is proposed that this second complex is a hemiketal formed from attack of the active site serine on the carbonyl carbon of the inhibitor. The accumulation of this intermediate may be a general feature of reactions of serine proteases and chloromethyl ketones derived from specific peptides and accounts for the very low Ki values observed for these reactions. The solvent deuterium isotope effect (SIE) on the inactivation step (ki) is 1.58 +/- 0.07 and is consistent with rate-limiting, general-catalyzed attack of the active site His on the methylene carbon of the inhibitor with displacement of chloride anion. The general catalyst is thought to be the active site Asp. In contrast, the SIE on the second-order rate constant for HLE inactivation, ki/Ki, is inverse and equals 0.64 +/- 0.05.(ABSTRACT TRUNCATED AT 250 WORDS)     

Kinetic studies on the interaction of eglin c with human leukocyte elastase and cathepsin G

We have investigated the inhibition of human leukocyte elastase and cathepsin G by recombinant Eglin c under near physiological conditions. The association rate constants k on of Eglin c for elastase and cathepsin G were 1.3 X 10(7) M-1 s-1 and 2 X 10(6) M-1 s-1, respectively. Under identical conditions, the k on for the association of human plasma alpha 1-proteinase inhibitor with the two leukocproteinases were 2.4 X 10(7) M-1 s-1 and 10(6) M-1 s-1, respectively. The consistency of these data could be verified using a set of competition experiments. The elastase-Eglin c interaction was studied in greater detail. The dissociation rate constant k off was determined by trapping of free elastase from an equilibrium mixture of elastase and Eglin c with alpha 1-proteinase inhibitor or alpha 2-macroglobulin. The rate of dissociation was very low (k off = 3.5 X 10(-5) s-1). The calculated equilibrium dissociation constant of the complex, Ki(calc) = k off/k on, was found to be 2.7 X 10(-12) M. Ki was also measured by adding elastase to mixtures of Eglin c and substrate and determining the steady-state rates of substrate hydrolysis. The Ki determined from these experiments (7.5 X 10(-11) M) was significantly higher than Ki(calc). This discrepancy might be explained by assuming that the interaction of Eglin c with elastase involves two steps: a fast binding reaction followed by a slow isomerization step. From the above kinetic constants it may be inferred that at a therapeutic concentration of 5 X 10(-7) M, Eglin c will inhibit leukocyte elastase in one second and will bind this enzyme in a "pseudo-irreversible" manner.     

Inhibition of neutrophil cathepsin G by oxidized mucus proteinase inhibitor. Effect of heparin.

Oxidation of mucus proteinase inhibitor (MPI) transforms Met73, the P'1 residue of its active center into methionine sulfoxide and lowers its affinity for neutrophil elastase [Boudier, C., and Bieth, J. G. (1994) Biochem. J. 303, 61-68]. Here, we show that the oxidized inhibitor has also a decreased affinity for neutrophil cathepsin G and pancreatic chymotrypsin. The Ki of the oxidized MPI-cathepsin G complex (1.2 microM) is probably too high to be compatible with significant inhibition of cathepsin G in inflammatory lung secretions. Stopped-flow kinetics shows that, within the inhibitor concentration range used, the mechanism of inhibition of cathepsin G and chymotrypsin by oxidized MPI is consistent with a one-step reaction, [equation in text] whereas the inhibition of elastase takes place in two steps, [equation in text]. Heparin, which accelerates the inhibition of the three proteinases by native MPI, also favors their interaction with oxidized MPI. Flow calorimetry shows that heparin binds oxidized MPI with Kd, Delta H degrees, and Delta S degrees values close to those reported for native MPI. In the presence of heparin, oxidized MPI inhibits cathepsin G via a two-step reaction characterized by Ki = 0.22 microM, k2 = 0.1 s-1, k-2 = 0.023 s-1, and Ki = 42 nM. Under these conditions, in vivo inhibition of cathepsin G is again possible. Heparin also improves the inhibition of chymotrypsin and elastase by oxidized MPI by increasing their kass or k2/Ki and decreasing their Ki. Our data suggest that oxidation of MPI during chronic bronchitis may lead to cathepsin G-mediated lung tissue degradation and that heparin may be a useful adjuvant of MPI-based therapy of acute lung inflammation in cystic fibrosis.     

Synthetic inhibitors of human leukocyte elastase, Part 3. Peptides with alkyl groups at the N- or C-terminus. Non-toxic competitive inhibitors of human leukocyte elastase

A series of carboxy-alkylamidated and N-acetylated amino acids and peptides were synthesized and examined for their ability to inhibit human leukocyte elastase. The Boc-amino acid alkylamides were found to be potent specific and competitive inhibitors of this enzyme. They were found not to or only poorly inhibit several other serine proteinases such as bovine trypsin, alpha-chymotrypsin, porcine pancreatic elastase and human leukocyte cathepsin G at concentrations less than 10(-4) M. Specificity and maximum inhibition of human leukocyte elastase were achieved when the N-terminus of the amino acid was protected by a t-butyloxy-carbonyl (Boc) group, the oligopeptide fragment consisted of valine residues and when the alkyl chain was between 10 and 12 carbon atoms in length and attached to the C-terminus of the peptide fragment. Highest inhibition was obtained with the compound Boc-[Val]3-NH[CH2]11--CH3 (Ki = 0.21 microM). These specific inhibitors were also found to be non-toxic after oral administration to mice and rats (LD50 greater than 3.0 g/kg body weight).     

Kinetics of the inhibition of human pancreatic elastase by recombinant eglin c. Influence of elastin.

Recombinant eglin c is a potent reversible inhibitor of human pancreatic elastase. At pH 7.4 and 25 degrees C, kass. = 7.3 x 10(5) M-1.s-1, kdiss. = 2.7 x 10(-4) s-1 and Ki = 3.7 x 10(-10) M. Stopped-flow kinetic indicate that the formation of the stable enzyme-inhibitor complex is not preceded by a fast pre-equilibrium complex or that the latter has a dissociation constant greater than 0.3 microM. The elastase-eglin c complex is much less stable at pH 5.0 and 25 degrees C, where kdiss. = 1.1 x 10(-2) s-1 and Ki = 7.3 x 10(-8) M. At pH 7.4 the activation energy for kass. is 43.9 kJ.mol-1 (10.5 kcal.mol-1). The kass. increases between pH 5.0 and 8.0 and remains essentially constant up to pH 9.0. This pH-dependence could not be described by a simple ionization curve. Both alpha 2-macroglobulin and alpha 1-proteinase inhibitor are able to dissociate the elastase-eglin c complex, as evidenced by measurement of the enzymic activity of alpha 2-macroglobulin-bound elastase or by polyacrylamide-gel electrophoresis of mixtures of alpha 1-proteinase inhibitor and elastase-eglin c complex. The rough estimate of kdiss. obtained with the alpha 2-macroglobulin dissociation experiment (1.6 x 10(-4) s-1) was of the same order of magnitude as the constant measured with the progress curve method. Eglin c strongly inhibits the solubilization of human aorta elastin by human pancreatic elastase. The extent of inhibition is the same whether elastase is added to a suspension of elastin and eglin c or whether elastase is preincubated with elastin for 3 min before addition of eglin c. However, the efficiency of the inhibitor sharply decreases if elastase is reacted with elastin for more prolonged periods.     

Synthesis of peptide fragments related to eglin c and examination of their inhibitory effect on human leukocyte elastase, cathepsin G and alpha-chymotrypsin

Various kinds of peptide fragments related to eglin c were prepared by the conventional solution method and their inhibitory effects on human leukocyte elastase, cathepsin G and alpha-chymotrypsin were examined. Peptide (31-40) inhibited cathepsin G (Ki = 2.3 x 10(-4) M), peptide (41-49) potently inhibited cathepsin G and alpha-chymotrypsin (Ki = 4.2 x 10(-5) M and 2.0 x 10(-5) M, respectively), and peptide (60-63) inhibited leukocyte elastase (Ki = 1.6 x 10(-4) M), whereas, peptide (31-35) weakly inhibited both elastase and cathepsin G (Ki = 2.1 x 10(-3) M and 7.3 x 10(-4) M, respectively).