A dipeptidocarboxypeptidase from E. coli

Thioproline formation as a driver of formaldehyde toxicity in Escherichia coli

Formaldehyde (HCHO) is a reactive carbonyl compound that formylates and cross-links proteins, DNA, and small molecules. It is of specific concern as a toxic intermediate in the design of engineered pathways involving methanol oxidation or formate reduction. The interest in engineering these pathways is not, however, matched by engineering-relevant information on precisely why HCHO is toxic or on what damage-control mechanisms cells deploy to manage HCHO toxicity. The only well-defined mechanism for managing HCHO toxicity is formaldehyde dehydrogenase-mediated oxidation to formate, which is counterproductive if HCHO is a desired pathway intermediate. We therefore sought alternative HCHO damage-control mechanisms via comparative genomic analysis. This analysis associated homologs of the Escherichia coli pepP gene with HCHO-related one-carbon metabolism. Furthermore, deleting pepP increased the sensitivity of E. coli to supplied HCHO but not other carbonyl compounds. PepP is a proline aminopeptidase that cleaves peptides of the general formula X-Pro-Y, yielding X + Pro-Y. HCHO is known to react spontaneously with cysteine to form the close proline analog thioproline (thiazolidine-4-carboxylate), which is incorporated into proteins and hence into proteolytic peptides. We therefore hypothesized that certain thioproline-containing peptides are toxic and that PepP cleaves these aberrant peptides. Supporting this hypothesis, PepP cleaved the model peptide Ala-thioproline-Ala as efficiently as Ala-Pro-Ala in vitro and in vivo, and deleting pepP increased sensitivity to supplied thioproline. Our data thus (i) provide biochemical genetic evidence that thioproline formation contributes substantially to HCHO toxicity and (ii) make PepP a candidate damage-control enzyme for engineered pathways having HCHO as an intermediate.

Crystal structure of a peptidyl-dipeptidase K-26-DCP from Actinomycete in complex with its natural inhibitor

Several soil‐derived Actinobacteria produce secondary metabolites that are proven specific and potent inhibitors of the human angiotensin‐I‐converting enzyme (ACE), a key target for the modulation of hypertension through its role in the renin–angiotensin–aldosterone system. K‐26‐DCP is a zinc dipeptidyl carboxypeptidase (DCP) produced by Astrosporangium hypotensionis, and an ancestral homologue of ACE. Here we report the high‐resolution crystal structures of K‐26‐DCP and of its complex with the natural microbial tripeptide product K‐26. The experimental results provide the structural basis for better understanding the specificity of K‐26 for human ACE over bacterial DCPs.

Database

Structural data are available in the PDB under the accession numbers 5L43 and 5L44.

Divergent evolution of the M3A family of metallopeptidases in plants

Plants, as stationary organisms, have developed mechanisms allowing them efficient resource reallocation and a response to changing environmental conditions. One of these mechanisms is proteome remodeling via a broad peptidase network present in various cellular compartments including mitochondria and chloroplasts. The genome of the model plant Arabidopsis thaliana encodes as many as 616 putative peptidase-coding genes organized in 55 peptidase families. In this study, we describe the M3A family of peptidases, which comprises four members: mitochondrial and chloroplastic oligopeptidase (OOP), cytosolic oligopeptidase (CyOP), mitochondrial octapeptidyl aminopeptidase 1 (Oct1) and plant-specific protein of M3 family (PSPM3) of unknown function. We have analyzed the evolutionary conservation of M3A peptidases across plant species and the functional specialization of the three distinct subfamilies. We found that the subfamily-containing OOP and CyOP-like peptidases, responsible for oligopeptide degradation in the endosymbiotic organelles (OOP) or in the cytosol (CyOP), are highly conserved in all kingdoms of life. The Oct1-like peptidase subfamily involved in pre-protein maturation in mitochondria is conserved in all eukaryotes, whereas the PSPM3-like protein subfamily is strictly conserved in higher plants only and is of unknown function. Specific characteristics within PSPM3 sequences, i.e. occurrence of a N-terminal transmembrane domain and amino acid changes in distal substrate-binding motif, distinguish PSPM3 proteins from other members of M3A family. We performed peptidase activity measurements to analyze the role of substrate-binding residues in the different Arabidopsis M3A paralogs.

Novel approaches for the identification of inhibitors of leishmanial dipeptidylcarboxypeptidase

INTRODUCTION

Leishmaniasis imposes a substantial burden of mortality and morbidity affecting 12 million globally and continues to be a neglected tropical disease. Control of the disease is mainly based on chemotherapy, which relies on a handful of drugs with serious limitations. Over the last decade, target-based drug discovery is also being employed in addition to the random screening of compounds. Leishmanial dipeptidylcarboxypeptidase (LDCP), an angiotensin converting enzyme (ACE) related metallopeptidase, has been recently identified as a novel drug target for antileishmanial chemotherapy.

AREAS COVERED

This article examines dipeptidylcarboxypeptidase (DCP) of Leishmania donovani and of other sources from the international literature regarding their biochemical and structural characterization in comparison to mammalian ACE. Furthermore, the author discusses the identification of LdDCP specific inhibitors by virtual screening and their effect on parasite multiplication. Finally, the review looks ahead at areas for further exploration of DCP inhibitors in Leishmania chemotherapy.

EXPERT OPINION

The first step in targeted screening is to identify a suitable drug target and its validation followed by its use in high throughput screening of compounds. Limited studies on LDCP inhibitors have established a good correlation between parasite enzyme inhibition and their biological activity. This suggests that there is a potential for LDCP inhibitors as new antileishmanial drugs.

Angiotensin-converting enzyme C-terminal catalytic domain is the main site of angiotensin I cleavage in vivo

Angiotensin-converting enzyme (ACE) plays a central role in the production of the vasoconstrictor angiotensin II. ACE is a single polypeptide, but it contains 2 homologous and independent catalytic domains, each of which binds zinc. To understand the in vivo role of these 2 domains, we used gene targeting to create mice with point mutations in the ACE C-domain zinc-binding motif. Such mice, termed ACE13/13, produce a full-length ACE protein with tissue expression identical to wild-type mice. Analysis of ACE13/13 mice showed that they produce ACE having only N-domain catalytic activity, as determined by the hydrolysis of domain specific substrates and by chloride sensitivity. ACE13/13 mice have blood pressure and blood angiotensin II levels similar to wild-type mice. However, plasma renin concentration is increased 2.6-fold and blood angiotensin I levels are increased 7.5-fold. Bradykinin peptide levels are not different from wild-type levels. ACE13/13 mice have a reduced increase of blood pressure after intravenous infusion of angiotensin I. ACE13/13 mice have a normal renal structure, but they are not able to concentrate urine after dehydration as effectively as wild-type mice. This study shows that the C-domain of ACE is the predominant site of angiotensin I cleavage in vivo. Although mice lacking C-domain activity have normal physiology under laboratory conditions, they respond less well to the stress of dehydration.

Cloning and characterization of angiotensin converting enzyme related dipeptidylcarboxypeptidase from Leishmania donovani

We report the first identification, gene cloning, recombinant expression and biochemical characterization of an angiotensin converting enzyme (ACE) related dipeptidylcarboxypeptidase (DCP) in a protozoan parasite. The mammalian counterpart of this enzyme, peptidyl dipeptidase A (a carboxyl dipeptidase) also known as ACE leads to the cleavage of angiotensin I to produce a potent vasopressor. The catalytic enzyme activity of its Escherichia coli DCP counter part can be inhibited by the antihypertensive drug captopril, suggesting that this class of enzymes constitutes a novel target for drugs and vaccines. By utilizing a DNA microarray expression profiling approach, we identified a gene encoding a DCP enzyme for the kinetoplast protozoan Leishmania donovani (LdDCP) that was differentially expressed in promastigote and amastigote stages of the parasite life cycle. Both RNA and protein levels of LdDCP are higher in axenic amastigotes compared to promastigotes. Immuno-fluorescence analysis revealed the cytosolic expression of the protein. Primary structure analysis of LdDCP revealed the presence of an active Zn binding site. When expressed in E. coli, the recombinant enzyme showed carboxy-dipeptidase activity with synthetic substrates. Replacement of two histidine and one glutamic acid at positions 466, 470 and 467, respectively, with alanine residues in its active site resulted in loss of enzyme activity. Captopril, an ACE specific inhibitor was able both to reduce significantly LdDCP enzyme activity and to inhibit promastigote growth. Both its cytosolic location and close homology to DCPs from bacterial species suggests a role in parasite nutrition. Further, identification of LdDCP now provides an opportunity to investigate Leishmania peptidases for their potential as drug and vaccine targets.

Crystal structure of the E. coli dipeptidyl carboxypeptidase Dcp: further indication of a ligand-dependent hinge movement mechanism

Dcp from Escherichia coli is a 680 residue cytoplasmic peptidase, which shows a strict dipeptidyl carboxypeptidase activity. Although Dcp had been assigned to the angiotensin I-converting enzymes (ACE) due to blockage by typical ACE inhibitors, it is currently grouped into the M3 family of mono zinc peptidases, which also contains the endopeptidases neurolysin and thimet oligopeptidase (TOP). We have cloned, expressed, purified, and crystallized Dcp in the presence of an octapeptide "inhibitor", and have determined its 2.0A crystal structure using MAD methods. The analysis revealed that Dcp consists of two half shell-like subdomains, which enclose an almost closed two-chamber cavity. In this cavity, two dipeptide products presumably generated by Dcp cleavage of the octapeptide bind to the thermolysin-like active site fixed to side-chains, which are provided by both subdomains. In particular, an Arg side-chain backed by a Glu residue, together with two Tyr phenolic groups provide a charged anchor for fixing the C-terminal carboxylate group of the P2' residue of a bound substrate, explaining the strict dipeptidyl carboxypeptidase specificity of Dcp. Tetrapeptidic substrates are fixed only via their main-chain functions from P2 to P2', suggesting a broad residue specificity for Dcp. Both subdomains exhibit very similar chain folds as the equivalent but abducted subdomains of neurolysin and TOP. Therefore, this "product-bound" Dcp structure seems to represent the inhibitor/substrate-bound "closed" form of the M3 peptidases, generated from the free "open" substrate-accessible form by a hinge-bending mechanism. A similar mechanism has recently been demonstrated experimentally for ACE2.

Some molecular and inhibitory specifications of a dipeptidyl carboxypeptidase from the polychaete Neanthes virens resembling angiotensin I converting enzyme

Dipeptidyl carboxypeptidase (DCP) from the polychaete Neanthes virens, resembling mammalian angiotensin I converting enzyme (ACE), was studied to discover some of its molecular and inhibitory properties, as the first evidence of these in a marine invertebrate. Amino acid and carbohydrate contents were analyzed. The N-terminal amino acid sequence of N. virens DCP was (NH2)D-E-E-A-G-R-Q-W-L-A-E-Y-D-L-R-N-Q-T-V-L-. Peptide maps of N. virens DCP from lysyl endopeptidase digestion were different from rabbit p-ACE. The far-ultraviolet circular dichroic spectra of N. virens DCP indicated that the secondary structure of this enzyme seemed to be an alpha-helical structure and was similar to that of rabbit p-ACE, but the near-ultraviolet circular dichroic spectra of N. virens DCP indicated that the aromatic amino acid residue circumambience of this enzyme was different from rabbit p-ACE. The effects of several reagents for chemical modification of amino acids on the activity of N. virens DCP were tested. Arg, Tyr, Glu, and/or Asp, His, Trp, and Met caused loss of the activity. In addition, the IC50 and Ki values for a well-known ACE inhibitor, Val-Tyr, which was a competitive inhibitor of N. virens DCP, were 263 and 20 microM, respectively. These results suggested that N. virens DCP is different from mammalian ACE in the molecular and inhibitory properties, although the same substrate specificity was demonstrated in a previous paper.

Purification and characterization of a dipeptidyl carboxypeptidase from the polychaete Neanthes virens resembling angiotensin I converting enzyme

Dipeptidyl carboxypeptidase (DCP) is well known as a mammalian angiotensin I converting enzyme (ACE) which plays an important role in blood pressure homeostasis. DCP was purified from the whole body of a polychaete, Neanthes virens. The purified enzyme was homogeneous by SDS-PAGE, with a molecular mass of 71 kDa by SDS-PAGE and 69 kDa by gel filtration, indicating that it is monomeric. The isoelectric point was 4.5 and optimum pH for the activity was 8.0. It showed a specific activity of 466.8 U/mg, which is the highest of known DCPs. The enzyme hydrolyzed angiotensin I to angiotensin II and sequentially released Phe-Arg and Ser-Pro from the C-terminus bradykinin, but does not cleave imido-bonds. Activity was completely inhibited by 1 mM EDTA and 5 mM o-phenanthroline, but it was not affected by serine and aspartic protease inhibitors. The original activity of EDTA-inactivated DCP was restored by addition of cobalt, manganese or low concentrations of zinc. The Km and Vmax values of the enzyme for Bz-Gly-His-Leu were 0.56 mM and 600 mumol/min per mg, respectively. The Ki values for specific mammalian ACE inhibitors, such as captopril and lisinopril, were 1.38 and 2.07 nM, respectively. In conclusion, we have shown the existence of a DCP from the polychaete, N. virens, with similar properties to those of mammalian ACE.

dcp gene of Escherichia coli: cloning, sequencing, transcript mapping, and characterization of the gene product

Dipeptidyl carboxypeptidase is a C-terminal exopeptidase of Escherichia coli. We have isolated the respective gene, dcp, from a low-copy-number plasmid library by its ability to complement a dcp mutation preventing the utilization of the unique substrate N-benzoyl-L-glycyl-L-histidyl-L-leucine. Sequence analysis of a 2.9-kb DNA fragment revealed an open reading frame of 2,043 nucleotides which was assigned to the dcp gene by N-terminal amino acid sequencing and electrophoretic molecular mass determination of the purified dcp product. Transcript mapping by primer extension and S1 protection experiments verified the physiological significance of potential initiation and termination signals for dcp transcription and allowed the identification of a single species of monocistronic dcp mRNA. The codon usage pattern and the effects of elevated gene copy number indicated a relatively low level of dcp expression. The predicted amino acid sequence of dipeptidyl carboxypeptidase, containing a potential zinc-binding site, is highly homologous (78.8%) to the corresponding enzyme from Salmonella typhimurium. It also displays significant homology to the products of the S. typhimurium opdA and the E. coli prlC genes and to some metalloproteases from rats and Saccharomyces cerevisiae. No potential export signals could be inferred from the amino acid sequence. Dipeptidyl carboxypeptidase was enriched 80-fold from crude extracts of E. coli and used to investigate some of its biochemical and biophysical properties.

Cloning and nucleotide sequence of the Salmonella typhimurium dcp gene encoding dipeptidyl carboxypeptidase

Plasmids carrying the Salmonella typhimurium dcp gene were isolated from a pBR328 library of Salmonella chromosomal DNA by screening for complementation of a peptide utilization defect conferred by a dcp mutation. Strains carrying these plasmids overproduced dipeptidyl carboxypeptidase approximately 50-fold. The nucleotide sequence of a 2.8-kb region of one of these plasmids contained an open reading frame coding for a protein of 77,269 Da, in agreement with the 80-kDa size for dipeptidyl carboxypeptidase (determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and gel filtration). The N-terminal amino acid sequence of dipeptidyl carboxypeptidase purified from an overproducer strain agreed with that predicted by the nucleotide sequence. Northern (RNA) blot data indicated that dcp is not cotranscribed with other genes, and primer extension analysis showed the start of transcription to be 22 bases upstream of the translational start. The amino acid sequence of dcp was not similar to that of a mammalian dipeptidyl carboxypeptidase, angiotensin I-converting enzyme, but showed striking similarities to the amino acid sequence of another S. typhimurium peptidase encoded by the opdA (formerly optA) gene.

Cloning and nucleotide sequence of opdA, the gene encoding oligopeptidase A in Salmonella typhimurium

The opdA gene (formerly called optA) of Salmonella typhimurium encodes a metallopeptidase, oligopeptidase A (OpdA), first recognized by its ability to cleave and allow utilization of N-acetyl-L-Ala4 (E. R. Vimr, L. Green, and C. G. Miller, J. Bacteriol. 153:1259-1265, 1983). Derivatives of pBR328 carrying the opdA gene were isolated and shown to express oligopeptidase activity at levels approximately 100-fold higher than that of the wild type. These plasmids complemented all of the phenotypes associated with opdA mutations (failure to use N-acetyl-L-Ala4, defective phage P22 development, and diminished endopeptidase activity). The opdA region of one of these plasmids (pCM127) was defined by insertions of Tn1000 (gamma delta), and these insertions were used as priming sites to determine the nucleotide sequence of a 2,843-bp segment of the insert DNA. This region contained an open reading frame coding for a 680-amino-acid protein, the N terminus of which agreed with that determined for purified OpdA. This open reading frame contained both a sequence motif typical of Zn2+ metalloproteases and a putative sigma 32 promoter. However, no induction was detected upon temperature shift by using a beta-galactosidase operon fusion. The predicted OpdA sequence showed similarity to dipeptidyl carboxypeptidase, the product of the S. typhimurium gene dcp, and to rat metallopeptidase EC 3.4.24.15., which is involved in peptide hormone processing.

Analysis of peptide metabolism by ruminal microorganisms

Methods were developed for the determination of oligoalanine and other short-chain peptides and peptide analogs in ruminal fluid by using reverse-phase high-pressure liquid chromatography. Chromatographic analysis of the breakdown of (Ala)3 and (Ala)4 in ruminal fluid in vitro revealed that the predominant mechanism of hydrolysis was a dipeptidyl peptidase-like activity. Hydrolysis proceeded from the N terminal of the peptide chain; N-acetyl-(Ala)3 was broken down at 11% of the rate of breakdown of (Ala)3 or (Ala)3-p-nitroanilide. (Ala)2-p-nitroanilide was hydrolyzed most rapidly of the arylamide substrates tested, but fluorogenic 4-methoxy-2-naphthylamide (MNA) compounds were more convenient and potentially more versatile substrates than p-nitroanilides. Gly-Arg-MNA was the most rapidly hydrolyzed dipeptidyl peptidase substrate, suggesting that ruminal peptidase activity was predominantly of a type I specificity.

A peptidyl dipeptidase-4 from Pseudomonas maltophilia: purification and properties

A peptidyl dipeptidase-4 (bacterial PDP-4) was purified to near homogeneity from a supernatant of Pseudomonas maltophilia extracellular medium. Bacterial PDP-4 is a single-polypeptide-chain enzyme, 82 kDa, with an alkaline isoelectric point. Peptides susceptible to hydrolysis by bacterial PDP-4 include angiotensin 1, bradykinin, enkephalins, atriopeptin 2, and smaller synthetic peptides. N-acylated tripeptides are hydrolyzed, but free tripeptides are not. A free carboxy terminus is required for hydrolysis. Peptides with ultimate and penultimate Pro residues are not hydrolyzed. The enzyme does not require an anion for activity. Bacterial PDP-4 was inhibited by EDTA and the dipeptide Phe-Arg. Thiorphan was an inhibitor only at levels well above those required for inhibition of neutral metalloendopeptidase (NEP), an enzyme for which thiorphan is specific. A second NEP and thermolysin inhibitor, phosphoramidon, did not inhibit bacterial PDP-4. The potent angiotensin-converting enzyme inhibitor lisinopril was not inhibitory. Bacterial PDP-4 is distinguished from a similar enzyme from Escherichia coli, which is not susceptible to EDTA inhibition, and one from Corynebacterium equi, which hydrolyzes free tripeptides. These data indicate that the bacterial PDP-4 catalytic site is unlike those of other enzymes that function either wholly or in part as peptidyl dipeptidases.

Role of chloride ion as an allosteric activator of angiotensin-converting enzyme

The nature of chloride ion as an activator of angiotensin-converting enzyme was studied by a series of kinetic experiments with hog plasma enzyme preparation. The enzyme required the presence of chloride ion for its full catalytic activity, but its requirement of monovalent anion was not absolute. The KA value for the enzyme-chloride binding was estimated to be about 150 mM in all cases regardless of the peptide substrates employed. In the presence of chloride ion, the activity of the enzyme was increased, but its optimum pH was shifted gradually to the alkaline region up to pH 8.2 depending on the concentration of chloride ion. In addition, in the presence of chloride ion, the apparent Km values were reduced markedly while the Vmax values were not much altered; for example, for the hydrolysis of angiotensin I decapeptide, the Km value decreased by a factor of 50 while only an 18% increase in Vmax was observed when the enzyme was saturated with chloride ion. The result suggests that chloride ion acts as a conformational modifier inducing the affinity of synergistic binding of substrate.

Dipeptidyl carboxypeptidase-deficient mutants of Salmonella typhimurium

Mutants of Salmonella typhimurium deficient in dipeptidyl carboxypeptidase have been isolated by screening for clones unable to use N-acetyl-L-alanyl-L-alanyl-L-alanine (AcAla3) as the sole nitrogen source. An insertion of the transposable element Tn10 near dcp (the locus coding for dipeptidyl carboxypeptidase) has been isolated and used to map the locus in the interval between purB and trp, an otherwise genetically silent region of the S. typhimurium map. All dcp mutants could still grow using N-acetyl-L-alanyl-L-alanyl-L-alanyl-L-alanine (AcAla4) as the sole nitrogen source. Crude extracts from the dcp mutants failed to hydrolyze AcAla3 but retained approximately 80% of the wild-type activity toward AcAla4. Several lines of evidence indicate that hydrolysis of AcAla4 in the dcp mutant results from the action of a new peptidase distinct from dipeptidyl carboxypeptidase. A mutant strain lacking dipeptidyl carboxypeptidase in addition to peptidases N, A, B, and D showed reduced protein breakdown during carbon starvation compared with a strain lacking only peptidases N, A, B, and D.

Degradation of abnormal proteins in peptidase-deficient mutants of Salmonella typhimurium

The degradation of abnormal proteins produced as a result of incorporation of the arginine analog L-canavanine or generated by exposure to puromycin was studied in wild-type and multiply peptidase-deficient strains of Salmonella typhimurium. Both types of abnormal protein were rapidly degraded during growth of Pep+ strains of this organism. Peptidase--deficient mutants (lacking peptidases N, A, B, and D) could also degrade these abnormal proteins, although the rate of production of trichloroacetic acid-soluble degradation products was slower in the mutant strain than in a strain carrying a normal complement of peptidases. Analysis of these trichloroacetic acid-soluble degradation products of ion-exchange chromatography showed that free amino acid was the major breakdown product produced by the wild-type strain. The acid-soluble degradation product produced by the mutant strain, however, was a complex mixture that contained a variety of small peptides as well as free amino acids. These results indicate that the same group of peptidases shown previously to function in the degradation of exogenously supplied peptides and in protein turnover during carbon starvation also lie on the pathway by which abnormal proteins are degraded.

Tripeptide-specific aminopeptidase from Escherichia coli AJ005

A tripeptidase, TP, from the ribosome-free fraction of Escherichia coli AJ005, a peptidase-deficient mutant of strain K-12, has been obtained using gel electrophoresis and chromatography on DEAE-Sephadex A-50, hydroxylapatite, and Sephadex G-200. Characterization studies on tripeptidase TP, freed of other detectable peptidases, indicate that this enzyme is capable of cleaving an amino-terminal leucine, lysine, methionine, or phenylalanine residue from certain tripeptides. Only one band of activity toward several tripeptides (and no activity toward dipeptides) was detected following gel electrophoresis of this preparation. Tripeptidase TP, the only strain AJ005 peptidase known to attack trilysine, was inactive toward all dipeptides, peptide amides, substituted peptides, esters, and tetrapeptides tested as substrates. Trilysine cleavage is optimal at about pH 8.5, as determined in Tris, borate, or phosphate buffers. Tripeptidase TP activity tested under a number of conditions was not inhibited by soybean trypsin inhibitor (3 mg/mL), phenylmethanesulfonyl fluoride (25 micrometer), or iodoacetate (9 mM). p-Mercuribenzoate (10 micrometer), divalent copper, cobalt, calcium (2.5 mM), zinc (25 micrometer), and mercury (10 micrometer) are inhibitory. Based on Sephadex G-200 chromatography tripeptidase TP has a particle weight of approximately 80 000 daltons. An apparent Km of 5.3 mM was determined for methionylglycylglycine cleavage.

An intramolecularly quenched fluorescent tripeptide as a fluorogenic substrate of angiotensin-I-converting enzyme and of bacterial dipeptidyl carboxypeptidase

The N-acyltripeptide 2-aminobenzoylglycyl-p-nitrophenylalanylproline was synthesized and applied as a substrate in the assay of angiotensin-I-converting enzyme from calf lung and human serum, and of the bacterial dipeptidyl carboxypeptidase from Escherichia coli. This compound belongs to a new class of substrates for proteolytic enzymes, having the general structure F--X--Q in which fluorescence of group F is quenched by intramolecular interaction with the group Q. Enzymatic cleavage of the peptide chain (X stands for one or more amino acid residues) generates the unquenched F-containing derivative and the resulting fluorescence is used for quantitative measurement of the hydrolysis rate. Cleavage of the Gly-Phe(NO2) peptide bond in the weakly fluorescent 2-amino-benzoylglycyl-p-nitrophenylalanylproline molecule results in appearance of the 71 times higher fluorescence (lambdamax = 415 nm) of 2-aminobenzoylglycine. Continuous recording of the rising fluorescence allows convenient, sensitive and specific determination of the enzymatic activity, applicable to crude enzyme preparations and human serum. The activity of the mammalian enzyme, measured by this method, is enhanced by Cl- ions and inhibited by low concentrations of EDTA and [Asn1, Val5]angiotensin II. Kinetic measurements showed Michaelis-Menten behavior, Km = 0.21 +/- 0.1 mM and 0.16 +/- 0.1 mM for the calf lung and the bacterial enzyme respectively.

The specificity of cathepsin B. Hydrolysis of glucagon at the C-terminus by a peptidyldipeptidase mechanism

The manner in which human liver cathepsin B (EC 3.4.22.1) digests glucagon was determined. After reaction of the proteinase with the substrate for 24h, more than 15 products were formed. During the first 7 h of reaction, eight products were formed; seven of these were dipeptides that originated from the C-terminal portion of the glucagon molecule, whereas the eighth peptide was the remaining large fragment of the hormone, consisting of residues 1-19. Measurement of the rate of formation of the products showed that cathepsin B degraded glucagon by a sequential cleavage of dipeptides from the C-terminal end of the molecule. Cathepsin B from both rat liver and bovine spleen was shown to hydrolyse glucagon by the same mechanism.

Low molecular weight angiotensin I converting enzyme from rat lung

A low molecular weight angiotensin I converting enzyme (light angiotensin enzyme) was isolated from a homogenate of rat lung subjected to dialysis against sodium acetate at pH 4.8. This enzyme has a molecular weight of 84 000 on Sephadex G-200 and a molecular weight of 91 000 on SDS-poly-acrylamide gel as compared with a molecular weight of 139 000 for angiotensin I converting enzyme on SDS-polyacrylamide. Light angiotensin enzyme was activated by NaCl and inhibited by EDTA, angiotensin II, and bradykinin potentiating factor nonapeptide. Light angiotensin enzyme cross-reacted with antibody prepared against angiotensin I converting enzyme and stained with periodic acid-Schiff reagent as a glycoprotein. The evidence suggests that light angiotensin enzyme is a fragment of the higher molecular weight enzyme.

Conversion of angiotensin I to angiotensin II

The angiotensin I converting enzyme has two important functions: it inactivates bradykinin and converts angiotensin I to angiotensin II. Inhibition of the enzyme blocks the renin-angiotensin system and decreases systemic blood pressure if the pressure is maintained or increased by renin. The enzyme occurs in a variety of tissues and cell forms. The vascular endothelial cells of the lung and of peripheral blood vessels, and the epithelial cells of the kidney tubules are major sources of the enzyme. In addition to inactivating hypotensive peptides and activating a hypertensive one in the systemic circulation, the enzyme may affect organ functions by hydrolyzing peptides that are formed and released locally.

An extracellular aminopeptidase from Clostridium histolyticum

An aminopeptidase was isolated from the culture filtrate of Clostridium histolyticum and purified to homogeneity. Absence of endopeptidase activity in the purified preparation was demonstrated. Gel filtration on a calibrated column indicates an apparent molecular weight of 340000 for the native enzyme. Gel electrophoresis of the denatured enzyme in the presence of dodecylsulfate in constant acrylamide concentration and in a concentration gradient, resulted in the appearance of a single component for which a molecular weight of 51000 and 59000 respectively, was calculated. From mobilities of crosslinked and denatured protein species a molecular weight of 56000 was obtained for the monomer. Specificity studies show that the enzyme cleaves all types of N-terminel amino acid residues including proline and hydroxyproline from small peptides and from polypeptides. The peptide bond formed between an N-terminal amino acid residue and proline is not cleaved by the enzyme. The combined action of aminopeptidase-P and clostridal aminopeptidase leads to complete hydrolysis of the proline-rich nonapeptide bradykinin. Low rates of hydrolysis was observed for charged residues, and amides of amino acids. Kinetic studies with five tripeptides of the general structure X-Gly-Gly, where X stands for Leu, Phe, Val, Ala, or Pro, show a decrease in Km with the increasing size of the hydrophobic side chain of X. The highest Kcat values are observed with proline and alanine. In the series Pro-Gly, Pro-Gly-Pro, Pro-Gly-Pro-Pro, the last peptide is the best substrate, indicating an active site complementary to at least four amino acid residues. The enzymatic activity is dependent on the presence of divalent cations, maximal activation being reached with Mn2+ and Co2+. The optimal pH for the Mn2+ and Co2+- activated enzyme is 8.6 and 8.2 respectively. The optimal temperature is 40 degrees C. Inhibition of the aminopeptidase was achieved with Zn2+, Cu2+ and p-mercuribenzoate, but not with diisopropylphosphofluoridate.