Catalytic activity of carboxypeptidase B and of carboxypeptidase Y with anisylazoformyl substrates

Isolation and Characterization of NpCI, a New Metallocarboxypeptidase Inhibitor from the Marine Snail Nerita peloronta with Anti-Plasmodium falciparum Activity

Metallocarboxypeptidases are zinc-dependent peptide-hydrolysing enzymes involved in several important physiological and pathological processes. They have been a target of growing interest in the search for natural or synthetic compound binders with biomedical and drug discovery purposes, i.e., with potential as antimicrobials or antiparasitics. Given that marine resources are an extraordinary source of bioactive molecules, we screened marine invertebrates for new inhibitory compounds with such capabilities. In this work, we report the isolation and molecular and functional characterization of NpCI, a novel strong metallocarboxypeptidase inhibitor from the marine snail Nerita peloronta. NpCI was purified until homogeneity using a combination of affinity chromatography and RP-HPLC. It appeared as a 5921.557 Da protein with 53 residues and six disulphide-linked cysteines, displaying a high sequence similarity with NvCI, a carboxypeptidase inhibitor isolated from Nerita versicolor, a mollusc of the same genus. The purified inhibitor was determined to be a slow- and tight-binding inhibitor of bovine CPA (Ki = 1.1·× 10^-8 mol/L) and porcine CPB (Ki = 8.15·× 10^-8 mol/L) and was not able to inhibit proteases from other mechanistic classes. Importantly, this inhibitor showed antiplasmodial activity against Plasmodium falciparum in an in vitro culture (IC₅₀ = 5.5 μmol/L), reducing parasitaemia mainly by inhibiting the later stages of the parasite's intraerythrocytic cycle whilst having no cytotoxic effects on human fibroblasts. Interestingly, initial attempts with other related proteinaceous carboxypeptidase inhibitors also displayed similar antiplasmodial effects. Coincidentally, in recent years, a metallocarboxypeptidase named PfNna1, which is expressed in the schizont phase during the late intraerythrocytic stage of the parasite's life cycle, has been described. Given that NpCI showed a specific parasiticidal effect on P. falciparum, eliciting pyknotic/dead parasites, our results suggest that this and related inhibitors could be promising starting agents or lead compounds for antimalarial drug discovery strategies.

Identification of ex vivo catabolites of peptides with doping potential in equine plasma by HILIC-HRMS

Bioactive peptides pose a great threat to sports integrity. The detection of these peptides is essential for enforcing their prohibition in sports. Identifying the catabolites of these peptides that are formed ex vivo in plasma may improve their detection. In the present study, the stability of 27 bioactive peptides with protection at both termini in equine plasma was examined under different incubation conditions, using HILIC coupled to HRMS. Of the 27 peptides, 13 were stable after incubation at 37°C for 72 hr, but the remaining 14 were less stable. Ex vivo catabolites of these 14 peptides were detected using their theoretical masses generated in silico, their appearance was monitored over the time course of incubation, and their identity was verified by their product ion spectra. Catabolites identified for chemotactic peptide, DALDA, dmtDALDA, deltorphins I and II, Hyp⁶ -dermorphin, Lys⁷ -dermorphin, and dermorphin analog are novel. A d-amino acid residue at position 2 or 1 of a peptide or next to its C-terminus protected the relevant terminal from degradation by exopeptidases, but such a residue at position 3 did not. A pGlu residue or N-methylation at the N-terminus of a peptide did not protect its N-terminal. Ethylamide at the C-terminus of a peptide provided the C-terminal protection from attacks by carboxypeptidases. The C-terminal Lys amide in DALDA, dmtDALDA, and Lys⁷ -dermorphin was susceptible to cleavage by plasma enzymes, which is the first report, to the authors' knowledge. The results from the present study provide insights into the stability of peptides in plasma.

A comprehensive approach to detecting multitudinous bioactive peptides in equine plasma and urine using hydrophilic interaction liquid chromatography coupled to high resolution mass spectrometry

Bioactive peptides possess pharmacological effects and can be illicitly used in sports. To deter such misuse, an untargeted method using high resolution mass spectrometry (HRMS) has been developed for comprehensive detection of multitudinous exogenous peptides in equine plasma and urine. Forty-four peptides were extracted using mixed-mode solid-phase extraction (SPE) from plasma and urine, separated with a hydrophilic interaction liquid chromatography (HILIC) column, and detected on an HRMS instrument. Ammonium formate as a mobile phase additive had effects on HILIC retention and charge state distribution of the peptides. The acetonitrile percentage in the reconstitution solution affected the solubility of peptide neat standards and peptides in plasma and urine extracts differently. The stability of the peptides in plasma at ambient temperature was assessed. The limit of detection (LOD) was 10-50 pg/mL for most of the peptides in plasma, and ≤ 500 pg/mL for the remaining. LOD was 100-400 pg/mL for the majority of the analytes in urine, and ≤ 4000 pg/mL for the others. The method was used successfully to analyze incurred plasma and urine samples from research horses administered dermorphin. Even in the absence of reference standards, dermorphin metabolites (aFGYPS-NH₂ , YaFG, and YaF) were identified. These results demonstrate that data generated with this method can be retrospectively reviewed for peptides that are unknown at the time of sample analysis without requiring re-analysis of the sample. This method provides a powerful novel tool for detection of numerous bioactive peptides and their metabolites in equine plasma and urine for doping control.

Fluorescence Detection of Prostate Cancer by an Activatable Fluorescence Probe for PSMA Carboxypeptidase Activity

Prostate cancer (PCa) is a common malignant tumor among adult males, and convenient intraoperative detection of PCa would reduce the risk of leaving positive surgical margins, especially during nerve-sparing procedures. To achieve rapid, fluorescence-based visualization of PCa, we focused on the glutamate carboxypeptidase (CP) activity of prostate-specific membrane antigen (PSMA), a type II transmembrane glycoprotein that is attracting attention as a PCa biomarker. Based on our finding that aryl glutamate conjugates with an azoformyl linker are recognized by PSMA and have a sufficiently low LUMO (lowest unoccupied molecular orbital) energy level to quench the fluorophore through photoinduced electron transfer, we designed and synthesized a first-in-class activatable fluorescence probe for CP activity of PSMA. The developed probe allowed us to visualize the CP activity of PSMA in living cells and in clinical specimens from PCa patients and is expected to be useful for rapid intraoperative detection and diagnosis of PCa.

Tri-domain bifunctional inhibitor of metallocarboxypeptidases A and serine proteases isolated from marine annelid Sabellastarte magnifica

This study describes a novel bifunctional metallocarboxypeptidase and serine protease inhibitor (SmCI) isolated from the tentacle crown of the annelid Sabellastarte magnifica. SmCI is a 165-residue glycoprotein with a molecular mass of 19.69 kDa (mass spectrometry) and 18 cysteine residues forming nine disulfide bonds. Its cDNA was cloned and sequenced by RT-PCR and nested PCR using degenerated oligonucleotides. Employing this information along with data derived from automatic Edman degradation of peptide fragments, the SmCI sequence was fully characterized, indicating the presence of three bovine pancreatic trypsin inhibitor/Kunitz domains and its high homology with other Kunitz serine protease inhibitors. Enzyme kinetics and structural analyses revealed SmCI to be an inhibitor of human and bovine pancreatic metallocarboxypeptidases of the A-type (but not B-type), with nanomolar K(i) values. SmCI is also capable of inhibiting bovine pancreatic trypsin, chymotrypsin, and porcine pancreatic elastase in varying measures. When the inhibitor and its nonglycosylated form (SmCI N23A mutant) were overproduced recombinantly in a Pichia pastoris system, they displayed the dual inhibitory properties of the natural form. Similarly, two bi-domain forms of the inhibitor (recombinant rSmCI D1-D2 and rSmCI D2-D3) as well as its C-terminal domain (rSmCI-D3) were also overproduced. Of these fragments, only the rSmCI D1-D2 bi-domain retained inhibition of metallocarboxypeptidase A but only partially, indicating that the whole tri-domain structure is required for such capability in full. SmCI is the first proteinaceous inhibitor of metallocarboxypeptidases able to act as well on another mechanistic class of proteases (serine-type) and is the first of this kind identified in nature.

A novel metallocarboxypeptidase-like enzyme from the marine annelid Sabellastarte magnifica--a step into the invertebrate world of proteases

After screening 25 marine invertebrates, a novel metallocarboxypeptidase (SmCP) has been identified by activity and MS analytical approaches, and isolated from the marine annelid Sabellastarte magnifica. The enzyme, which is a minor component of the molecularly complex animal body, as shown by 2D gel electrophoresis, has been purified from crude extracts to homogeneity by affinity chromatography on potato carboxypeptidase inhibitor and by ion exchange chromatography. SmCP is a protease of 33792 Da, displaying N-terminal and internal sequence homologies with M14 metallocarboxypeptidase-like enzymes, as determined by MS and automated Edman degradation. The enzyme contains one atom of Zn per molecule, is activated by Ca2+ and is drastically inhibited by the metal chelator 1,10-phenanthroline, as well as by excess Zn2+ or Cu2+, but moderately so by EDTA. SmCP is also strongly inhibited by specific inhibitors of metallocarboxypeptidases, such as benzylsuccinic acid and the protein inhibitors found in potato and leech (i.e. recombinant forms, both at nanomolar levels). The enzyme displays high peptidase efficiency towards pancreatic carboxypeptidase-A synthetic substrates, such as those with hydrophobic residues at the C-terminus but, remarkably, also towards the acidic ones. This property, previously described as for carboxypeptidase O-like activity, has been shown on long peptide substrates by MS. The results obtained in the present study indicate that SmCP is a novel member of the M14 metallocarboxypeptidases family (assignable to the M14A or pancreatic-like subfamily) with a wider specificity that has not been described previously.

An assay for measuring functional activated thrombin-activatable fibrinolysis inhibitor in plasma

Thrombin-activatable fibrinolysis inhibitor (TAFI), also called procarboxypeptidase U (proCPU), is a plasma zymogen that can be activated by thrombin, the thrombin-thrombomodulin complex, or plasmin. The activated form of TAFI (TAFIa, CPU) removes C-terminal lysine residues of plasmin-modified fibrin (FN') that mediates a positive feedback mechanism in plasminogen (Pg) activation, thereby attenuating fibrinolysis. The plasma concentration of TAFI is approximately 75 nM. Because the half-maximal effect of TAFIa occurs at 1 nM, only approximately 1.3% of TAFI needs to be activated to exert an effect on clot lysis. The assay is performed by mixing soluble FN' covalently attached to a quencher and fluorescein-labeled Pg. The sample containing TAFIa is then added, and the rate of fluorescence increase due to removal of C-terminal lysine from FN' and loss of Pg binding is measured with a fluorescence plate reader. The assay was shown to be sensitive for TAFIa at a concentration as low as 12 pM. The intraassay variability and interassay variability of the assay were 6.3 and 8.3%, respectively. This assay was not confounded by the naturally occurring TAFI Thr325Leu polymorphism that affects the thermal stability of TAFIa or endogenous plasminogen in plasma.

Measurement of procarboxypeptidase U (TAFI) in human plasma: a laboratory challenge

BACKGROUND

The importance of carboxypeptidase U (CPU) as a novel regulator of the fibrinolytic rate has attracted much interest during recent years. CPU circulates in plasma as a zymogen, proCPU, that can be activated by thrombin, thrombin-thrombomodulin (T-Tm), or plasmin. Given that the proCPU concentration in plasma is far below its K(m) for activation by the T-Tm complex, the formation of CPU will be directly proportional to the proCPU concentration. A low or high proCPU plasma concentration might therefore tip the balance between profibrinolytic and antifibrinolytic pathways and thereby cause a predisposition to bleeding or thrombosis.

CONTENT

To measure plasma proCPU concentrations, different methods have been developed based on 2 different principles: antigen determination and measurement of CPU activity after quantitative conversion of the proenzyme to its active form by addition of T-Tm. The major drawbacks that should be kept in mind when analyzing clinical samples by both principles are reviewed.

CONCLUSIONS

proCPU is a potential prothrombotic risk factor. Evaluation of its relationship with thrombosis requires accurate assays. Many assays used in different clinical settings are inadequately validated, forcing reconsideration of conclusions made in these reports.

Reversible inhibitors of TAFIa can both promote and inhibit fibrinolysis

The plasma carboxypeptidase activated thrombin-activable fibrinolysis inhibitor (TAFIa), is thermally unstable at 37 degrees C, with a half-life of 8 or 15 min depending on the isoform. The arginine analog, 2-guanidinoethylmercaptosuccinate (GEMSA), not only inhibits TAFIa but also slows the spontaneous inactivation of the enzyme, thereby reducing the activity of TAFIa, while extending its apparent half-life. Because, as shown in previous work, the ability of TAFIa to prolong clot lysis can be more dependent on its half-life than its concentration, in this study we determined whether reversible inhibitors of TAFIa could paradoxically prolong clot lysis. Potato tuber carboxypeptidase inhibitor (PTCI) or GEMSA were titrated into normal pooled human plasma, in the presence of soluble thrombomodulin. Both inhibitors mediate a biphasic antifibrinolytic effect, prolonging clot lysis at lower concentrations and enhancing clot lysis at higher concentrations. The antifibrinolytic effect of GEMSA is maximized at 1 mmol L-1, increasing clot lysis time from 100 min to 350 min. The antifibrinolytic effect of PTCI is maximized at 100 nmol L-1, increasing clot lysis time from 100 min to 240 min. To further characterize the nature of this biphasic effect, TAFI at various concentrations was added to TAFI-immunodepleted human plasma in the presence of PTCI or GEMSA. The magnitude of the effect depends on the concentration of TAFIa, the concentration of inhibitor, and the potency of the inhibitor. We propose that the biphasic antifibrinolytic effect is mediated by the dynamic equilibrium of free TAFIa that inactivates quickly, and TAFIa bound to inhibitor that inactivates slowly. TAFIa inhibitors used as therapeutic agents might not only enhance lysis at higher concentrations, but also stabilize fibrin clots at intermediate concentrations.

Anisylazoformylarginine: a superior assay substrate for carboxypeptidase B type enzymes

Anisylazoformylarginine (CH(3)OC(6)H(4)-N=N-CO-Arg-OH) is rapidly hydrolyzed at the acyl-arginine linkage by the zinc-enzyme porcine carboxypeptidase B. The catalytic reaction is readily monitored spectrophotometrically by disappearance of the intense absorption (349 nm, epsilon 19,100) of the azo chromophore, which chemically fragments after substrate cleavage.

Amino acid residues in the P6-P'3 region of thrombin-activable fibrinolysis inhibitor (TAFI) do not determine the thrombomodulin dependence of TAFI activation

Thrombin bound to thrombomodulin activates thrombin-activable fibrinolysis inhibitor (TAFI) and protein C much more efficiently than thrombin alone. Although thrombomodulin has been proposed to alter the thrombin active site, the recently determined structure of the thrombin-thrombomodulin complex does not support this proposal. In this study, the contribution of amino acids near the activation site of TAFI toward thrombomodulin dependence was determined, utilizing four variants of TAFI with specific substitutions in the P6-P'3 region surrounding the Arg-92 cleavage site. Two point mutants had either the Ser-90 or Asp-87 of TAFI replaced with Ala, a third mutant had the thrombin activation site of the fibrinogen Bbeta-chain substituted into positions 91-95 of TAFI, and a fourth mutant had the thrombin activation site of protein C substituted into positions 90-95 of TAFI. Each of these mutants was expressed, purified, and characterized with respect to activation kinetics and functional properties of the enzyme. Even though fibrinogen is poorly cleaved by thrombin-thrombomodulin, the fibrinogen activation site does not significantly alter the thrombomodulin dependence of TAFI activation. The TAFI variant with the protein C activation sequence is only slowly activated by thrombin-thrombomodulin, and not at all by free thrombin. Mutating Asp-87 to Ala increases the catalytic efficiency of activation 3-fold both in the presence and absence of thrombomodulin, whereas mutating Ser-90 to Ala effects only minor kinetic differences compared with wild type TAFI. The thermal stabilities and antifibrinolytic properties of the enzymes were not substantially altered by any of the mutations that allowed for efficient activation of the enzyme. We conclude that residues in the P6-P'3 region of TAFI do not determine the thrombomodulin dependence of activation, which lends support to the argument that the role of thrombomodulin is to optimally orient thrombin and its substrate, rather than to allosterically alter the specificity of the thrombin active site.

Two naturally occurring variants of TAFI (Thr-325 and Ile-325) differ substantially with respect to thermal stability and antifibrinolytic activity of the enzyme

Thrombin-activable fibrinolysis inhibitor (TAFI) is a carboxypeptidase B-like zymogen that is activated to TAFIa by plasmin, thrombin, or the thrombin-thrombomodulin complex. The enzyme TAFIa attenuates clot lysis by removing lysine residues from a fibrin clot. Screening of nine human cDNA libraries indicated a common variation in TAFI at position 325 (Ile-325 or Thr-325). This is in addition to the variation at amino acid position 147 (Ala-147 or Thr-147) characterized previously. Thus, four variants of TAFI having either Ala or Thr at position 147 and either Thr or Ile at position 325 were stably expressed in baby hamster kidney cells and purified to homogeneity. The kinetics of activation of TAFI by thrombin/thrombomodulin were identical for all four variants; however, Ile at position 325 extended the half-life of TAFIa from 8 to 15 min at 37 degrees C, regardless of the residue at position 147. In clot lysis assays with thrombomodulin and the TAFI variants, or with pre-activated TAFI variants, the Ile-325 variants exhibited an antifibrinolytic effect that was 60% greater than the Thr-325 variants. Similarly, in the absence of thrombomodulin, the Ile-325 variants exhibited an antifibrinolytic effect that was 30-50% greater than the Thr-325 variants. In contrast, the variation at position 147 had little if any effect on the antifibrinolytic potential of TAFIa. The increased antifibrinolytic potential of the Ile-325-containing TAFI variants reflects the fact that these variants have an increased ability to mediate the release of lysine from partially degraded fibrin and suppress plasminogen activation. These findings imply that individuals homozygous for the Ile-325 variant of TAFI would likely have a longer lived and more potent TAFIa enzyme than those homozygous for the Thr-325 variant.