Activated human plasma carboxypeptidase B is retained in the blood by binding to alpha2-macroglobulin and pregnancy zone protein

α2-Macroglobulins: Structure and Function

α₂-macroglobulins are broad-spectrum endopeptidase inhibitors, which have to date been characterised from metazoans (vertebrates and invertebrates) and Gram-negative bacteria. Their structural and biochemical properties reveal two related modes of action: the "Venus flytrap" and the "snap-trap" mechanisms. In both cases, peptidases trigger a massive conformational rearrangement of α₂-macroglobulin after cutting in a highly flexible bait region, which results in their entrapment. In some homologs, a second action takes place that involves a highly reactive β-cysteinyl-γ-glutamyl thioester bond, which covalently binds cleaving peptidases and thus contributes to the further stabilization of the enzyme:inhibitor complex. Trapped peptidases are still active, but have restricted access to their substrates due to steric hindrance. In this way, the human α₂-macroglobulin homolog regulates proteolysis in complex biological processes, such as nutrition, signalling, and tissue remodelling, but also defends the host organism against attacks by external toxins and other virulence factors during infection and envenomation. In parallel, it participates in several other biological functions by modifying the activity of cytokines and regulating hormones, growth factors, lipid factors and other proteins, which has a great impact on physiology. Likewise, bacterial α₂-macroglobulins may participate in defence by protecting cell wall components from attacking peptidases, or in host-pathogen interactions through recognition of host peptidases and/or antimicrobial peptides. α₂-macroglobulins are more widespread than initially thought and exert multifunctional roles in both eukaryotes and prokaryotes, therefore, their on-going study is essential.

Fibroblastic synoviocytes secrete plasma proteins via α2 -macroglobulins serving as intracellular and extracellular chaperones

Changes in plasma protein levels in synovial fluid (SF) have been implicated in osteoarthritis and rheumatoid arthritis. It was previously thought that the presence of plasma proteins in SF reflected ultrafiltration or extravasation from the vasculature, possibly due to retraction of inflamed endothelial cells. Recent proteomic analyses have confirmed the abundant presence of plasma proteins in SF from control and arthritic patients. Systematic depletion of high-abundance plasma proteins from SF and conditioned media from synoviocytes cultured in serum, and protein analysis under denaturing/reducing conditions have limited our understanding of sources and the native structures of "plasma protein" complexes in SF. Using Western blotting, qPCR, and mass spectrometry, we found that Hig-82 lapine fibroblastic synovicytes cultured under serum-free conditions expressed and secreted plasma proteins, including the cytokine-binding protein secreted phosphoprotein 24 kDa (Spp24) and many of the proteases and protease inhibitors found in SF. Treating synoviocytes with TGF-β1 or BMP-2 for 24 h upregulated the expression of plasma proteins, including Spp24, α2 -HS-glycoprotein, α1 -antitrypsin, IGF-1, and C-reactive protein. Furthermore, many of the plasma proteins of mass <151 kDa were secreted as disulfide-bound complexes with members of the α2 -macroglobulin (A2M) family, which serve as intracellular and extracellular chaperones, not protease inhibitors. Using brefeldin A to block vesicular traffic and protease inhibitors to inhibit endogenous activation of naïve A2M, we demonstrated that the complexes were formed in the endoplasmic reticulum lumen and that Ca(2+) cysteine protease-dependent processes are involved.

Selective modulation of thrombin-activatable fibrinolysis inhibitor (TAFI) activation by thrombin or the thrombin-thrombomodulin complex using TAFI-derived peptides

BACKGROUND

Thrombin-activatable fibrinolysis inhibitor (TAFI) is a risk factor for coronary heart disease. TAFI is proteolytically activated by thrombin, the thrombin-thrombomodulin complex and plasmin. Once active, it dampens fibrinolysis and inflammation. The aim of this study was to generate TAFI-derived peptides that specifically modulate TAFI activation and activity.

METHODS

Thirty-four overlapping TAFI peptides, and modifications thereof, were synthesized. The effects of these peptides on TAFI activation and TAFIa activity were determined. In addition, the binding of the peptides to thrombin were determined.

RESULTS

Four peptides (peptides 2, 18, 19 and 34) inhibited TAFI activation and two peptides (peptides 14 and 24) inhibited TAFIa activity directly. Peptide 2 (Arg12-Glu28) and peptide 34 (Cys383-Val401) inhibited TAFI activation by the thrombin-thrombomodulin complex with IC50 values of 7.3 ± 1.8 and 6.1 ± 0.9 μm, respectively. However, no inhibition was observed in the absence of thrombomodulin. This suggests that the regions Arg12-Glu28 and Cys383-Val401 in TAFI are involved in thrombomodulin-mediated TAFI activation. Peptide 18 (Gly205-Ser221) and peptide 19 (Arg214-Asp232) inhibited TAFI activation by thrombin and the thrombin-thrombomodulin complex. Furthermore, these peptides bound to thrombin (KD : 1.5 ± 0.4 and 0.52 ± 0.07 μm for peptides 18 and 19, respectively), suggesting that Gly205-Asp232 of TAFI is involved in binding to thrombin. Peptide 14 (His159-His175) inhibited TAFIa activity. The inhibition was TAFIa specific, because no effect on the homologous enzyme carboxypeptidase B was observed.

CONCLUSIONS

Thrombin-activatable fibrinolysis inhibitor-derived peptides show promise as new tools to modulate TAFI activation and TAFIa activity. Furthermore, these peptides revealed potential binding sites on TAFI for thrombin and the thrombin-thrombomodulin complex.

Elevated cytokines, thrombin and PAI-1 in severe HCPS patients due to Sin Nombre virus

Sin Nombre Hantavirus (SNV, Bunyaviridae Hantavirus) is a Category A pathogen that causes Hantavirus Cardiopulmonary Syndrome (HCPS) with case fatality ratios generally ranging from 30% to 50%. HCPS is characterized by vascular leakage due to dysregulation of the endothelial barrier function. The loss of vascular integrity results in non-cardiogenic pulmonary edema, shock, multi-organ failure and death. Using Electric Cell-substrate Impedance Sensing (ECIS) measurements, we found that plasma samples drawn from University of New Mexico Hospital patients with serologically-confirmed HCPS, induce loss of cell-cell adhesion in confluent epithelial and endothelial cell monolayers grown in ECIS cultureware. We show that the loss of cell-cell adhesion is sensitive to both thrombin and plasmin inhibitors in mild cases, and to thrombin only inhibition in severe cases, suggesting an increasing prothrombotic state with disease severity. A proteomic profile (2D gel electrophoresis and mass spectrometry) of HCPS plasma samples in our cohort revealed robust antifibrinolytic activity among terminal case patients. The prothrombotic activity is highlighted by acute ≥30 to >100 fold increases in active plasminogen activator inhibitor (PAI-1) which, preceded death of the subjects within 48 h. Taken together, this suggests that PAI-1 might be a response to the severe pathology as it is expected to reduce plasmin activity and possibly thrombin activity in the terminal patients.

α-2-Macroglobulin: a physiological guardian

Alpha macroglobulins are large glycoproteins which are present in the body fluids of both invertebrates and vertebrates. Alpha-2-macroglobulin (α2 M), a key member of alpha macroglobulin superfamily, is a high-molecular weight homotetrameric glycoprotein. α2 M has many diversified and complex functions, but it is primarily known by its ability to inhibit a broad spectrum of proteases without the direct blockage of the protease active site. α2 M is also known to be involved in the regulation, transport, and a host of other functions. For example, apart from inhibiting proteinases, it regulates binding of transferrin to its surface receptor, binds defensin and myelin basic protein, etc., binds several important cytokines, including basic fibroblast growth factor (bFGF), platelet-derived growth factor (PDGF), nerve growth factor (NGF), interleukin-1β (IL-1β), and interleukin-6 (IL-6), and modify their biological activity. α2 M also binds a number of hormones and regulates their activity. α2 M is said to protect the body against various infections, and hence, can be used as a biomarker for the diagnosis and prognosis of a number of diseases. However, this multipurpose antiproteinse is not "fail safe" and could be damaged by reactive species generated endogenously or exogenously, leading to various pathophysiological conditions.

Design and characterization of a selenium-containing inhibitor of activated thrombin-activatable fibrinolysis inhibitor (TAFIa), a zinc-containing metalloprotease

Available therapies for thromboembolic disorders include thrombolytics, anticoagulants, and antiplatelets, but these are associated with complications such as bleeding. To develop an alternative drug which is clinically safe, we focused on activated thrombin-activatable fibrinolysis inhibitor (TAFIa) as the target molecule. TAFIa is a zinc-containing carboxypeptidase that significantly inhibits fibrinolysis. Here we designed and synthesized selenium-containing compounds 5-13 to discover novel TAFIa inhibitors having a superior zinc-coordinating group. Compounds 5-13 significantly inhibited TAFIa activity (IC(50) 2.2 × 10(-12) M - 2.6 × 10(-6) M). We found that selenol is a better functional group than thiol for coordinating to zinc at the active site of TAFIa. Furthermore, compound 12, which has an amino-chloro-pyridine ring, was found to be a potent and selective TAFIa inhibitor that lacks carboxypeptidase N inhibitory activity. Therefore, compound 12 is a promising candidate for the treatment of thromboembolic disorders. This is the first report of a selenium-containing inhibitor for TAFIa.

Non-invasive detection of candidate pregnancy protein biomarkers in the feces of captive polar bears (Ursus maritimus)

Currently, there is no method of accurately and non-invasively diagnosing pregnancy in polar bears. Specific proteins may exhibit altered profiles in the feces of pregnant bears, but predicting appropriate candidate proteins to investigate is speculative at best. The objective of this study was to identify potential pregnancy biomarker proteins based on their increased abundance in the feces of pregnant polar bears compared to pseudopregnant females (controls) using two-dimensional in-gel electrophoresis (2D-DIGE) and mass spectrometry (MS). Three 2D-DIGE gels were performed to evaluate fecal protein profiles from controls (n=3) and pregnant polar bears (n=3). There were 2224.67±52.39 (mean±SEM) spots resolved per gel. Of these, only five proteins were elevated in the pregnant group (P<0.05), and seven additional spots tended to be higher (0.05<P<0.10). All 12 were submitted for MS analysis and the identities of 11 were ascertained with a >99.9% confidence interval. The 11 spots represented seven distinct proteins, five of which were significantly more abundant in the pregnant group: IgGFc-binding protein, filamin-C, carboxypeptidase B, transthyretin, and immunoglobulin heavy chain variable region. To our knowledge, this was the first study that employed 2D-DIGE to identify differentially expressed proteins in fecal samples to characterize a physiological condition other than those related to gastrointestinal disorders. These promising results provided a strong foundation for ensuing efforts to develop a non-invasive pregnancy assay for use in both captive and wild polar bears.

Flexibility of the thrombin-activatable fibrinolysis inhibitor pro-domain enables productive binding of protein substrates

We have previously reported that thrombin-activatable fibrinolysis inhibitor (TAFI) exhibits intrinsic proteolytic activity toward large peptides. The structural basis for this observation was clarified by the crystal structures of human and bovine TAFI. These structures evinced a significant rotation of the pro-domain away from the catalytic moiety when compared with other pro-carboxypeptidases, thus enabling access of large peptide substrates to the active site cleft. Here, we further investigated the flexible nature of the pro-domain and demonstrated that TAFI forms productive complexes with protein carboxypeptidase inhibitors from potato, leech, and tick (PCI, LCI, and TCI, respectively). We determined the crystal structure of the bovine TAFI-TCI complex, revealing that the pro-domain was completely displaced from the position observed in the TAFI structure. It protruded into the bulk solvent and was disordered, whereas TCI occupied the position previously held by the pro-domain. The authentic nature of the presently studied TAFI-inhibitor complexes was supported by the trimming of the C-terminal residues from the three inhibitors upon complex formation. This finding suggests that the inhibitors interact with the active site of TAFI in a substrate-like manner. Taken together, these data show for the first time that TAFI is able to form a bona fide complex with protein carboxypeptidase inhibitors. This underlines the unusually flexible nature of the pro-domain and implies a possible mechanism for regulation of TAFI intrinsic proteolytic activity in vivo.

Insights into the molecular inactivation mechanism of human activated thrombin-activatable fibrinolysis inhibitor

SUMMARY BACKGROUND

Thrombin-activatable fibrinolysis inhibitor (TAFI) is a validated target for thrombotic diseases. TAFI is converted in vivo to activated TAFI (TAFIa) by removal of its pro-domain. Whereas TAFI is stable and persists in the circulation, possibly in complex with plasminogen, TAFIa is unstable and poorly soluble, with a half-life of minutes.

OBJECTIVES

In order to study the molecular determinants of this instability, we studied the influence of protein inhibitors on human TAFIa.

RESULTS

We found that protein inhibitors significantly reduced the instability and insolubility of TAFIa. In addition, we solved the 2.5-A resolution crystal structure of human TAFIa in complex with a potent protein inhibitor, tick-derived carboxypeptidase inhibitor, which gives rise to a stable and soluble TAFIa species. The structure revealed a significant reduction in the flexibility of dynamic segments when compared with the structures of bovine and human TAFI. We also identified two latent hotspots, loop Lbeta2beta3 and segment alpha5-Lalpha5beta7-beta7, where conformational destabilization may begin. These hotspots are also present in TAFI, but the pro-domain may provide sufficient stabilization and solubility to guarantee protein persistence in vivo. When the pro-domain is removed, the free TAFIa moiety becomes unstable, its activity is suppressed, and the molecule becomes insoluble.

CONCLUSIONS

The present study corroborates the function of protein inhibitors in stabilizing human TAFIa and it provides a rigid and high-resolution mold for the design of small molecule inhibitors of this enzyme, thus paving the way for novel therapy for thrombotic disorders.

Biochemical characterization of bovine plasma thrombin-activatable fibrinolysis inhibitor (TAFI)

Background

TAFI is a plasma protein assumed to be an important link between coagulation and fibrinolysis. The three-dimensional crystal structures of authentic mature bovine TAFI (TAFIa) in complex with tick carboxypeptidase inhibitor, authentic full lenght bovine plasma thrombin-activatable fibrinolysis inhibitor (TAFI), and recombinant human TAFI have recently been solved. In light of these recent advances, we have characterized authentic bovine TAFI biochemically and compared it to human TAFI.

Results

The four N-linked glycosylation sequons within the activation peptide were all occupied in bovine TAFI, similar to human TAFI, while the sequon located within the enzyme moiety of the bovine protein was non-glycosylated. The enzymatic stability and the kinetic constants of TAFIa differed somewhat between the two proteins, as did the isoelectric point of TAFI, but not TAFIa. Equivalent to human TAFI, bovine TAFI was a substrate for transglutaminases and could be proteolytically cleaved by trypsin or thrombin/solulin complex, although small differences in the fragmentation patterns were observed. Furthermore, bovine TAFI exhibited intrinsic activity and TAFIa attenuated tPA-mediated fibrinolysis similar to the human protein.

Conclusion

The findings presented here suggest that the properties of these two orthologous proteins are similar and that conclusions reached using the bovine TAFI may be extrapolated to the human protein.

The crystal structure of thrombin-activable fibrinolysis inhibitor (TAFI) provides the structural basis for its intrinsic activity and the short half-life of TAFIa

Mature thrombin-activable fibrinolysis inhibitor (TAFIa) is a highly unstable metallocarboxypeptidase that stabilizes blood clots by clipping C-terminal lysine residues from partially degraded fibrin. In accordance with its in vitro antifibrinolytic activity, animal studies have reported that inhibition of mature TAFI aids in the prevention of thrombosis. The level of TAFI activity is stringently regulated through (i) controlled proteolytic truncation of the zymogen (TAFI), generating the mature enzyme, TAFIa, and (ii) the short half-life of TAFIa. TAFI itself exhibits an intrinsic enzymatic activity, which is likely required to provide a baseline level of antifibrinolytic activity. The novel crystal structure presented here reveals that the active site of TAFI is accessible, providing the structural explanation for the its intrinsic activity. It also supports the notion that an "instability region" exists, in agreement with site-directed mutagenesis studies. Sulfate ions, bound to this region, point toward a potential heparin-binding site and could explain how heparin stabilizes TAFIa.

Prostate cancer serum biomarker discovery through proteomic analysis of alpha-2 macroglobulin protein complexes

Alpha-2 macroglobulin (A2M) functions as a universal protease inhibitor in serum and is capable of binding various cytokines and growth factors. In this study, we investigated if immunoaffinity enrichment and proteomic analysis of A2M protein complexes from human serum could improve detection of biologically relevant and novel candidate protein biomarkers in prostate cancer. Serum samples from six patients with androgen-independent, metastatic prostate cancer and six control patients without malignancy were analyzed by immunoaffinity enrichment of A2M protein complexes and MS identification of associated proteins. Known A2M substrates were reproducibly identified from patient serum in both cohorts, as well as proteins previously undetected in human serum. One example is heat shock protein 90 alpha (HSP90α), which was identified only in the serum of cancer patients in this study. Using an ELISA, the presence of HSP90α in human serum was validated on expanded test cohorts and found to exist in higher median serum concentrations in prostate cancer (n = 18) relative to control (n = 13) patients (median concentrations 50.7 versus 27.6 ng/mL, respectively, p = 0.001). Our results demonstrate the technical feasibility of this approach and support the analysis of A2M protein complexes for proteomic-based serum biomarker discovery.

Thrombin-activable Fibrinolysis Inhibitor (TAFI) Zymogen Is an Active Carboxypeptidase

Thrombin-activable fibrinolysis inhibitor (TAFI) is a carboxypeptidase found in human plasma, presumably as an inactive zymogen. The current dogma is that proteolytic activation by thrombin/thrombomodulin generates the active enzyme (TAFIa), which down-regulates fibrinolysis by removing C-terminal lysine residues from partially degraded fibrin. In this study, we have shown that the zymogen exhibits continuous and stable carboxypeptidase activity against large peptide substrates, and we suggest that the activity down-regulates fibrinolysis in vivo.

Post-translational modifications of human thrombin-activatable fibrinolysis inhibitor (TAFI): evidence for a large shift in the isoelectric point and reduced solubility upon activation

Thrombin-activable fibrinolysis inhibitor (TAFI) is distinct from pancreatic procarboxypeptidase B in several ways. The enzymatic activity of TAFIa is unstable and decays with a half-life of a few minutes. During this study, we observed that (i) the isoelectric point (pI) of TAFI shifts dramatically from pH 5 toward pH 8 upon activation and (ii) TAFIa is significantly less soluble than TAFI. The structural bases for these observations were investigated by characterizing all post-translational modifications, including attached glycans and disulfide connectivity. The analyses revealed that all five potential N-glycosylation sites were utilized including Asn22, Asn51, Asn63, Asn86 (located in the activation peptide), and Asn219 (located in the catalytic domain). Asn219 was also found in an unglycosylated variant. Four of the glycans, Asn51, Asn63, Asn86, and Asn219 displayed microheterogeneity, while the glycan attached to Asn22 appeared to be homogeneous. In addition, bisecting GlcNAc attached to the trimannose core was detected, suggesting an origin other than the liver. Monosaccharide composition and LC-MS/MS analyses did not produce evidence for O glycosylation. TAFI contains eight cysteine residues, of which two, Cys69 and Cys383, are not involved in disulfides and contain free sulfhydryl groups. The remaining six cystines form disulfides, including Cys156-Cys169, Cys228-Cys252, and Cys243-Cys257. This pattern is homologous to pancreatic procarboxypeptidase B, and it is therefore unlikely that permutations in the cysteine connectivity are responsible for the enzymatic instability. LC-MS/MS analyses covering more than 90% of the TAFI amino acid sequence revealed no additional modifications. When these results are taken together, they suggest that the inherent instability of TAFIa is not caused by post-translational modifications. However, after activation, TAFIa loses 80% of the attached glycans, generating a large shift in pI and a propensity to precipitate. These changes are likely to significantly affect the properties of TAFIa as compared to TAFI.

Generation of a stable activated thrombin activable fibrinolysis inhibitor variant

Activated thrombin activable fibrinolysis inhibitor (TAFIa), generated upon activation of TAFI, exerts an antifibrinolytic effect. TAFIa is a thermolabile enzyme, inactivated through a conformational change. The objective of the current study was to generate a stable variant of human TAFIa. Using a site-directed as well as a random mutagenesis approach to generate a library of TAFI mutants, we identified two mutations that increase TAFIa stability, i.e. a Ser305 to Cys and a Thr329 to Ile mutation, respectively. Combining these mutations in TAFI-Ala147-Ile325, the most stable isoform of TAFIa (half-life of 9.4 +/- 0.4 min), revealed a TAFIa half-life of 70 +/- 3.1 min (i.e. an 11-fold increase versus 6.3 +/- 0.3 min for TAFIa-Ala147-Thr325, the most frequently occurring isoform of TAFI in humans) at 37 degrees C. Moreover, clot lysis (induced by tissue plasminogen activator) experiments in which TAFI-Ala147-Cys305-Ile325-Ile329 was added to TAFI-depleted plasma revealed a 50% clot lysis time of 313 +/- 77 min (i.e. a 3.0-fold increase versus 117 +/- 10 min for TAFI-Ala147-Thr325). The availability of a more stable TAFIa variant will facilitate the search for inhibitors and allow further structural analysis to elucidate the mechanisms of the instability of TAFIa.

Development of ELISAs measuring the extent of TAFI activation

OBJECTIVE

To date, quantitation of TAFI antigen levels has been mainly focused on "total" antigen levels and has been shown to yield ambiguous results because of the existence of different isoforms and various degrees of activation. Our objective was to develop assays that allow measuring the extent of TAFI activation.

METHODS AND RESULTS

A variety of enzyme-linked immunosorbent assays (ELISAs) were evaluated for their preferential reactivity toward TAFI before and after activation, and toward the recombinantly expressed activation peptide. Three ELISAs with distinct reactivities were selected: recognizing either exclusively nonactivated TAFI, the released activation peptide, or exclusively TAFIa (activated TAFI). Evaluation of TAFI activation during clot lysis revealed that decreases of TAFI levels are associated with increases of the released activation peptide and TAFIa levels. In addition, antigenic measurement of TAFIa parallels activity measured by chromogenic assay. Analyzing plasma samples revealed that subjects with hyperlipidemia had significantly higher plasma levels of both the activation peptide (109.2 versus 95.5; P<0.001) and TAFIa (112.1 versus 103.3; P=0.03), and not of TAFI antigen (92.5 versus 87.9; P=0.07) (results in % of plasma pooled from normolipidemic subjects).

CONCLUSIONS

ELISAs that allow to measure the extent of TAFI activation were developed. These ELISAs constitute more sensitive markers in studies on the relationship between TAFI and cardiovascular diseases.

alpha2-Macroglobulin in the marine fish Sparus aurata

The alpha2-macroglobulin proteinase inhibitors (alpha2Ms) are a family of plasma proteins with the unique ability to inhibit a broad spectrum of proteinases, but are also known as binding proteins for many growth factors and cytokines, including growth hormone and members of the transforming growth factor-beta superfamily. A partial cDNA (475 amino acids) encoding the C-terminus of alpha2M was cloned from the liver of the marine teleostean fish Sparus aurata. The deduced amino acid sequence of the cloned fragment showed 58-60% similarity to carp alpha2Ms. Northern blot analysis of hepatic alpha2M revealed a transcript of about 5 kb. A transcript of a similar size was detected in 1-day larvae. Steady state levels of alpha2M in larvae increased gradually on subsequent days post-hatching. alpha2M expression in embryos was determined by RT-PCR and started in embryos aged 8 h post-fertilization, but not earlier. RT-PCR of muscle RNA detected alpha2M also in fish muscle, albeit with a lower expression than in the liver. Immunoreactive-alpha2M was found in yolk syncytial layer of 3-day larvae and in livers from larvae and adults. Immunoreactive-alpha2M was also identified in soluble total proteins from young larvae with a pattern resembling that of plasma. These data demonstrate that the alpha2M gene is expressed early in fish development. Moreover, in addition to its major expression in liver, alpha2M is expressed also in fish muscle.

Antithrombotic agents in the treatment of severe sepsis

Sepsis, a systemic inflammatory syndrome, is a response to infection and when associated with multiple organ dysfunction is termed severe sepsis. It remains a leading cause of mortality in the critically ill. The response to the invading microorganisms may be considered as a balance between a pro-inflammatory and an anti-inflammatory reaction. While an inadequate pro-inflammatory reaction and a strong anti-inflammatory response could lead to overwhelming infection and the death of the patient, a strong and uncontrolled pro-inflammatory response, manifested by the release of pro-inflammatory mediators may lead to microvascular thrombosis and multiple organ failure. Endotoxin triggers sepsis via the release of various mediators such as tumour necrosis factor-alpha and interleukin-1 (IL-1). These cytokines activate the complement and coagulation systems, release adhesion molecules, prostaglandins, leukotrienes, reactive oxygen species and nitric oxide. Other mediators involved in the sepsis syndrome include IL-1, -6 and -8; arachidonic acid metabolites; platelet activating factor; histamine; bradykinin; angiotensin; complement components and vasoactive intestinal peptide. These pro-inflammatory responses are counteracted by IL-10. Most of the trials targeting the different mediators of the pro-inflammatory response have failed due to a lack of correct definition of sepsis. Understanding the exact pathophysiology of the disease will enable more advanced treatment options. Targeting the coagulation system with various anticoagulant agents including, activated protein C, and tissue factor pathway inhibitor (TFPI) is a rational approach. Many clinical trials have been conducted to evaluate these agents in severe sepsis. While trials on antithrombin and TFPI were not so successful, the double-blind, placebo-controlled, Phase III trial of recombinant human activated Protein C Worldwide Evaluation in Severe Sepsis (PROWESS) was successful, creating a significant decrease in mortality when compared to the placebo group. A better understanding of the pathophysiologic mechanism of severe sepsis will provide better treatment options, and combination antithrombotic treatment may provide a multipronged approach for the treatment of severe sepsis.

Migration of the activation peptide of thrombin-activatable fibrinolysis inhibitor (TAFI) during SDS-polyacrylamide gel electrophoresis

Thrombin-activatable fibrinolysis inhibitor (TAFI) is a plasma zymogen, which upon activation is capable of delaying fibrinolysis. We investigated the migration and detection of the activation peptide of TAFI during SDS-polyacrylamide gel electrophoresis (SDS-PAGE). Purified TAFI before and after activation by thrombin/thrombomodulin was electrophoresed on 4-20% polyacrylamide gels and stained with Coomassie blue as well as Western blotting. Before activation, Coomassie blue staining resulted in one main band of TAFI. After activation, a sharp band corresponding to TAFIa was observed. No distinct activation peptide was detected, in agreement with the literature. Western blotting using a polyclonal anti-TAFI antibody, on the other hand, showed one additional broad band with an Mr of about 33 000 after TAFI activation. N-terminal sequence analysis confirmed that this band represented the activation peptide of TAFI. In addition, we tested the reactivity of two anti-TAFI monoclonal antibodies (MA-T3D8 and MA-T18A8) towards TAFI before and after activation by Western blotting. Both monoclonal antibodies recognized TAFI. After activation of TAFI, MA-T3D8 reacted with TAFIa, while MA-T18A8 reacted with the activation peptide. We identify the 33 000 band as the activation peptide of TAFI and exemplify the use of this information for the characterization of monoclonal antibodies against TAFI.

New insights into factors affecting clot stability: a role for thrombin activatable fibrinolysis inhibitor (TAFI; plasma procarboxypeptidase B, plasma procarboxypeptidase U, procarboxypeptidase R)

The thrombin-catalyzed conversion of plasma fibrinogen into fibrin and the development of an insoluble fibrin clot are the final steps in the coagulation cascade during hemostasis. The delicate balance between clot formation and fibrinolysis, which determines clot stability, is controlled by a complex interplay between fibrin and other molecular and cellular components of the hemostatic system, including thrombin activatable fibrinolysis inhibitor (TAFI). TAFI is activated by thrombin and has an important role in the stability of the fibrin clot, which is reviewed here. In particular, the role of TAFI in fibrinolysis and those characteristics of the protein that affect clot stability are described. In addition, the importance of TAFI in the coagulation process and how changes in its availability may contribute to bleeding or thrombotic disorders are discussed.

Preferential detection of pro-carboxypeptidase R by enzyme-linked immunosorbent assay

We generated two monoclonal antibodies (mAbs), 2A16 and 10G1, against pro-carboxypeptidase R (proCPR), also known as thrombin activatable fibrinolysis inhibitor (TAFI). By use of these mAbs, we developed a sandwich enzyme-linked immunosorbent assay (ELISA) system to detect proCPR. Since the amount of the antigen detectable by the ELISA was essentially the same in fresh plasma and serum incubated at 37 C for 1 hr, we concluded that the ELISA system detected not only proCPR, but also inactivated CPR generated from proCPR. However, an appreciable amount of proCPR remained unactivated in serum. For extensive activation of proCPR in plasma, thrombin and thrombomodulin complexes (TTM) can be used together with CaCl2. Following extensive conversion of proCPR to CPR by T-TM and CaCl2, converting plasma to serum (T-TM serum), antigenicity became undetectable by ELISA. Further analysis revealed that 2A16 reacts only with proCPR although 10G1 reacts with proCPR, active CPR and inactivated CPR. Therefore, we concluded that the ELISA system preferentially detects proCPR and not CPR. Our sandwich ELISA system utilizing 2A16 and 10G1 provides a suitable method for detecting proCPR and can be used to determine levels of proCPR in plasma samples from patients.

Thrombin-activatable fibrinolysis inhibitor (TAFI, plasma procarboxypeptidase B, procarboxypeptidase R, procarboxypeptidase U)

Recently, a new inhibitor of fibrinolysis was described, which downregulated fibrinolysis after it was activated by thrombin, and was therefore named TAFI (thrombin-activatable fibrinolysis inhibitor; EC 3.4.17.20). TAFI turned out to be identical to the previously described proteins, procarboxypeptidase U, procarboxypeptidase R, and plasma procarboxypeptidase B. Activated TAFI (TAFIa) downregulates fibrinolysis by the removal of carboxy-terminal lysines from fibrin. These carboxy-terminal lysines are exposed upon limited proteolysis of fibrin by plasmin and act as ligands for the lysine-binding sites of plasminogen and tissue-type plasminogen activator (t-PA). Elimination of these lysines by TAFIa abrogates the fibrin cofactor function of t-PA-mediated plasminogen activation, resulting in a decreased rate of plasmin generation and thus downregulation of fibrinolysis. In this review, the characteristics of TAFI are summarized, with an emphasis on the pathways leading to activation of TAFI and the role of TAFIa in the inhibition of fibrinolysis. However, it cannot be ruled out that TAFI has other, as yet undefined, functions in biology.

Development of a genotype 325-specific proCPU/TAFI ELISA

OBJECTIVE

A Thr/Ile polymorphism at position 325 in the coding region of proCPU has been reported. Immunological assays, fully characterized (including genotype dependency), are required for the quantitation of proCPU levels.

METHODS AND RESULTS

We have generated a panel of monoclonal antibodies against human, plasma-derived proCPU. Two combinations exhibiting distinct reactivities were selected for measurement of proCPU in plasma. T12D11/T28G6-HRP yielded values of 10.1+/-3.1 microg/mL (mean+/-SD, n=86; normal donors), and T32F6/T9G12-HRP yielded values of 5.4+/-3.0 microg/mL. Grouping according to the 325 genotype demonstrated that T12D11/T28G6-HRP was independent to this polymorphism whereas T32F6/T9G12-HRP revealed a complete lack of reactivity with the Ile/Ile genotype (ie, 0.0+/-0.0, 4.2+/-1.7, and 7.3+/-2.9 microg/mL for the Ile/Ile, Ile/Thr, and Thr/Thr isoforms, respectively). Commercially available antigen assays appeared to be partially dependent on the 325 genotype (eg, 44+/-8.9% and 100+/-30% for the Ile/Ile and Thr/Thr isoforms, respectively).

CONCLUSIONS

Our data demonstrate that great care should be taken when evaluating proCPU antigen values as a putative causative agent or as a diagnostic risk marker for cardiovascular events.

Thrombin-activatable fibrinolysis inhibitor (TAFI, plasma procarboxypeptidase B, procarboxypeptidase R, procarboxypeptidase U)

Recently, a new inhibitor of fibrinolysis was described. This inhibitor downregulated fibrinolysis after it was activated by thrombin, and was therefore named TAFI (thrombin-activatable fibrinolysis inhibitor; EC 3.4.17.20). TAFI turned out to be identical to previously described proteins, procarboxypeptidase U, procarboxypeptidase R, and plasma procarboxypeptidase B. In this overview, the protein will be referred to as TAFI. TAFI is a procarboxypeptidase and a member of the family of metallocarboxypeptidases. These enzymes are circulating in plasma and are present in several tissues such as pancreas. In this review, we will describe the properties of basic carboxypeptidases with the emphasis on the role of TAFI in coagulation and fibrinolysis. It cannot be ruled out, however, that TAFI has other, yet undefined, functions in biology.

Thrombin activatable fibrinolysis inhibitor and an antifibrinolytic pathway

Coagulation and fibrinolysis are processes that form and dissolve fibrin, respectively. These processes are exquisitely regulated and protect the organism from excessive blood loss or excessive fibrin deposition. Regulation of these cascades is accomplished by a variety of mechanisms involving cellular responses, flow, and protein-protein interactions. With respect to regulation mediated by protein-protein interaction, the coagulation cascade appears to be more complex than the fibrinolytic cascade because it has more components. Yet each cascade is regulated by initiators, cofactors, feedback reactions, and inhibitors. Coagulation is also controlled by an anticoagulant pathway composed of (minimally) thrombin, thrombomodulin, and protein C.(1) Protein C is converted by the thrombin/thrombomodulin complex to activated protein C (APC), which catalyzes the proteolytic inactivation of the essential cofactors required for thrombin formation, factors Va and VIIIa. An analogous antifibrinolytic pathway has been identified recently. This pathway provides an apparent symmetry between coagulation and fibrinolysis and is also composed of thrombin, thrombomodulin, and a zymogen that is activated to an enzyme. The enzyme proteolytically inactivates a cofactor to attenuate fibrinolysis. However, unlike APC, which is a serine protease, the antifibrinolytic enzyme is a metalloprotease that exhibits carboxypeptidase B-like activity. Within a few years of each other, 5 groups independently described a molecule that accounts for this antifibrinolytic activity. We refer to this molecule as thrombin activatable fibrinolysis inhibitor (TAFI), a name that is based on functional properties by which it was identified, assayed, and purified. (Because of the preferences of some journals "activatable" is occasionally referred to as "activable.") This review will encompass a historical account of efforts to isolate TAFI and characterize it with respect to its activation, activity, regulation, and potential function in vivo.

Proteolytic activation of purified human procarboxypeptidase U

Carboxypeptidase U (CPU, EC 3.4.17.20) is a recently described basic carboxypeptidase which circulates in plasma as an enzymatically inactive precursor procarboxypeptidase U (proCPU), also known as plasma carboxypeptidase B precursor or thrombin activatable fibrinolysis inhibitor (TAFI). The activation of the zymogen proceeds through a proteolytic cleavage at Arg-92. The active form - CPU - is able to retard the initial phase of fibrinolysis by cleaving C-terminal lysine residues exposed on fibrin partially degraded by the action of plasmin. These C-terminal lysine residues are essential for the high affinity binding of plasminogen to fibrin and the subsequent activation to plasmin. In this report, the activation of purified human proCPU was studied using trypsin and some key proteases of the coagulation and fibrinolytic cascade, i.e., kallikrein, plasmin and thrombin. The most efficient activation is obtained in the presence of thrombin in complex with thrombomodulin. After in vitro activation, CPU is unstable at 37 degrees C (T(1/2)=15 min). Its stability can be improved dramatically using lower temperatures.

Human procarboxypeptidase U, or thrombin-activable fibrinolysis inhibitor, is a substrate for transglutaminases. Evidence for transglutaminase-catalyzed cross-linking to fibrin

Procarboxypeptidase U (EC 3.4.17.20) (pro-CpU), also known as plasma procarboxypeptidase B and thrombin-activable fibrinolysis inhibitor, is a human plasma protein that has been implicated in the regulation of fibrinolysis. In this study, we show that pro-CpU serves as a substrate for transglutaminases. Both factor XIIIa and tissue transglutaminase catalyzed the polymerization of pro-CpU and the cross-linking to fibrin as well as the incorporation of 5-dimethylaminonaphthalene-1-sulfonyl cadaverine (dansylcadaverine), [14C]putrescine, and dansyl-PGGQQIV. These findings show that pro-CpU contains both amine acceptor (Gln) and amine donor (Lys) residues. The amine acceptor residues were identified as Gln2, Gln5, and Gln292, suggesting that both the activation peptide and the mature enzyme participate in the cross-linking reaction. These observations imply that transglutaminases may mediate covalent binding of pro-CpU to other proteins and cell surfaces in vivo. In particular, factor XIIIa may cross-link pro-CpU to fibrin during the latter part of the coagulation cascade, thereby helping protect the newly formed fibrin clot from premature plasmin degradation. Moreover, the cross-linking may facilitate the activation of pro-CpU, stabilize the enzymatic activity, and protect the active enzyme from further degradation.

Plasma and recombinant thrombin-activable fibrinolysis inhibitor (TAFI) and activated TAFI compared with respect to glycosylation, thrombin/thrombomodulin-dependent activation, thermal stability, and enzymatic properties

Thrombin-activable fibrinolysis inhibitor (TAFI) is a human plasma zymogen similar to pancreatic pro-carboxypeptidase B. Cleavage of the zymogen by thrombin/thrombomodulin generates the enzyme, activated TAFI (TAFIa), which retards fibrin clot lysis in vitro and likely modulates fibrinolysis in vivo. In the present work we stably expressed recombinant TAFI in baby hamster kidney cells, purified it to homogeneity from conditioned serum-free medium, and compared it to plasma TAFI (pTAFI) with respect to glycosylation and kinetics of activation by thrombin/thrombomodulin. Although rTAFI is glycosylated somewhat differently than pTAFI, cleavage products with thrombin/thrombomodulin are indistinguishable, and parameters of activation kinetics are very similar with kcat = 0.55 s-1, K(m) = 0.54 microM, and Kd = 6.0 nM for rTAFI and kcat = 0.61 s-1, K(m) = 0.55 microM, and Kd = 6.6 nM for pTAFI. The respective TAFIa species also were prepared and compared with respect to thermal stability and enzymatic properties, including inhibition of fibrinolysis. The half-life of both enzymes at 37 degrees C is about 10 min, and the decay of enzymatic activity is associated with a quenching (to approximately 62% of the initial value at 60 min) of the intrinsic fluorescence of the enzyme. Stability was highly temperature-dependent, which, according to transition state theory, indicates both high enthalpy and entropy changes associated with inactivation (delta Ho++ approximately equal to 45 kcal/mol and delta So++ approximately equal to 80 cal/mol/K). Both species of TAFIa are stabilized by the competitive inhibitors 2-guanidinoethylmercaptosuccinic acid and epsilon-aminocaproic acid. rTAFIa and pTAFIa are very similar with respect to kinetics of cleavage of small substrates, susceptibility to inhibitors, and ability to retard both tPA-induced and plasmin-mediated fibrinolysis. These studies provide new insights into the thermal instability of TAFIa, a property which could be a significant regulator of its activity in vivo; in addition, they show that rTAFI and rTAFIa are excellent surrogates for the natural plasma-derived species, a necessary prerequisite for future studies of structure and function by site-specific mutagenesis.