Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound   Target Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound

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Query: EC:3.4.21.5 (thrombin)
33,306 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The thrombin-activatable fibrinolysis inhibitor (TAFI) pathway defines a novel molecular connection between blood coagulation and both fibrinolysis and inflammation. TAFI is a plasma zymogen that can be activated by thrombin, the thrombin-thrombomodulin complex, or plasmin. The activated form of TAFI (TAFIa) attenuates fibrinolysis by removing the carboxyl-terminal lysine residues from partially degraded fibrin that mediate positive feedback in the fibrinolytic cascade. A role for TAFIa in modulating inflammation is suggested by the ability of this enzyme to down-regulate pericellular plasminogen activation and to inactivate the inflammatory peptides bradykinin and the anaphylatoxins C3a and C5a. The focus of this review is on recent advances in the clinical measurement of the TAFI pathway in human subjects and what this has revealed in terms of the molecular genetics of TAFI, the biological variation in plasma TAFI antigen levels, potential regulators of expression of the gene encoding TAFI, and the TAFI pathway as a risk factor for the development of vascular diseases. Although this field is in its infancy, much recent progress has been made and the available data suggest that the TAFI pathway is an intriguing new player in a variety of physiological and pathophysiological contexts.
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PMID:Curiouser and curiouser: recent advances in measurement of thrombin-activatable fibrinolysis inhibitor (TAFI) and in understanding its molecular genetics, gene regulation, and biological roles. 1733 88

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.
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PMID:An assay for measuring functional activated thrombin-activatable fibrinolysis inhibitor in plasma. 1796 38

Activated thrombin-activable fibrinolysis inhibitor (TAFIa) plays a significant role in the prolongation of fibrinolysis. During fibrinolysis, plasminogen is activated to plasmin, which lyses a clot by cleaving fibrin after selected arginine and lysine residues. TAFIa attenuates fibrinolysis by removing the exposed C-terminal lysine residues. It was recently reported that TAFI zymogen possesses sufficient carboxypeptidase activity to attenuate fibrinolysis through a mechanism similar to TAFIa. Here, we show with a recently developed TAFIa assay that when thrombin is used to clot TAFI-deficient plasma supplemented with TAFI, there is some TAFI activation. The extent of activation was dependent upon the concentration of zymogen present in the plasma, and lysis times were prolonged by TAFIa in a concentration-dependent manner. Potato tuber carboxypeptidase inhibitor, an inhibitor of TAFIa but not TAFI, abolished the prolongation of lysis in TAFI-deficient plasma supplemented with TAFI zymogen. In addition, TAFIa but not TAFI catalyzed release of plasminogen bound to soluble fibrin degradation products. The data presented confirm that TAFI zymogen is effective in cleaving a small substrate but does not play a role in the attenuation of fibrinolysis because of its inability to cleave plasmin-modified fibrin degradation products.
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PMID:Thrombin-activable fibrinolysis inhibitor zymogen does not play a significant role in the attenuation of fibrinolysis. 1825 11

Thrombin is a key hemostatic enzyme, which propagates its own generation by activating factors V, VIII, and XI. Sustained thrombin generation also activates thrombin-activatable fibrinolysis inhibitor (TAFI), which stabilizes fibrin clot against fibrinolysis. Recombinant activated factor VII (rFVIIa) is considered a novel hemostatic intervention for refractory bleeding, but rebleeding episodes related to fibrinolysis still occur. The present study aimed to investigate the antifibrinolytic effects of rFVIIa in relation to thrombin generation. Using thrombelastography, the effects of rFVIIa on thrombin-activated fibrin formation and on fibrinolysis induced by tissue plasminogen activator were evaluated in various factor-deficient plasma samples. A Thrombinoscope was used to quantitate thrombin generation. Thrombin increased antifibrinolytic activity in a concentration-dependent manner as demonstrated by a longer clot lysis time. In plasma deficient in factors V, VIII, IX, X, or XI, clot lysis occurred early (< 20 min), and rFVIIa addition had minimal effect, except for improved antifibrinolytic effect in factor-XI-deficient plasma. A normal clot lysis time was observed in factor-XIII-deficient or dual antithrombin/factor-VIII-deficient plasma. Inhibition of TAFI increased the rate of fibrinolysis. Thrombin generation was delayed or decreased in single factor-deficient plasma except for factor XIII deficiency. After rFVIIa addition, the peak thrombin generation reached over 100 nmol/l in factor-XI-deficient plasma, but not in plasma deficient in factors V, VIII, IX, or X. Thrombin generation and subsequent activation of TAFI were important for clot stability. We conclude that rFVIIa therapy does not compensate for increased susceptibility to fibrinolysis due to lack of factor(s) necessary for the formation of tenase and prothrombinase.
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PMID:Effects of recombinant activated factor VII on thrombin-mediated feedback activation of coagulation. 1827 34

The presence of fibrin degradation products, thrombin-like enzyme, prothrombin fragments, thrombin-activatable fibrinolysis inhibitor, plasmin and other active components of blood coagulation and fibrinolysis in seminal plasma has been reported. In the present study we investigate the presence of thrombomodulin in human semen. Using an Imubind thrombomodulin enzyme-linked immunosorbent assay (American Diagnostica Inc., Stamford, Connecticut, USA), seminal thrombomodulin levels were measured in 47 semen specimens obtained from subfertile individuals, normally fertile individuals, semen donors as well as vasectomized individuals, and in a further group defined by normality in several parameters derived from the World Health Organization fertility criteria. Conventional semen parameters were analysed in all semen samples. Thrombomodulin is quantifiable in human semen at a concentration lower than that normally found in citrated blood plasma samples. Slightly higher levels were seen for fertile stratifications compared with infertile individuals but without significant difference, given the numbers accrued. A vasectomized group showed the lowest value. In conclusion, our results establish the presence of thrombomodulin in human semen and suggest its production both upstream and downstream from the level of a vasectomy lesion.
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PMID:Detection and quantification of thrombomodulin in human semen. 1827 35

Procoagulant state, inflammation, and endothelial dysfunction have been documented in metabolic syndrome. Endothelial dysfunction is a strong predictor of cardiovascular events. Studies on the association of thrombin-activatable fibrinolysis inhibitor and thrombosis are still controversial, but substantial evidence suggests that increased thrombin-activatable fibrinolysis inhibitor or thrombin-activatable fibrinolysis inhibits or protects against arterial thrombosis. This study aimed to assess concomitantly the effects of fenofibrate therapy on thrombin-activatable fibrinolysis inhibitor concentrations and endothelial functions in patients with metabolic syndrome. Twenty-five patients (16 women; mean age 50.4 +/- 7.0) were enrolled in the study. Plasma thrombin-activatable fibrinolysis inhibitor, C-reactive protein, and fibrinogen levels were measured before fenofibrate administration and after 8 weeks of fenofibrate treatment. Endothelial function was assessed by endothelial-dependent flow-mediated dilatation from brachial artery. Pretreatment (baseline) thrombin-activatable fibrinolysis inhibitor level was 52.3 (1.2-119.7) decreasing to 7.7 (0.9-51.2; P < 0.001) after 8 weeks of fibrate treatment. Endothelial functions, which were measured with flow-mediated dilatation, were significantly improved after treatment (mean flow-mediated dilatation was 6.76 +/- 2.21 at baseline and 10.66 +/- 1.17% after 8 week of fenofibrate treatment, P < 0.001). Fenofibrate decreases thrombin-activatable fibrinolysis inhibitor levels and improves endothelial function in metabolic syndrome and, thus, suggests a potential for protection against cardiovascular effects. Further studies are warranted to confirm the effects of fibrates on thrombin-activatable fibrinolysis inhibitor and for conclusive evidence on the association between thrombin-activatable fibrinolysis inhibitor and thrombosis.
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PMID:Fenofibrate improves endothelial function and decreases thrombin-activatable fibrinolysis inhibitor concentration in metabolic syndrome. 1846 53

Thrombin-activatable fibrinolysis inhibitor (TAFI) is a pro-metallocarboxypeptidase that can be proteolytically activated (TAFIa). TAFIa is unique among carboxypeptidases in that it spontaneously inactivates with a short half-life, a property that is crucial for its role in controlling blood clot lysis. We studied the intrinsic instability of TAFIa by solving crystal structures of TAFI, a TAFI inhibitor (GEMSA) complex and a quadruple TAFI mutant (70-fold more stable active enzyme). The crystal structures show that TAFIa stability is directly related to the dynamics of a 55-residue segment (residues 296-350) that includes residues of the active site wall. Dynamics of this flap are markedly reduced by the inhibitor GEMSA, a known stabilizer of TAFIa, and stabilizing mutations. Our data provide the structural basis for a model of TAFI auto-regulation: in zymogen TAFI the dynamic flap is stabilized by interactions with the activation peptide. Release of the activation peptide increases dynamic flap mobility and in time this leads to conformational changes that disrupt the catalytic site and expose a cryptic thrombin-cleavage site present at Arg302. This represents a novel mechanism of enzyme control that enables TAFI to regulate its activity in plasma in the absence of specific inhibitors.
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PMID:Crystal structures of TAFI elucidate the inactivation mechanism of activated TAFI: a novel mechanism for enzyme autoregulation. 1880 67

This review considers the perhaps unappreciated role of contact pathway proteins in the pathogenesis of thrombotic/thromboembolic morbidity associated with mechanical circulatory support. Placement of ventricular assist devices (VADs) has been associated with consumption of circulating contact proteins and persistent generation of activated contact proteins such as Factor XII and high molecular weight kininogen. Importantly, activated contact proteins are absorbed to the surface of VADs via the Vroman effect. Further, hyperfibrinogenemia and persistent platelet activation exist in patients with VADs, likely contributing to speed of clot growth. Using thrombelastographic-based analyses, it has been determined that contact pathway protein activated coagulation results in a thrombus that develops strength at a significantly faster rate that tissue factor initiated coagulation. Further, thrombelastographic analyses that include the addition of tissue-type plasminogen activator have demonstrated that contact protein pathway activation results in thrombin activatable fibrinolysis inhibitor activation to a far greater extent than that observed with tissue factor initiated coagulation, resulting in a thrombus that takes significantly longer to lyse. These observations serve as the rational basis for clinical investigation to determine if regional suppression of thrombin generation with FXII/high molecular weight kininogen inhibition in concert with thrombin-activatable fibrinolysis inhibitor inhibition may decrease mechanical circulatory support-associated thrombotic morbidity.
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PMID:Mechanical circulatory device thrombosis: a new paradigm linking hypercoagulation and hypofibrinolysis. 1864 51

Thrombin-activatable procarboxypeptidase B (proCPB or thrombin-activatable fibrinolysis inhibitor or TAFI) is a plasma procarboxypeptidase that is activated by the thrombin-thrombomodulin complex on the vascular endothelial surface. The activated CPB removes the newly exposed carboxyl terminal lysines in the partially digested fibrin clot, diminishes tissue plasminogen activator and plasminogen binding, and protects the clot from premature lysis. We have recently shown that CPB is catalytically more efficient than plasma CPN, the major plasma anaphylatoxin inhibitor, in inhibiting bradykinin, activated complement C3a, C5a, and thrombin-cleaved osteopontin in vitro. Using a thrombin mutant (E229K) that has minimal procoagulant properties but retains the ability to activate protein C and proCPB in vivo, we showed that infusion of E229K thrombin into wild-type mice reduced bradykinin-induced hypotension but it had no effect in proCPB-deficient mice, indicating that the beneficial effect of E229K thrombin is mediated through its activation of proCPB and not protein C. Similarly proCPB-deficient mice displayed enhanced pulmonary inflammation in a C5a-induced alveolitis model and E229K thrombin ameliorated the magnitude of alveolitis in wild-type but not proCPB-deficient mice. ProCPB-deficient mice also displayed enhanced arthritis in an inflammatory arthritis model. Thus, our in vitro and in vivo data support the thesis that thrombin-activatable CPB has broad anti-inflammatory properties. By specific cleavage of the carboxyl terminal arginines from C3a, C5a, bradykinin and thrombin-cleaved osteopontin, it inactivates these active inflammatory mediators. Along with the activation of protein C, the activation of proCPB by the endothelial thrombin-thrombomodulin complex represents a homeostatic feedback mechanism in regulating thrombin's pro-inflammatory functions in vivo.
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PMID:Regulation of tissue inflammation by thrombin-activatable carboxypeptidase B (or TAFI). 1870 98

Thrombin-activatable fibrinolysis inhibitor (TAFI) is a metallocarboxypeptidase (MCP) that links blood coagulation and fibrinolysis. TAFI hampers fibrin-clot lysis and is a pharmacological target for the treatment of thrombotic conditions. TAFI is transformed through removal of its prodomain by thrombin-thrombomodulin into TAFIa, which is intrinsically unstable and has a short half-life in vivo. Here we show that purified bovine TAFI activated in the presence of a proteinaceous inhibitor renders a stable enzyme-inhibitor complex. Its crystal structure reveals that TAFIa conforms to the alpha/beta-hydrolase fold of MCPs and displays two unique flexible loops on the molecular surface, accounting for structural instability and susceptibility to proteolysis. In addition, point mutations reported to enhance protein stability in vivo are mainly located in the first loop and in another surface region, which is a potential heparin-binding site. The protein inhibitor contacts both the TAFIa active site and an exosite, thus contributing to high inhibitory efficiency.
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PMID:Structure of activated thrombin-activatable fibrinolysis inhibitor, a molecular link between coagulation and fibrinolysis. 1872 83


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