Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: EC:3.4.21.5 (thrombin)
 33,306 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Factor XI is a key component of the intrinsic pathway of blood coagulation in vitro. The poor correlation between the clinical bleeding diathesis in factor XI deficiency and abnormalities in clotting assays that measure intrinsic coagulation brings into question the role of this serine protease in in vivo hemostasis. The characterizations of the point mutations responsible for the majority of cases of severe factor XI deficiency in Ashkenazi Jews and subsequent epidemiologic studies have provided insight into the perplexing hemostatic abnormalities in this disorder. It appears that excessive bleeding in factor XI deficiency depends on the severity of the deficiency in certain situations and on the location of the hemostatic challenge in others. Additional coexisting abnormalities of hemostasis, such as von Willebrand's disease, may also be responsible for variation in clinical presentation, particularly in those individuals with mild factor XI deficiency. The absence of abnormal bleeding in congenital deficiency of factor XII, the protease that activates factor XI in the intrinsic cascade, has stimulated a search for other mechanisms for factor XI activation. Recent studies have pointed to the serine protease thrombin and autoactivation by activated factor XI as possible alternatives to factor XII as activators of factor XI. These findings suggest that factor XI, rather than operating in a pathway for the initiation of hemostasis, may function in the consolidation of clot formation after the initiation of the hemostatic process by other mechanisms.
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PMID:Advances and dilemmas in factor XI. 937 4

Recent in vitro studies have shown that fibrinolytic activity may be attenuated by a thrombin-activatable fibrinolysis inhibitor (TAFI), which is activated by thrombin, generated via the intrinsic pathway of coagulation in a factor XI-dependent way. Thus factor XI may play a role in the regulation of endogenous fibrinolysis. The aim of this study was to investigate the effect of in vivo inhibition of factor XI and TAFI in an experimental thrombosis model in rabbits. Incorporation of anti-factor XI antibodies in jugular vein thrombi resulted in an almost twofold increase in endogenous thrombolysis compared with a control antibody. A similar effect was observed when the anti-factor XI antibody was administered systemically. Inhibition of TAFI activity also resulted in a twofold increase in clot lysis whereas inhibition of both factor XI and TAFI activity had no additional effect. Thus, we provide the first in vivo evidence for enhanced thrombolysis through inhibition of clotting factor XI, demonstrating a novel role for the intrinsic pathway of coagulation. Furthermore we demonstrate that inhibition of TAFI had a similar effect on thrombolysis. We postulate that inhibition of factor XI activity enhances thrombolysis because of diminished indirect activation of TAFI.
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PMID:Enhancement of rabbit jugular vein thrombolysis by neutralization of factor XI. In vivo evidence for a role of factor XI as an anti-fibrinolytic factor. 942 60

Are truly inert biomaterials feasible? Recent mathematical models of coagulation which are reviewed here suggest that such materials are impossible. This conclusion, which is certainly consistent with our collective experimental evidence, arises from the calculation that conversion of Factor XI to XIa never drops to zero even at the highest flow rates and with virtually no Factor XIIa bound to a surface. Residual amounts of XIa are still formed which can in principle kick-off the coagulation cascade. Furthermore, if the flow rates and corresponding mass transfer coefficients are low and in spite of these near-vanishing levels of the initiating coagulants, the surprising result is that substantial amounts of thrombin are produced. On the contrary, under slightly higher flow conditions, there can be more substantial levels of initiating coagulants, yet paradoxically thrombin production is near zero. This article presents a theoretical understanding of the events which take place during the interaction of biomaterials with flowing blood. We follow these events from the time of first contact to the final production of thrombin. The effect of flow and surface activity on the contact phase reactions is examined in detail and the two are found to be intertwined. The common pathway is also examined and here the main feature is the existence of three flow dependent regions which produce either high or very low levels of thrombin, as well as multiple thrombin steady states. In a final analysis we link the two segments of the cascade and consider the events which result. In addition, we note that multiple steady states arise only in the presence of two (thrombin) feedback loops. Single loops or the bare cascade will produce only single steady states. With some imagination one can attribute to the feedback loops the role of providing the cascade with a mechanism to produce high thrombin levels in case of acute need (e.g. bleeding) or to allow levels to subside to 'stand-by' when there is no need for clotting. We present this as a partial answer to the question: Why is the coagulation cascade so complex and what is the importance of the feedback loops?
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PMID:Coagulation on biomaterials in flowing blood: some theoretical considerations. 943 Mar 33

To study the physiological significance of thrombin as an initiator of intrinsic blood coagulation, activated human platelets were compared with dextran sulfate as a surface for thrombin-catalyzed factor XI activation. Activated gel-filtered platelets promoted factor XI activation by thrombin at initial rates 2-5-fold greater than dextran sulfate in the presence of high molecular weight kininogen (HK, 45 nM), ZnCl2 (25 microM), and CaCl2 (2 mM), conditions optimal for factor XI binding to platelets. Physiological concentrations of HK (636 nM) inhibited factor XI activation by thrombin in a concentration-dependent manner, and this inhibition was reversed by prothrombin (1-3 microM) and by prothrombin fragment 1.2 (PF1.2), but not by prothrombin fragment 1 (PF1). Since prothrombin and PF1.2 (but not PF1) also displaced HK from its binding site on the Apple 1 domain of factor XI, we conclude that the Kringle II domain of prothrombin competes with HK for binding to the Apple 1 domain of factor XI. Prothrombin (1-3 microM) and PF1.2 (but not PF1) in the presence of CaCl2 (2 mM) were able to replace HK (45 nM) in the presence of ZnCl2 (25 microM) as a cofactor for the specific, reversible, high-affinity (Kd approximately 25 nM) binding of factor XI to 947 +/- 150 sites per platelet. This binding is mediated by residues Asn 235-Arg 266 in the Apple 3 domain since a conformationally constrained, synthetic peptide analogue of this sequence inhibits both factor XI binding to activated platelets and platelet-mediated, thrombin-catalyzed factor XI activation in the presence of prothrombin and CaCl2. Finally, prothrombin (1.2 microM) and CaCl2 (2 mM) could substitute for HK (45 nM) and ZnCl2 (25 microM) in promoting optimal rates of thrombin-catalyzed factor XI activation on the platelet surface, thereby initiating the intrinsic coagulation pathway by mechanisms completely independent of the contact phase proteins, factor XII, HK, and prekallikrein.
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PMID:Prothrombin is a cofactor for the binding of factor XI to the platelet surface and for platelet-mediated factor XI activation by thrombin. 1792 39

Platelet factor XI is an alternatively spliced product of the factor XI gene expressed specifically within megakaryocytes and platelets as an approximately 1.9-kb mRNA transcript (compared with approximately 2.1 kb in liver cells) lacking exon V. Flow cytometry with an affinity-purified factor XI antibody, with PAC1 antibody (to the GPIIb/IIIa complex on activated platelets), and with S12 antibody (to P-selectin, an alpha-granule membrane protein expressed on the platelet surface during secretion) on platelets activated with ADP, thrombin, thrombin receptor peptide (SFLLRN amide), or collagen at various concentrations exposed platelet factor XI and PAC1 antibody binding in parallel. Unactivated platelets expressed approximately 40% of total platelet factor XI but no PAC1 binding sites. Enhanced membrane exposure of platelet factor XI is independent of alpha-granule secretion, because ADP and collagen exposed platelet factor XI but no S12 binding sites. Platelets from four patients with plasma factor XI deficiency (<0.04 U/mL) had normal constitutive and activation-dependent expression of platelet factor XI. Well-washed platelets from normal and from factor XI-deficient donors incubated with low concentrations of thrombin (0. 05 to 0.1 U/mL) corrected the clotting defect observed with factor XI-deficient plasma. Thus, functionally active platelet factor XI is differentially expressed on platelet membranes in a tissue-specific manner both constitutively and in a concentration-dependent fashion by various agonists in the absence of detectable plasma factor XI.
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PMID:Tissue-specific expression of functional platelet factor XI is independent of plasma factor XI expression. 957 17

Tissue factor (TF)-induced coagulation was compared in contact pathway suppressed human blood from normal, factor VIII-deficient, and factor XI-deficient donors. The progress of the reaction was analyzed in quenched samples by immunoassay and immunoblotting for fibrinopeptide A (FPA), thrombin-antithrombin (TAT), factor V activation, and osteonectin. In hemophilia A blood (factor VIII:C <1%) treated with 25 pmol/L TF, clotting was significantly delayed versus normal, whereas replacement with recombinant factor VIII (1 U/mL) restored the clot time near normal values. Fibrinopeptide A release was slower over the course of the experiment than in normal blood or hemophilic blood with factor VIII replaced, but significant release was observed by the end of the experiment. Factor V activation was significantly impaired, with both the heavy and light chains presenting more slowly than in the normal or replacement cases. Differences in platelet activation (osteonectin release) between normal and factor VIII-deficient blood were small, with the midpoint of the profiles observed within 1 minute of each other. Thrombin generation during the propagation phase (subsequent to clotting) was greatly impaired in factor VIII deficiency, being depressed to less than 1/29 (<1.9 nmol TAT/L/min) the rate in normal blood (55 nmol TAT/L/min). Replacement with recombinant factor VIII normalized the rate of TAT generation. Thus, coagulation in hemophilia A blood at 25 pmol/L TF is impaired, with significantly slower thrombin generation than normal during the propagation phase; this reduced thrombin appears to affect FPA production and factor V activation more profoundly than platelet activation. At the same level of TF in factor XI-deficient blood (XI:C <2%), only minor differences in clotting or product formation (FPA, osteonectin, and factor Va) were observed. Using reduced levels of initiator (5 pmol/L TF), the reaction was more strongly influenced by factor XI deficiency. Clot formation was delayed from 11.1 to 15.7 minutes, which shortened to 9.7 minutes with factor XI replacement. The maximum thrombin generation rate observed ( approximately 37 nmol TAT/L/min) was approximately one third that for normal (110 nmol/L TAT/min) or with factor XI replacement (119 nmol TAT/L/min). FPA release, factor V activation, and release of platelet osteonectin were slower in factor XI-deficient blood than in normal blood. The data demonstrate that factor XI deficiency results in significantly delayed clot formation only at sufficiently low TF concentrations. However, even at these low TF concentrations, significant thrombin is generated in the propagation phase after formation of the initial clot in hemophilia C blood.
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PMID:Blood coagulation in hemophilia A and hemophilia C. 961 54

Evidence of factor XI (FXI) activation in vivo is scarce. In addition, it remains uncertain whether thrombin, factor XIIa (FXIIa), or perhaps another protease is responsible for FXI conversion. We investigated the activation of FXI in eight healthy volunteers after infusion of a low dose of endotoxin (4 ng/kg of body weight). Activation of prekallikrein FXII, FXI, and prothrombin was measured with sensitive enzyme-linked immunosorbent assays (ELISAs), and FXI activation was measured with a novel enzyme capture assay that detects noncomplexed FXIa. Activation of FXI was apparent with a significant plasma peak level of noncomplexed FXIa of 10 to 11 pmol/L at 1 and 2 hours after endotoxin infusion, followed by a gradual increase in FXIa-FXIa inhibitor complexes, measured in the ELISAs, with a summit of 11 to 15 pmol/L at 6 and 24 hours, respectively. In accordance with previous studies, thrombin generation was detected 1 hour after endotoxin infusion to become maximal after 3 to 4 hours. In contrast, we did not find any evidence of contact activation, because markers of activation of prekallikrein and FXII remained undetectable. From the FXIa data a theoretical model was constructed which suggested that inhibition of FXIa does not take place in the plasma compartment, but is localized on a surface. These data provide the first evidence for FXI activation in low-grade endotoxemia and suggest that FXI is activated independently of FXII.
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PMID:Activation of clotting factor XI without detectable contact activation in experimental human endotoxemia. 978 66

The glycosaminoglycan heparin enhances several reactions involving coagulation factor XI (FXI) including activation of FXI by factor XIIa, thrombin, and autoactivation; and inactivation of activated FXI (FXIa) by serine protease inhibitors. We examined the effect of heparin on inhibition of FXIa by the inhibitors C1-inhibitor (C1-INH) and antithrombin III (ATIII). Second order rate constants for inhibition in the absence of heparin were 1.57 x 10(3) and 0.91 x 10(3) M-1 s-1 for C1-INH and ATIII, respectively. Therapeutic heparin concentrations (0.1-1.0 units/ml) enhanced inhibition by ATIII 20-55-fold compared with 0.1-7.0-fold for C1-INH. For both inhibitors, the effect of heparin over a wide range of concentrations (10(-1) to 10(5) units/ml) produced bell-shaped curves, demonstrating that inhibition occurs by a template mechanism requiring both inhibitor and protease to bind to heparin. This implies that FXI/XIa contains structural elements that interact with heparin. Human FXI contains a sequence of amino acids (R250-I-K-K-S-K) in the apple 3 domain of the heavy chain that binds heparin (Ho, D., Badellino, K., Baglia, F., and Walsh, P. (1998) J. Biol. Chem. 273, 16382-16390). To determine the importance of this sequence to heparin-mediated reactions, recombinant FXI molecules with alanine substitutions for basic amino acids were expressed in 293 fibroblasts, and tested in heparin-dependent assays. Inhibition of FXIa by ATIII in the presence of heparin was decreased 4-fold by alanine substitution at Lys253 (A253), with smaller effects noted for mutants A255 and A252. FXI undergoes autoactivation to FXIa in the presence of heparin. The rate of autoactivation was decreased substantially for A253 with modest decreases for A255 and A252. Substituting all four charged residues in the sequence resulted in a profound decrease in autoactivation, significantly greater than for any single substitution. Relative affinity for heparin was tested by determining the concentration of NaCl required to elute FXIa from heparin-Sepharose. Wild type FXIa eluted from the column at 320 mM NaCl, whereas FXIa with multiple substitutions (A252-254 or A250-255) eluted at 230 mM NaCl. All proteins with single substitutions in charged amino acids eluted at intermediate NaCl concentrations. The data indicate that FXI/XIa must bind to heparin for optimal inhibition by ATIII and for autoactivation. Lys253 is the most important amino acid involved in binding, and Lys255 and Lys252 also have roles in interactions with heparin.
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PMID:Characterization of a heparin binding site on the heavy chain of factor XI. 981 19

Thrombin Activatable Fibrinolysis Inhibitor (TAFI) is a recently identified fibrinolysis inhibitor in plasma, that when converted to an enzyme potently attenuates fibrinolysis. It is activated by relatively high concentrations of thrombin that exceed the thrombin concentration required for fibrin formation. These high concentrations of thrombin are generated by the intrinsic pathway via activation of factor XI by thrombin. The down regulation of fibrinolysis by TAFI can be measured in a clot lysis assay. When the clot lysis times of healthy individuals were determined, large inter-individual differences were observed. To determine if differences in concentration of TAFI explain the variation in clot lysis between individuals, specific assays were developed for the measurement of TAFI antigen and activity in plasma. In normal plasma, there was a dose-dependent relationship between TAFI antigen and TAFI activity. There was also a correlation between clot lysis time and plasma TAFI antigen, indicating that the amount of TAFI that is activated during the clot lysis assay, is dependent on the concentration of TAFI. In the plasmas of 20 healthy individuals, clot lysis times, TAFI antigen and TAFI activity were determined. Both TAFI antigen and TAFI activity showed a significant correlation with the clot lysis time. No correlation between TAFI antigen and clot lysis time was found when the clot lysis time was determined in the presence of an antibody blocking the factor XI feedback loop. These results indicate that plasma TAFI levels influence the clot lysis time in healthy individuals in the presence of an intact intrinsic pathway of coagulation.
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PMID:Plasma TAFI levels influence the clot lysis time in healthy individuals in the presence of an intact intrinsic pathway of coagulation. 984 79

In this report we describe an in vitro model of blood coagulation reactions that mimics as closely as possible the in vivo condition. Our model indicates that the tissue factor-factor VIIa complex initiates coagulation by activating small amounts of both factor IX and factor X in the environment of the tissue factor bearing cell. Factor Xa and factor IXa formed in the initial reaction then play very distinct roles in the subsequent interactions of the clotting mechanism leading to a burst of thrombin generation on the platelet surface. Our results also indicate that factor XI can be activated by thrombin in the absence of factor XII and that the function of factor XI is simply to enhance conversion of factor IX to factor IXa resulting in enhanced thrombin generation on the platelet surface.
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PMID:Newer concepts of blood coagulation. 987 50


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