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:3.4.21.6 (thromboplastin)
13,278 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The effect of recombinant tissue factor pathway inhibitor (rTFPI) on extracellular matrix procoagulant activity was studied in a human ex vivo model of thrombogenesis. Extracellular matrix of endotoxin stimulated endothelial cells perfused with human non-anticoagulated blood at a wall shear rate of 100/s induced pronounced fibrin deposition, which covered 82 +/- 11% of the matrix surface within 5 min. Preincubating the matrix with the combination of rTFPI, factor VIIa (FVIIa) and factor Xa (FXa) reduced fibrin deposition to levels observed with matrix from non-stimulated endothelial cells (7 +/- 6% fibrin coverage, P < 0.001). Preincubation with rTFPI plus FXa also reduced fibrin deposition significantly, but to a lesser extent (41 +/- 6% fibrin coverage, P < 0.001). Preincubation with rTFPI alone, or with rTFPI plus FVIIa, did not affect fibrin deposition. The inhibition of thrombus formation on procoagulant extracellular matrix by rTFPI seemed to be FXa-dependent, and a result of TFPI's ability to bind and inhibit both TF activity as well as FXa. The results from this study suggest a future role for rTFPI as an agent for prevention of TF-induced vascular thrombosis.
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PMID:Tissue factor pathway inhibitor prevents thrombus formation on procoagulant subendothelial matrix. 786 81

Using a fast kinetic centrifugal analyzer, the inhibitory effects of glycosylated and unglycosylated full-length and truncated forms of TFPI on protease generation were studied in fibrinogen-deficient human plasma after extrinsic (EA) or intrinsic (IA) activation of coagulation. When the assay system was supplemented with increasing amounts of the TFPI variants the generation of both thrombin and factor Xa was inhibited in a concentration-dependent manner. Clear differences in the effectiveness of the TFPI variants were found. After EA, the unglycosylated full-length TFPI was most effective followed by the glycosylated full-length form. The C-terminal truncated TFPI showed the lowest inhibitory activity in this system. However, its efficiency increased several fold when coagulation was activated via the intrinsic pathway. Comparing the IC50 values after IA, the truncated TFPI was more effective than the unglycosylated full-length form and nearly as effective as the glycosylated full-length TFPI. After both EA and IA the thrombin generation inhibition by TFPI variants was more pronounced than the inhibition of factor Xa generation. The results show that chemical modifications of the TFPI structure can result in changes of TFPI's inhibitory properties to activated clotting factors leading to differences in protease generation inhibition.
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PMID:Inhibitory effects of TFPI variants on thrombin and factor Xa generation in fibrinogen-deficient human plasma. 790 Jan 3

The coagulation enzyme thrombin, a serine protease like all other coagulation factors, plays a central role in the hemostatic processes engaged after injurious events. It induces, with particular efficacy, the aggregation of blood platelets (primary hemostasis) and accounts, via splitting of fibrinogen to fibrin, for the event actually responsible for the coagulation of blood (secondary hemostasis). As is well-known, thrombin itself is generated by a cascade of activation events involving various coagulation factors (F). In this respect the "tissue factor" (TF, formerly known as thromboplastin), in combination with F VIIa, attains decisive significance, not only in the extrinsic pathway of coagulation (activation of F X-->Xa), but also in the intrinsic pathway (activation of F IX-->IXa). Under physiological circumstances, platelet aggregation and coagulation are restricted to the area of the vascular lesion, since the surrounding intact endothelium inhibits an intraluminal spreading of both processes. These "antithrombotic" features of the endothelium encompass antiaggregatory mechanisms (formation and release of prostacyclin [PGI2], adenosine, EDRF [NO], degradation of ADP and other nucleotides mediated by ecto-nucleotidases) as well as anti-coagulatory properties (formation and release of "tissue factor pathway inhibitor" [TFPI], which blocks the coagulation cascade by joining F Xa, TF and F VIIa into an inactive complex, thrombomodulin--thrombin induced activation of protein C, which, together with protein S, inactivates F Va and F VIIIa, thereby attenuating further generation of thrombin, and the heparan sulfate-enhanced activation of antithrombin III and heparin-cofactor II).(ABSTRACT TRUNCATED AT 250 WORDS)
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PMID:[Interaction of blood and the vascular wall: hemostatic aspects]. 815 53

Tissue factor pathway inhibitor is a multivalent, Kunitz-type proteinase inhibitor. It directly inhibits factor Xa and, in a factor Xa-dependent fashion, produces feedback inhibition of the factor VIIa/tissue factor catalytic complex which is responsible for the initiation of coagulation. Human recombinant TFPI (rTFPI) produced in Escherichia coli was used to define the kinetic constants describing the human factor Xa:TFPI interaction. The inactivation of factor Xa by E. coli-rTFPI is indistinguishable from that of rTFPI produced in mammalian SK-hepatoma cells, suggesting that post-translational modifications such as glycosylation and phosphorylation do not play a major role in the inhibitory process. The slow, tight-binding inhibition of factor Xa follows the scheme: [formula: see text] Where the enzyme (E) and inhibitor (I) form an initial, immediate collision complex (EI) that then isomerizes slowly to a tightened final EI* complex. In the absence of other additions, the initial Ki (=k2/k1) and final Ki* for the inhibition of factor Xa by E. coli-rTFPI are 1.24 nM and 26.4 pM, respectively. In the presence of calcium ions (5 mM) the interaction between factor Xa and rTFPI is substantially weaker, with a Ki of 42.7 nM and Ki* of 85.2 pM. The addition of other components of the prothrombinase complex produces enhanced factor Xa inhibition predominantly through an effect on the initial Ki. In the presence of calcium ions and saturating concentrations of phospholipids and factor Va, the Ki and Ki* for factor Xa inactivation are 2.04 nM and 52.3 pM. The enhancing effect of heparin on the inhibitory process is concentration dependent and exhibits an optimum, reminiscent of the "template" model for heparin's acceleration of thrombin and factor IXa inhibition by antithrombin III. At optimal concentrations, the major mechanism of heparin action is also a reduction in the Ki of the initial encounter complex between factor Xa and rTFPI.
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PMID:Kinetics of factor Xa inhibition by tissue factor pathway inhibitor. 826 29

Experiments were performed to evaluate activation of factor VII bound to relipidated tissue factor (TF) in suspension and to TF constitutively expressed on the surface of an ovarian carcinoma cell line (OC-2008). Activation was assessed by measuring cleavage of 125I-factor VII and by the ability of unlabeled factor VII to catalyze activation of a variant factor IX molecule that, after activation, cannot back-activate factor VII. Factor Xa was found to effectively activate factor VII bound to TF relipidated in either acidic or neutral phospholipid vesicles. Autoactivation of factor VII bound to TF in suspension was dependent on the preparation of TF apoprotein used and the technique of its relipidation. This highlights the need for caution in extrapolating data from TF in suspension to the activation of factor VII bound to cell surfaces during hemostasis. A relatively slow activation of factor VII bound to OC-2008 monolayers in the absence of added protease was observed consistently. Antithrombin in the presence or absence of heparin prevented this basal activation, whereas TF pathway inhibitor (TFPI/factor Xa complexes had only a limited inhibitory effect. Adding a substrate concentration of factor X markedly enhanced basal activation of factor VII, but both TFPI/factor Xa and antithrombin/heparin abolished this enhancement. Overall, our data are compatible with the hypothesis that not all factor VII/TF complexes formed at a site of tissue injury are readily activated to factor VIIa (VIIa)/TF complexes during hemostasis. The clinical significance of this is discussed.
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PMID:Studies of the activation of factor VII bound to tissue factor. 889 39

Hookworms are hematophagous nematodes that infect a wide range of mammalian hosts, including humans. There has been speculation for nearly a century as to the identity of the anticoagulant substances) used by these organisms to subvert host hemostasis. Using molecular cloning, we describe a family of potent small protein (75-84 amino acids) anticoagulants from the hookworm Ancylostoma caninum termed AcAP (A. caninum anticoagulant protein). Two recombinant AcAP members (AcAP5 and AcAP6) directly inhibited the catalytic activity of blood coagulation factor Xa (fXa), while a third form (AcAPc2) predominantly inhibited the catalytic activity of a complex composed of blood coagulation factor VIIa and tissue factor (fVIIa/TF). The inhibition of fVIIa/TF was by a unique mechanism that required the initial formation of a binary complex of the inhibitor with fXa at a site on the enzyme that is distinct from the catalytic center (exo-site). The sequence of AcAPc2 as well as the utilization of an exo-site on fXa distinguishes this inhibitor from the mammalian anticoagulant TFPI (tissue factor pathway inhibitor), which is functionally equivalent with respect to fXa-dependent inhibition of fIIa/TF. The relative sequence positions of the reactive site residues determined for AcAP5 with the homologous regions in AcAP6 and AcAPc2 as well as the pattern of 10 cysteine residues present in each of the inhibitors suggest that the AcAPs are distantly related to the family of small protein serine protease inhibitors found in the nonhematophagous nematode Ascaris lumbricoides var. suum.
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PMID:Anticoagulant repertoire of the hookworm Ancylostoma caninum. 870 Sep

The inhibition of thrombin generation (TG) was studied in plasma from human volunteers after single subcutaneous administrations of 4000, 8000 or 12,000 anti-Xa units (i.e., 6, 12 or 18 mg) of the synthetic pentasaccharide (SR 90107/ORG 31540) (SP). SP impaired TG in plasma for up to 18 h after injection, and the time-courses of TG and factor Xa inhibitions were similar. In untreated plasma supplemented in vitro with SP to obtain the same anti-Xa activity as in ex vivo samples, equivalent TG inhibitions were observed thus showing that no transformed SP molecules were involved in the TG inhibition ex vivo. Functional as well as immunological assay of TFPI indicated that subcutaneous injection of 12,000 anti-Xa units of SP did not induce any TFPI release, whereas under the same conditions, 13,000 IU of Fraxiparine produced a significant rise of TFPI in plasma. The plotting of TG inhibition versus SP concentration could be fitted with a good correlation (r = 0.94) to the graphical representation linking [ATIII-SP] to [SP]. These results demonstrate that following subcutaneous administration to man, SP inhibits TG ex vivo and likely in vivo exclusively through the same selective ATIII-mediated inhibition of factor Xa as the one elicited in vitro.
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PMID:The effect of the synthetic pentasaccharide SR 90107/ORG 31540 on thrombin generation ex vivo is uniquely due to ATIII-mediated neutralization of factor Xa. 877 23

The effects of the components of the protein C pathway on thrombin generation were studied in a reconstituted model in which thrombin is generated by factor VIIa and relipidated tissue factor (TF) via the activation of the purified coagulation factors X, IX, VIII, V, and prothrombin. The influence of protein C and soluble thrombomodulin on thrombin generation was correlated with factor Xa generation, factor V(a) and factor VIII(a) formation/inactivation, and protein C activation. Thrombin generation initiated by low concentrations of factor VIIa.TF (1.25 pM) occurs in an explosive fashion during a propagation phase which occurs after an initiation phase of approximately 1 min in which only traces of thrombin are formed. In the absence of other inhibitors, protein C (65 nM) in combination with high concentrations of soluble thrombomodulin (10 nM) resulted in a reduced rate of thrombin generation during the propagation phase without affecting the initiation phase; the activated protein C generated failed to neutralize prothrombinase activity and did not prevent prothrombin consumption. In the presence of plasma levels of the tissue factor pathway inhibitor (2. 5 nM recombinant TFPI), the protein C pathway reduced the rate of thrombin generation, initiated by 1.25 pM factor VIIa.TF, and completely eliminated prothrombinase activity at soluble thrombomodulin concentrations of >/=1 nM. The neutralization of prothrombinase activity coincided with cleavages at Arg-506 and subsequent cleavage at Arg-306 of the factor Va heavy chain by activated protein C. Thus, the protein C pathway combined with TFPI creates a minimal inhibitory potential required to shut down TF-initiated thrombin generation. The protein C pathway constituents did not influence factor Xa generation or factor VIIIa degradation over the interval in which prothrombinase activity was neutralized. Our data thus suggest that the protein C pathway regulates thrombin generation solely by the inactivation of factor Va. At low initiating factor VIIa.TF (1.25 pM) and high thrombomodulin concentrations (10 nM), the factor Va heavy chain is cleaved before significant amounts of light chain are generated. The ability of the protein C pathway to inhibit thrombin generation was greatly reduced when the reaction was initiated in the presence of factor Va, supporting the hypothesis that effective down-regulation of thrombin generation by the protein C pathway, in reactions initiated with the procofactor, occurs by prevention of the coexistence of the factor Va heavy and light chains.
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PMID:Inhibitory mechanism of the protein C pathway on tissue factor-induced thrombin generation. Synergistic effect in combination with tissue factor pathway inhibitor. 906 69

TF antigen and activity are found in abundance in human atherosclerotic plaques, particularly in the lipid-rich core. TF is also readily induced in the arterial wall by balloon injury and accumulates in the resulting neointima. In chronic atherosclerosis, the macrophage is likely to be the major source of TF within the plaque. TF accumulates as an early event associated with the migration of monocytes to the vessel wall in response to chemoattractants, such as MCP-1, and their differentiation into macrophages. As SMC become activated in the developing plaque, they provide a second source of TF. Macrophages and SMC accumulate lipid and become foam cells, ultimately degenerating into a necrotic core rich in TF. Spontaneous plaque rupture or acute interventions expose active TF in the core to circulating blood, triggering thrombosis. In acute arterial injury, SMC appear to be the chief source of TF. In normal vessels, the induction of TF in the medial SMC is not sufficient to generate fibrin, presumably because the TF is not readily accessible on the luminal surface. In contrast, endothelial denudation of previously injured arteries may expose intimal TF to circulating blood, resulting in rapid fibrin deposition. In advanced human atherosclerosis, it is likely that even in areas that do not contain "unstable" or "stable" plaques, the vessel wall is not normal and more closely resembles that of a previously injured artery possessing an active intima. Interventions, such as balloon angioplasty, coronary atherectomy, or stent placement may expose intimal TF, leading to fibrin deposition. As the initiator of coagulation, TF is a potential target for inhibiting the thrombotic complications of atherosclerosis. TFPI (reviewed in 52) is currently under clinical investigation as an anticoagulant and its effects on intimal hyperplasia in animal models are being studied. Direct factor Xa inhibitors, such as tick anticoagulant peptide (TAP) and leech anticoagulant peptide (ATS), are also under investigation (53-54). Finally, the recent crystallization of TF (55) and the TF:VIIa (56) should provide important new insights into the design of molecules for directly inhibiting TF.
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PMID:Tissue factor in the pathogenesis of atherosclerosis. 919 53

Tissue factor (TF) pathway inhibitor (TFPI) regulates factor X activation through the sequential inhibition of factor Xa and the VIIa.TF complex. Factor Xa formation was studied in a purified, reconstituted system, at plasma concentrations of factor X and TFPI, saturating concentrations of factor VIIa, and increasing concentrations of TF reconstituted into phosphatidylcholine:phosphatidylserine membranes (TF/PCPS) or PC membranes (TF/PC). The initial rate of factor Xa formation was equivalent in the presence or absence of 2.4 nM TFPI. However, reaction extent was small (<20%) relative to that observed in the absence of TFPI, implying the rapid inhibition of VIIa.TF during factor X activation. Initiation of factor Xa formation using increasing concentrations of TF/PCPS or TF/PC in the presence of TFPI yielded families of progress curves where both initial rate and reaction extent were linearly proportional to the concentration of VIIa.TF. These observations were consistent with a kinetic model in which the rate-limiting step represents the initial inhibition of newly formed factor Xa. Numerical analyses of progress curves yielded a rate constant for inhibition of VIIa.TF by Xa.TFPI (>10(8) M-1.s-1) that was substantially greater than the value (7.34 +/- 0.8 x 10(6) M-1.s-1) directly measured. Thus, VIIa.TF is inhibited at near diffusion-limited rates by Xa.TFPI formed during catalysis which cannot be explained by studies of the isolated reaction. We propose that the predominant inhibitory pathway during factor X activation may involve the initial inhibition of factor Xa either bound to or in the near vicinity of VIIa.TF on the membrane surface. As a result, VIIa.TF inhibition is unexpectedly rapid, and the concentration of active factor Xa that escapes regulation is linearly dependent on the availability of TF.
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PMID:Regulation of extrinsic pathway factor Xa formation by tissue factor pathway inhibitor. 946 88


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