Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: EC:3.4.21.68 (tissue plasminogen activator)
 11,311 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Human blood coagulation factor XI is a dimer composed of two identical subunits. Each subunit contains four apple domains as tandem repeats followed by a serine protease region. A disulfide bridge between Cys321 of each fourth apple domain links the subunits together. The role of Cys321 in the dimerization of factor XI was examined by mutagenesis followed by expression of its cDNA in baby hamster kidney cells. The recombinant proteins were then purified from the tissue culture medium and shown to have full biological activity. Normal recombinant factor XI was secreted as a dimer as determined by SDS-PAGE, while recombinant factor XI-Cys321 Ser migrated as a monomer under these conditions. Gel filtration studies, however, revealed that each protein existed as a dimer under native conditions, indicating that the disulfide bond between Cys321 of each factor XI monomer was not necessary for dimer formation. The fourth apple domain (apple4) of factor XI was then introduced into tissue plasminogen activator (tPA) to investigate its role in the dimerization of other polypeptide chains. The fusion protein, containing apple4 (apple4-tPA), formed dimers as detected by SDS-PAGE and gel filtration. Furthermore, dimerization was specific to apple4, while apple3 had no effect on dimerization. These data further indicated that the apple4 domain of factor XI mediates dimerization of the two subunits and the interchain disulfide bond involving Cys321 was not essential for dimer formation.
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PMID:Apple four in human blood coagulation factor XI mediates dimer formation. 158 18

Factor XI (plasma thromboplastin antecedent) is a plasma glycoprotein that participates in the early phase of blood coagulation. The gene for the human protein has been isolated from two different lambda phage genomic libraries. Four independent recombinant lambda phage carrying overlapping DNA inserts that coded for the entire gene for factor XI were isolated and characterized by restriction mapping, Southern blotting, and selective DNA sequencing to establish the number and location of the intron-exon boundaries. The gene for human factor XI was 23 kilobases in length and consisted of 15 exons (I-XV) and 14 introns (A-N). Exon I coded for the 5' untranslated region, and exon II coded for the signal peptide. The next eight exons (III-X) coded for the four tandem repeats of 90 or 91 amino acids that were present in the amino-terminal region of the mature protein. Each of these tandem repeats was coded by two exons that were interrupted by a single intron, and these introns were located in essentially the same position within each of the four tandem repeats. The carboxyl-terminal region of the protein, which contained the catalytic chain, was coded by five exons (XI-XV) that were interrupted by four introns. The last four introns were located in the same positions as those in the genes for human tissue plasminogen activator and human urokinase.
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PMID:Organization of the gene for human factor XI. 282 46

Recently, an alternative pathway for factor XI activation has been described in which factor XI is activated by thrombin. Patients with a factor XI deficiency bleed mostly from tissues with high local fibrinolytic activity. Therefore, the role of thrombin-mediated factor XI activation in both fibrin formation and fibrinolysis was studied in a plasma system. Clotting was induced by the addition of tissue factor or thrombin to recalcified plasma in the presence or absence of tissue-type plasminogen activator, after which clot formation and lysis were measured using turbidimetry. Thrombin-mediated activation of factor XI was found to take place in plasma under physiologic conditions in the absence of a dextran sulfate-like cofactor. At high tissue factor concentrations, no effect of factor XI was seen on the rate of fibrin formation. Decreasing amounts of tissue factor resulted in a gradually increasing contribution of factor XI to the rate of fibrin formation. In addition, thrombin-mediated factor XI activation resulted in an inhibition of tissue-type plasminogen activator-induced lysis of the clot. This inhibition occurred even at tissue factor concentrations at which no effect of factor XI was observed on fibrin formation. Trace amounts of activated factor XI (1.25 pmol/L, representing 0.01% activation) were capable of completely inhibiting fibrinolysis in our system. The inhibitory effect was found to be mediated by thrombin that is additionally generated in a factor XI-dependent manner via the intrinsic pathway and is capable of protecting the clot against lysis. We also observed that formation of additional thrombin continued after the clot had been formed. We conclude that thrombin-mediated factor XI activation can take place in plasma. The presence of factor XI during coagulation results in the formation of additional thrombin within the clot capable of protecting this clot from fibrinolytic attack. The large amounts of thrombin that are formed by the intrinsic pathway via factor XI may play an important role in the procoagulant and thrombogenic state of clots and may therefore have important clinical and therapeutic implications.
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PMID:Feedback activation of factor XI by thrombin in plasma results in additional formation of thrombin that protects fibrin clots from fibrinolysis. 757 97

We investigated the age-related changes in blood coagulation, fibrinolysis, and platelet aggregation in male WBN/Kob rats, animals that exhibit spontaneously diabetes mellitus at more than 6 months of age. The rats aged 6 months or more showed significant hyperglycemia, hypoinsulinemia, and hyperlipidemia. As changes in coagulation parameters, the data indicated significant increases in factors II, V, VII, VIII, IX, X, and XII activities; a significant decrease in antithrombin III activity in rats more than 6 months of age; significant increases in fibrinogen level and factor XI activity; and significant decreases in prothrombin time and activated partial thromboplastin time in those more than 9 months of age. As changes in fibrinolytic parameters, the animals showed significant decreases in plasminogen and tissue-type plasminogen activator, and significant increases in alpha2-plasmin inhibitor and plasminogen activator inhibitor at more than 6 months of age. In addition, there were significant correlations between the plasma levels of coagulation/fibrinolytic markers and the 4-hour fasting glucose or lipids. Furthermore, they displayed significant increases in ADP- or collagen-induced platelet aggregation and in cholesterol/phospholipid molar ratio in platelets at more than 9 months of age. The increase in cholesterol/phospholipid ratio may be responsible for hyperaggregation of platelets in diabetic animals. These findings suggest that WBN/Kob rats are suitable for research on blood coagulation abnormalities in diabetes. However, further studies are needed to clarify the details of the mechanisms involved.
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PMID:Age-related changes in coagulation, fibrinolysis, and platelet aggregation in male WBN/Kob rats. 1089 50

The effect of oral contraceptives (OC) on fibrinolytic parameters was investigated in a cycle-controlled cross-over study in which 28 non-OC using women were randomly prescribed either a representative of the so-called second (30 microg ethinylestradiol, 150 microg levonorgestrel) or third generation OC (30 microg ethinylestradiol, 150 microg desogestrel) and who switched OC after a two month wash out period. During the use of OC, the levels of tissue-type plasminogen activator (tPA) activity, plasminogen, plasmin-alpha2-antiplasmin complexes and D-dimer significantly increased (by 30 to 80%), while the levels of plasminogen activator inhibitor- (PAI-1) antigen, PAI-1 activity and tPA antigen significantly decreased (25 to 50%), suggesting an increase in endogenous fibrinolytic activity. These OC-induced changes were not different between the two contraceptive pills. TAFI (thrombin-activatable fibrinolysis inhibitor) levels increased on levonorgestrel, and even further increased on desogestrel. A clot lysis assay that probes both fibrinolytic activity and the efficacy of the coagulation system to generate thrombin necessary to down regulate fibrinolysis via TAFI showed no change of the clot lysis time during OC use. This finding suggests that the OC-induced increase in endogenous fibrinolytic activity is counteracted by an increased capacity of the coagulation system to down regulate fibrinolysis via TAFI. Indeed we observed that during OC use there was a significant increase of F1+2 generation during clot formation. When these assays were performed in the presence of an antibody against factor XI, we observed that the clot lysis time was significantly increased during OC use and that the increase in F1+2 generation during OC therapy was due to a factor XI-independent process, which was significantly higher on desogestrel than on levonorgestrel. These data indicate that the OC-induced inhibition of endogenous fibrinolysis takes place in a factor XI-independent way and is more pronounced on desogestrel than on levonorgestrel-containing OC.
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PMID:Increased fibrinolytic activity during use of oral contraceptives is counteracted by an enhanced factor XI-independent down regulation of fibrinolysis: a randomized cross-over study of two low-dose oral contraceptives. 1092 60

The incomplete penetrance of thrombosis in familial protein C deficiency suggests disease occurs when this deficit is combined with additional abnormalities in the hemostatic system. The pattern of inherited thrombophilia in the Vermont II kindred, which is affected by a clinically dominant type I protein C deficiency, provides strong evidence for a second unidentified gene that segregates independently of protein C deficiency and increases susceptibility to thrombosis. To test the second gene hypothesis, thirty-four candidate genes for proteins involved in hemostasis or inflammation were tested as the unknown defect, using highly polymorphic short tandem repeat (STR) markers in an informative subset (n = 31) of the kindred. The genes considered are; alpha-fibrinogen, beta-fibrinogen, gamma-fibrinogen, prothrombin, tissue factor, factor V, protein S, complement component 4 binding protein, factor XI, factor XII, factor XIIIa, factor XIIIb, histidine rich glycoprotein, high molecular weight kininogen, kallikrein, von Willebrands factor, platelet factor 4, thrombospondin, antithrombin III, alpha-1-antitrypsin, thrombomodulin, plasminogen, tissue plasminogen activator, urokinase plasminogen activator, plasminogen activator inhibitor-1, plasminogen activator inhibitor-2, protein C inhibitor, alpha-2-plasmin inhibitor, kallistatin, lipoprotein a, interleukin 6, interleukin 1, cystathionine-beta-synthase, and methylenetetrahydrofolate reductase. Mutations in many of these genes have been previously established as independent risk factors for thrombosis. However, linkage analysis provided no evidence to implicate any of the candidate genes as the second inherited factor that promotes thrombophilia in this kindred.
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PMID:Genetic screening of candidate genes for a prothrombotic interaction with type I protein C deficiency in a large kindred. 1120 93

Changes in plasma hemostatic and fibrinolytic proteins were determined during courses of a murine model of fatal and non-fatal Rocky Mountain spotted fever. C3H/HeN mice were infected with Rickettsia conorii and coagulation and histopathologic studies were performed at prescribed periods of time. A significant decrease in plasma factor VIII activity and rise in plasma factor V procoagulant activity correlated with a fatal infection. Factor VII levels were unchanged; factor XI levels dropped early in the course in the lethally infected animals, but returned to normal. Factor XII, high molecular weight kininogen, and prekallikrein levels were unchanged by the sublethal infection. Prekallikrein levels fell during the lethal infection. Antithrombin concentrations were decreased significantly in all animals, but plasma plasminogen levels did not change in either group of animals. Nonocclusive thrombi were microscopically observed rarely and only in animals surviving a sublethal infection. A fall in tissue plasminogen activator activity and a rise in plasminogen activator inhibitor activity highly correlated with a lethal outcome. Lethal infection with R. conorii is associated with primary endothelial cell injury resulting in decreased tissue plasminogen activator and increased plasminogen activator inhibitor.
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PMID:Hemostatic/fibrinolytic protein changes in C3H/HeN mice infected with Rickettsia conorii--a model for Rocky Mountain spotted fever. 1158 21

When fibrin deposition and removal are properly balanced, the organism is protected from both a catastrophic loss of blood at the site of injury and the inappropriate loss of fluidity within the vascular system. When these activities are not properly balanced, however, severe bleeding or thromboses can occur. Myocardial infarction is a common and morbid consequence of the latter. The thrombin/thrombomodulin complex plays an essential role in regulating this balance because it generates both an anticoagulant substance, activated protein C, and an antifibrinolytic substance, activated TAFI (thrombin activatable fibrinolysis inhibitor, also known as plasma carboxypeptidase B or carboxypeptidase U). Thus, the coagulation and fibrinolytic cascades are explicitly linked by virtue of thrombin catalyzed activation of TAFI, either by the thrombin/thrombomodulin complex or, in the absence of thrombomodulin, by the massive amounts of thrombin generated through the factor XI-dependent pathway after clotting. Some potential targets for diagnosis, prognosis and therapy related to the balance between fibrin formation and removal include: development of a convenient global assay for plasma fibrinolytic potential; an assay for plasma or urine thrombomodulin that had been oxidized at methionine 388 and thereby has lost its capacity to stimulate activation of protein C but not TAFI; an assay for activated TAFI; discovery of a means for tapping the tremendous potential of the vasculature to acutely release tissue-type plasminogen activator; and an assessment of the potential role of polymorphisms in the TAFI gene which might influence TAFI levels or the properties TAFIa. In addition, a much fuller and quantitative understanding of the properties of the coagulation and tibrinolytic cascades is needed in order to optimize diagnosis, prognosis and therapy in disorders such as myocardial infarction that are related to the balance between fibrin formation and removal.
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PMID:Myocardial infarction and the balance between fibrin deposition and removal. 1166 89

TAFI (thrombin activatable fibrinolysis inhibitor) down regulates fibrinolysis after activation by relatively high concentrations of thrombin generated during coagulation via thrombin mediated factor XI activation and subsequent activation of the intrinsic pathway. It is this secondary burst of thrombin that is severely diminished in haemophilia A, a deficiency of coagulation factor VIII. We therefore investigated the role of TAFI in haemophilia A by measuring the clot lysis times of tissue factor induced fibrin formation and tPA mediated fibrinolysis. In haemophilia A plasma clot lysis times were normal at relatively high tissue factor concentrations but severely decreased at moderate to low tissue factor concentrations, indicating that the thrombin generation via the extrinsic pathway was insufficient to activate TAFI. Addition of factor VIII, TAFI or thrombomodulin restored the clot lysis times at low tissue factor concentrations. This confirms the hypothesis that the bleeding disorder in haemophilia A is not merely a defect in the initial clot formation but is in fact a triple defect: reduced thrombin formation via the extrinsic pathway at low tissue factor concentrations, a reduced secondary burst of thrombin generation via the intrinsic pathway and a defective down regulation of the fibrinolytic system by the intrinsic pathway.
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PMID:The defective down regulation of fibrinolysis in haemophilia A can be restored by increasing the TAFI plasma concentration. 1168 21

Factor XI (FXI), the zymogen of the blood coagulation protease FXIa, and the structurally homologous protein plasma prekallikrein circulate in plasma in noncovalent complexes with H-kininogen (HK). HK binds to the heavy chains of FXI and of prekallikrein. Each chain contains four apple domains (F1-F4 for FXI and P1-P4 for prekallikrein). Previous studies indicated that the HK-binding site on FXI is located in F1, whereas the major HK-binding site on prekallikrein is in P2. To determine the contribution of each FXI apple domain to HK-FXI complex formation, we examined binding of recombinant single apple domain-tissue plasminogen activator fusion proteins to HK. The order of affinity from highest to lowest is F2 F4 > F1 F3. Monoclonal antibodies against F2 are superior to F4 or F1 antibodies as inhibitors of HK binding to FXI. Antibody alphaP2, raised against prekallikrein, cross-reacts with FXI F2 and inhibits FXI-HK binding with an IC(50) of 8 nm. HK binding to a platelet-specific FXI variant lacking the N-terminal half of F2 is reduced > 5-fold compared with full-length FXI. A chimeric FXI molecule in which F2 is replaced by P2 is cleaved within P2 during activation by factor XIIa, resulting in greatly reduced HK binding capacity. In contrast, wild-type FXI is not cleaved within F2, and its binding capacity for HK is unaffected by factor XIIa. Our data show that HK binding to FXI involves multiple apple domains, with F2 being most important. The findings demonstrate a similarity in mechanism for FXI and prekallikrein binding to HK.
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PMID:Characterization of the H-kininogen-binding site on factor XI: a comparison of factor XI and plasma prekallikrein. 1173 91


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