Fibrinolysis proteins

Figure 2.19 Organization of polypeptide chains into domains. Small protein molecules like the epidermal growth factor, EGF, comprise only one domain. Others, like the serine proteinase chymotrypsin, are arranged in two domains that are required to form a functional unit (see Chapter 11). Many of the proteins that are involved in blood coagulation and fibrinolysis, such as urokinase, factor IX, and plasminogen, have long polypeptide chains that comprise different combinations of domains homologous to EGF and serine proteinases and, in addition, calcium-binding domains and Kringle domains.

Urokinase-type plasminogen activator (uPA, urokinase) is synthesized by endothelial and tumor cells as a single-chain glycoprotein (scuPA) without catalytic activity. When it is converted to a two-chain protein (tcuPA) by plasmin, an active serine protease center develops, which activates plasminogen. Thus, uPA (55 kDa) results in the amplification of fibrinolysis. 

Basic aspects of the proteins of the blood coagulation system and of fibrinolysis are described in this chapter. Some fundamental aspects of platelet biology are also presented. Hemorrhagic and thrombotic states can cause serious medical emergencies, and thromboses in the coronary and cerebral arteries are major causes of death in many parts of the world. Rational management of these conditions requires a clear understanding of the bases of blood clotting and fibrinolysis. 

Figure 51-7. Scheme of sites of action of streptokinase, tissue plasminogen activator (t-PA), urokinase, plasminogen activator inhibitor, and Kj-antiplasmin (the last two proteins exert inhibitory actions). Streptokinase forms a complex with plasminogen, which exhibits proteolytic activity this cleaves some plasminogen to plasmin, initiating fibrinolysis. 

The inflammatory process in sepsis is linked to the coagulation system. Pro-inflammatory mediators maybe procoagulant and antifibrinolytic, whereas anti-inflammatory mediators may be fibrinolytic. A key factor in the inflammation of sepsis is activated protein C, which enhances fibrinolysis and inhibits inflammation. Protein C levels are decreased in septic patients. 

The antithrombotic factors produced by endothelial cells are thrombomodulin (TM) and protein S (PS), components of the vitamin K-dependent protein C (PC) anticoagulant pathway, inhibiting F-Va-F-Villa (E15) tissue plasminogen activator (tPA), responsible for fibrinolysis (N2, LI8) and the lipoprotein-associated coagulation inhibitor (LACI), which inhibits F-VIIa-TF complex and F-Xa (B51). 

Protein C and Protein S. PC and PS may inhibit thrombin formation and complex with PAI, thereby promoting fibrinolysis. During sepsis, PC activity is significantly reduced, either by consumption or by TM down-regulation, while increased levels of ClbBP inhibit PS. Infusion of activated PC and PS protected animals from the lethal effect of bacteria. Administration of PC and PS concentrates should be studied carefully in septic patients before their use is recommended (F14). 

Although the fibrinolytic pathway is activated when thrombin binds to thrombomodulin, the thrombin-thrombomodulin complex, in addition to activating protein C (APC), activates a fibrinolysis inhibitor called the thrombin-activatable fibrinolysis inhibitor (TAFIa). Thus plasmin generation and, in turn, fibrinolysis are... 

By far the most widely measured marker of hemostatic activation is D-dimer, which is a product formed by the action of plasmin on cross-linked fibrin (95). D-dimer levels in plasma are generally elevated in DIC. The consumption of platelets and coagulation proteins as a result of thrombin generation leads to the deposition of fibrin thrombi at multiple organ sites. This triggers fibrinolysis with an increase in the formation of fibrin degradation products, which can cause bleeding at multiple sites. Because DIC can have a variety of causes and may coexist with systemic fibrinolysis, such as in pulmonary embolism or deep vein thrombosis, the d-Dimer test is not specific for DIC (95). 

Administration of activated protein C (drotrecogin) to promote fibrinolysis and associated antiinflammatory mechanisms may be beneficial in patients with an APACHE II (Acute Physiology and Chronic Health Evaluation II) score greater than 25. This agent reduced mortality in severe sepsis but poses an increased risk of serious bleeding. 

The fibrin thrombus resulting from blood clotting (see p. 290) is dissolved again by plasmin, a serine proteinase found in the blood plasma. For this purpose, the precursor plasminogen first has to be proteolyti-cally activated by enzymes from various tissues. This group includes the plasminogen activator from the kidney (urokinase) and tissue plasminogen activator (t-PA) from vascular endothelia. By contrast, the plasma protein a2-antiplasmin, which binds to active plasmin and thereby inactivates it, inhibits fibrinolysis. 

Partially responsible for inhibition of the extrinsic pathway. Inactivates factors V and VIII and promotes fibrinolysis. Activity declines following warfarin administration. cofactor to accelerate the anticoagulant activity of protein C. Decreased levels occur following warfarin administration. ... 

Fibrinolysis. The hydrolysis of an elastic, filamentous protein (fibrin) derived from fibrinogen by the action of thrombin, which releases fibrinopeptides A and B (co-fibrins A and B) from fibrinogen in co-agulation of the blood. 

Mechanism of Action A tissue plasminogen activator that acts as a CV-thrombolytic by binding to the fibrin in a thrombus and converting entrapped plasminogen to plas-min. This process initiates fibrinolysis. Therapeutic Effect Degrades fibrin clots, fibrinogen, and other plasma proteins. 

Anistreplase (streptokinase- plasminogen complex) streptococci protein streptococci plaminogen, which is then cleaved to form plasmin. Plasmin mediate fibrinolysis. Systemic lytic state and immunogenicity may limit its use. Because antistreplase is aheadly lined to plaminogen, the onset of fibrinolytic is faster. These enzymes are not fibrin-specific. 

A third underlying mechanism seems to involve a reduction in concentrations of free protein S, again more pronounced with third-generation products. When protein S falls, the antifibrinolytic effect of the so-called thrombin-activated fibrinolysis inhibitor is increased in other words, fibrinolysis is impeded, with an increased risk of clotting problems (104). Again, however, these are recent methods, which were not available when the third-generation products were launched. 

Nicolaes GA, Thomassen MC, Tans G, Rosing J, Hemker HC. Effect of activated protein C on thrombin generation and on the thrombin potential in plasma of normal and APC-resistant individuals. Blood Coagul Fibrinolysis 1997 8(l) 28-38. 

Antifibrinolytic compounds can block the conversion of plasminogen to plasmin, or directly bind to the active site of plasmin to inhibit fibrinolysis. The plasma protein, a 2-macroglobulin, is a primary physiological inhibitor of plasmin. Plasmin released from fibrin is also very rapidly inactivated by a2-antiplasmin, which plays a role in the regulation of the fibrinolytic process (Aoki and Harpel, 1984). 2-anti plasmin inactivates plasmin in a very rapid reaction, interferes with plasminogen binding to fibrin, and is ligated to fibrin by Factor Xllla (Sakata and Aoki, 1980). After a2-antiplasmin is covalendy linked to fibrin s G-terminal a chain, it retains it ability to inhibit plasmin, a function that helps to prevent premature clot lysis. 

Thrombin activable fibrinolysis inhibitor (TAFI) is a plasma protein that is activated by thrombin in the presence of thrombomodulin to a labile carboxypeptidase-B-like enzyme that inhibits fibrinolysis. When TAFIa is included in a clot undergoing lysis induced by tPA and plasminogen, the time to achieve lysis is prolonged and free lysine and arginine are released (Wang et al., 1998). TAFIa retards the fibrin-enhanced activation of plasminogen by tPA and inhibits the accumulation of plasminogen at the lysis front (Sakharov et al., 1997). 

Yasuda, S., Bohgaki, M., Atsumi, T., and T. Koike, 2005, Pathogenesis of antiphospholipid antibodies impairment of fibrinolysis and monocyte activation via die p38 mitogen-activated protein kinase pathway.Immunobiology. 210(10) 775-80. 

Mannucci PM, Mari D, Merati G, Peyvandi F, Tagliabue L, Sacchi E, Taioli E, Sansoni P, Bertolini S, Franceschi C. Gene polymorphisms predicting high plasma levels of coagulation and fibrinolysis proteins. A study in centenarians. Arterioscler Thromb Vase Biol 1997 17 755-759. 

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