Of fibrinogen

Thrombin, the two-chain derivative of the prothrombin molecule, has a molecular weight of approximately 37,000 daltons. Its proteolytic properties induce the conversion of fibrinogen to fibrin to produce the initial visible manifestation of coagulation, the soluble fibrin clot. In addition, thrombin influences the activity of Factors V, VIII, and XIII and plasmin. Thrombin affects platelet function by inducing viscous metamorphosis and the release reaction with subsequent aggregation. 

FIGURE 15.5 The cascade of activation steps leading to blood clotting. The intrinsic and extrinsic pathways converge at Factor X, and the final common pathway involves the activation of thrombin and its conversion of fibrinogen into fibrin, which aggregates into ordered filamentous arrays that become cross-linked to form the clot. 

Fibrin is formed from fibrinogen synthesized by the liver and secreted into the circulation. The conversion of fibrinogen to fibrin is initiated by a serine protease, thrombin. Thrombin, at the same time, can activate a transglutaminase enzyme, factor XIII present in... 

Heparin inhibits the formation of fibrin clots, inhibits the conversion of fibrinogen to fibrin, and inactivates several of the factors necessary for the clotting of blood. Heparin cannot be taken orally because it is inactivated by gastric acid in the stomach therefore, it must be given by injection. Heparin has no effect on clots that have already formed and aids only in preventing the formation of new blood clots (thrombi). The LMWHs act to inhibit clotting reactions by binding to antithrombin HI, which inhibits the synthesis of factor Xa and the formation of thrombin. 

Brown AEX, Litvinov RI, Discher DE et al (2007) Forced unfolding of the coiled- coils of fibrinogen by single molecule afm. Biophys J 92 L39-L42... 

Initiation of the fibrin clot in response to tissue injury is carried out by the extrinsic pathway. How the intrinsic pathway is activated in vivo is unclear, but it involves a negatively charged surface. The intrinsic and extrinsic pathways converge in a final common path-vray involving the activation of prothrombin to thrombin and the thrombin-catalyzed cleavage of fibrinogen to form the fibrin clot. The intrinsic, extrinsic, and final common pathways are complex and involve many different proteins (Figure 51-1 and Table 51-1). In... 

Thrombin (34 kDa), a serine protease formed by the prothrombinase complex, hydrolyzes the four Arg-Gly bonds between the fibrinopeptides and the a and P portions of the Aa and BP chains of fibrinogen (Figure 51-5A). The release of the fibrinopeptides by thrombin generates fibrin monomer, which has the subunit stmc-... 

Figure 51-4. Diagrammatic representation (not to scaie) of fibrinogen showing pairs of Aa, B 3, and y chains iinked by disuifide bonds. (FPA, fibrinopeptide A FPB, fibrinopeptide B.)...

Figure 51-5. Formation of a fibrin clot. A Thrombin-induced cleavage of Arg-Gly bonds of the Aaand B(3 chains of fibrinogen to produce fi-brinopeptides (left-hand side) and the a and p chains of fibrin monomer (right-hand side). B Cross-linking of fibrin molecules by activated factor XIII (factor Xllla).

A number of laboratory tests are available to measure the phases of hemostasis described above. The tests include platelet count, bleeding time, activated partial thromboplastin time (aPTT or PTT), prothrombin time (PT), thrombin time (TT), concentration of fibrinogen, fibrin clot stabifity, and measurement of fibrin degradation products. The platelet count quantitates the number of platelets, and the bleeding time is an overall test of platelet function. aPTT is a measure of the intrinsic pathway and PT of the extrinsic pathway. PT is used to measure the effectiveness of oral anticoagulants such as warfarin, and aPTT is used to monitor heparin therapy. The reader is referred to a textbook of hematology for a discussion of these tests. 

W. L., and Rivera, K. E., Facile purification of fibrinogen fragments using a computer-based model with general applicability to the generation of salt gradients, Prot. Expression Purif., 14, 71, 1998. 

Cryoprecipitate Component of blood obtained by freezing and thawing plasma that is rich in clotting factors. One unit of cryoprecipitate derived from a unit of whole blood contains a volume of 10 to 20 mL, 80 to 100 units of factor VIII which consists of both the procoagulant activity and the von Willebrand factor, 150 to 250 mg of fibrinogen, 50 to 100 units of factor XIII, and 50 to 60 mg of fibronectin. 

Ginseng Rg2 has shown inhibitory effects on platelet aggregation similar to aspirin, and R0 reportedly inhibits the conversion of fibrinogen to fibrin [27]. The amount of ginseng administered may also influence the effect(s) produced. In rats and mice, small doses of ginseng extract result in increased spontaneous motor activity, whereas larger doses produce an inhibitory effect on the central nervous system [28]. 

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