Fibrin clot lysis

In vitro assays do not use any whole-cell or animal-based components. The fibrin clot lysis assay, as established for tissue plasminogen activators and described for alteplase in the USP, is an example of this type of potency testing [5]. By means of defined standard materials, a fibrin clot is formed and the time to complete lysis is characterized as measure of potency, compared to a reference standard with defined activity. The LAL-test is a well-established and internationally harmonized in vitro alternative to detect or quantify bacterial endotoxins, using Limulus amebocyte lysate (LAL) obtained from the aqueous extracts of circulating amebocytes of horseshoe crab (Limulus polyphemus or Tachypleus tri-dentatus) which has been prepared and characterized appropriately [5]. Two types of technique may be used for this test gel-clot techniques, which are based on gel formation and photometric techniques. 

Scanu (S9) confirmed that Lp(a) binds to fibrin and competes with plasminogen and tPA. Therefore, the activation of plasminogen is prohibited, a process involving the temairy complex of tPA, plasminogen, and fibrinogen. As a result, clot lysis in vitro is diminished (Fig. 10). 

The increased platelet adhesion onto HCP surfaces is mainly due to the high surfacial concentration of fibrinogen associated with immobilized heparin. Therefore one of the solutions to the problem could have been provided by simultaneous immobilization of heparin and proteolytic enzymes which could hydrolyze the adsorbed fibrinogen. The immobilized enzyme could also provide the lysis of the fibrin clot, in case it was formed, for instance in stagnation regions of the complicated devices. 

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. 

Sakharov, D. V., Nagelkerke, J. F., and Rijken, D. C. (1996). Rearrangements of the fibrin network and spatial distribution of fibrinolytic components during plasma clot lysis. Study with confocal microscopy. / Biol. Chem. 271, 2133-2138. 

SidelmannJJ, Gram J, Jespersen J, Kluft C. Fibrin clot formation and lysis basic mechanisms. Semin Thromb Hemost. 2000 26 605-618. 

The potency of urokinase is tieieiroined by comparing its capacity to activate plasminogen to form plasmin with the same capacity of a reference preparation of urokinase calibrated in LU. the formation of plasmin is measured by the determination of the lysis time of a fibrin clot in given conditions. 

Fibrinolysis has previously been reviewed.70 80 The physiologic process in which a fibrin clot is dissolved and the regulation of that process is not completely understood. The main proenzyme in the blood is plasminogen, a Ps globulin, which when activated is converted to the fibrin lysis enzyme plasmin, a globulin. Plasmin is usually bound by many other proteins and inactivated in the plasma. If this did not occur, pro-... 

Reteplase acts from within the clot, and so must penetrate the cloi before it is active. TPA binds to the fibrin matrix on the outside of the clot. Thus, higher clot lysis rates are reported for reteplase than for TPA. 

A series of events such as the platelet aggregation, hbrinogenesis, hbrin adhesion, hbrin aggregation, and vascular inner wall injury are implicated in the thrombosis. Thus the thrombosis, antithrombosis, and thrombolysis may relate to antiplatelet aggregation [96], chemical- and electrical-induced blood vessel injury [97, 98], thread-induced fibrin or platelet adhesion [99], euglobulin clot lysis time [100,101], fibrinolytic area [102], and reduction of thrombus mass [103,104]. 

Plasmin, fibrinolysin, a serine protease catalyzing Lys-Xaa and Arg-Xaa bond cleavage similar to that of trypsin. Plasmin is the key protease in blood clot lysis, and its major natural substrates are fibrinogen and fibrin. Human plasmin is derived from plasminogen, and is a two-chain protein consisting of the A or H chain (Mr 65 kDa) and the B or L chain (Mr 27.7 kDa). The active site is located in the B chain. The various molecular forms of plasmin are inactivated by protein inhibitors such as the Kunitz type, serpins, soybean and limabean trypsin inhibitors. The most important, fast-acting inhibitor of plasmin... 

Describe the lysis of fibrin clots by plasmin and the activation of plasminogen by tissue-type plasminogen activator (TPA). 

The role of the fibrinolytic system is to dissolve any clots that are formed within the intact vascular system and so restrict clot formation to the site of injury. The digestion of the fibrin and hence its lysis is catalysed by the proteolytic enzyme, plasmin, another serine proteinase. Plasmin is formed from the inactive precursor, plasminogen, by the activity of yet other proteolytic enzymes, urokinase, streptokinase and tissue plasminogen activator (tPA) which are also serine proteinases. These enzymes only hydrolyse plasminogen that is bound to the fibrin. Any plasmin that escapes into the general circulation is inactivated by binding to a serpin (Box 17.2). 

The hbrinolytic system (Fig. 22.2) is involved in restricting clot propagation in the blood and in the removal of fibrin as wounds heal. Treatment of patients with hbrinolytic (thrombolytic) drugs that activate the hbrinolytic system is not a substitute for the anticoagulant drugs. The purpose of thrombolytic therapy is rapid lysis of already formed clots. 

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). 

Regulation of the fibrinolytic system is useful in therapeutics. Increased fibrinolysis is effective therapy for thrombotic disease. Tissue plasminogen activator (t-PA), urokinase, and streptokinase all activate the fibrinolytic system (Figure 34-3). Conversely, decreased fibrinolysis protects clots from lysis and reduces the bleeding of hemostatic failure. Aminocaproic acid is a clinically useful inhibitor of fibrinolysis. Heparin and the oral anticoagulant drugs do not affect the fibrin-olytic mechanism. 

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