Fibrin fibrinolysins

Although the exact action of the thrombolytic dragp is slightly different, these drugs break down fibrin clots by converting plasminogen to plasmin (fibrinolysin). Plasmin is an enzyme that breaks down the fibrin of a blood clot. This reopens blood vessels after their occlusion and prevents tissue necrosis. 

Fibrinolysins are produced by both staphylococci (staphylokinase) and streptococci (streptokinase). These toxins dissolve fibrin clots, formed by the host around wounds and lesions to seal them, by indirect activation of plasminogen, thereby increasing the likelihood of organism spread. Streptokinase m be employed elinieally in conjunction with streptodomase (Chapter 25) in the treatment of thrombosis. 

Fig. 8. Kinetics of unstabilized fibrin hydrolysis with complexes of immobilized heparin with fibrinogen (1), thrombin (2), fibrinolysin (plasmin) (3), and serum albumin (4),06)...

The breakdown of blood clots is illustrated in the lower part of Figure 2 5-1. Tissue plasminogen activator (t-PA) converts plasminogen to plasmin. Plasmin, also known as fibrinolysin, is an enzyme that directly breaks down the fibrin mesh, thus destroying the clot. 

Thrombolytics facilitate the breakdown and dissolution of clots that have already formed. These drugs work by converting plasminogen (profibrinolysin) to plasmin (fibrinolysin).34 As shown in Figure 25-1, plas-min is the active form of an endogenous enzyme that breaks down fibrin clots. Thrombolytic drugs activate this enzyme by various mechanisms and can be used to dissolve clots that have already formed, thus reopening occluded blood vessels. 

An abscess begins by the combined action of inflammatory cells (such as neutrophils), bacteria, fibrin, and other inflammatory mediators. Bacteria may release heparinases that cause local thrombosis and tissue necrosis or fibrinolysins, collagenases, or other enzymes that allow extension of the process into surrounding tissues. Neutrophils gathered in the abscess cavity die in 3 to 5 days, releasing lysosomal enzymes that liquefy the core of the abscess. A mature abscess may have a fibrinous capsule that isolates bacteria and the liquid core from antimicrobials and immunologic defenses. 

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... 

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