Enzymes Plasmin

Factor Xlla converts prekallikrein to kallikrein and kallikrein cleaves HK to generate bradykinin. There is also an important positive feedback in the system in which the kallikrein generated rapidly converts unactivated factor XII to activated factor XII, and the rate of this reaction is hundreds of times faster than the rate of autoactivation [11]. Therefore, much of the unactivated factor XII can be cleaved and activated by kallikrein. Cl inhibitor inhibits all functions of factor Xlla and it is one of two major plasma kallikrein inhibitors. Thus all functions of kallikrein are also inhibited, including the feedback activation of factor XII, the cleavage of HK, and the activation of plasma pro-urokinase [66] to lead to plasmin formation. Cl inhibitor also inhibits the fibrinolytic enzyme plasmin, although it is a relatively minor inhibitor compared to a2-antiplasmin or a2-macroglobulin. 

The measurement of plasminogen activator inhibitor-1 (PAI-1), which complexes with tissue plasminogen activator (t-PA) and thus affects the ability of the latter to activate fibrinolysis, is useful in the assessment of fibrinolytic disorders (93). The complex formed by the fibrinolytic enzyme plasmin with its inhibitor... 

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). 

Fig. 1. Basic scheme of fibrin polymerization and fibrinolysis. The clot is formed on the conversion of fibrinogen to fibrin by cleavage of the fibrinopeptides by thrombin, followed by stabilization of the network with isopeptide bonds by the transglutaminase Factor XHIa. The clot is dissolved through proteolysis by the enzyme plasmin, which is activated on the fibrin surface by plasminogen activators. This process is controlled by several inhibitory reactions (black arrows).

The central process of fibrinolysis is conversion of inactive plasminogen to the proteolytic enzyme plasmin. Injured cells release activators of plasminogen. Plasmin remodels the thrombus and limits the extension of thrombosis by proteolytic digestion of fibrin. 

So far the endogenous fibrinolytic enzyme plasmin could not successfully used m thrombolysis because unan injection, it was immediately blocked... 

The breakdown, or lysis, of formed clots depends on production of a proteolytic enzyme, plasmin, from the inactive precursor plasminogen, when coagulation is initiated. 

Formation of the proteolytic plasma enzyme, plasmin, out of its precursor, plasminogen, by adding kaolin or chloroform to plasma requires factor XII (42) as well as at least one cofactor. Chloroform may well create an insoluble protein film at the plasma/chloroform interface, again interposing more physical events. At least one Hageman factor cofactor has been purified (43). It is removed from plasma by activating powders such as glass and is perhaps responsible for the lysine esterase activity present in unactivated plasma (44). Thus, particular amounts of certain interfaces may simultaneously add and remove enzyme activities. 

An important application of fibrinolytic drugs has been to dissolve thrombi in acutely occluded coronary arteries, thereby to restore blood supply to ischaemic myocardium, to limit necrosis and to improve prognosis. The approach is to give a plasminogen activator intravenously by infusion or by bolus injection in order to increase the formation of the fibrinolytic enzyme plasmin. Those currently available include ... 

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-... 

Under pathological conditions, e.g., (DIC), fibrinogen can be digested by the fibrinolytic enzyme plasmin. This leads to bleeding, even though clotting is also occurring. 

The determination of plasminogen levels and the enzyme, plasmin, derived from this zymogen has been considered difiicult because of the presence of potent inhibitors in human plasma. The older methods, utilizing natural substrates such as casein, lack the desired sensitivity, are lengthy and involve procedures not easily adapted for the routine laboratory or for interlaboratory comparison (F3). Because of the above concerns and the general interest in the fibrinolytic system, substantial research has been devoted to develop a synthetic substrate method for these two components. 

Latallo had developed a similar assay using the enzyme plasmin and soybean trypsin inhibitor in place of aprotinin (L4). 

So far, the endogenous fibrinolytic enzyme plasmin could not successfully be used in thrombolysis because upon injection, it was immediately blocked by endogenous inhibitors, particularly by (X2-antiplasmin. Plasmin became fibrinolytically effective only after consumption of these inhibitors. To obtain a fibrinolytic effect, large quantities of plasmin were required, which in turn led to hyperplasminemia. This caused proteolytic degradation of fibrinogen and several clotting factors and hence hemordiagic complications. 

Once the damage is repaired, the clot is dissolved by the action of a proteolytic enzyme plasmin. Plasmin is normally found circulating in the blood as its inactive precursor plasminogen. It is activated by tissue plasminogen activator as healing takes place. 

These drugs are used to destroy thrombi that are occluding blood vessels. They are most effective given soon after the thrombus has formed, for example within 3 hours of a myocardial infarction. All of the drugs in use activate plasminogen resulting in the production of the active enzyme plasmin. 

Whether the removal of fibrinogen by intact plasma represents physical or enzymatic displacement by high molecular weight kininogen and factor XII, or an indirect result of activation of plasminogen to the fibrino(gen)olytic enzyme plasmin via the activation of factor XII (14), remains to be studied. 

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