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Biological Role of Fibrinolysis

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Biological Role of Fibrinolysis J Clin Pathol: first published as 10.1136/jcp.33.Suppl_14.1 on 1 January 1980. Downloaded from J Clin Pathol, 33, Suppl (Roy Coll Path), 14, 1-4 Scientific basis Biological role of fibrinolysis JF DAVIDSON AND ISOBEL D WALKER From the Thrombosis Research Group, Department of Haematology, Glasgow Royal Infirmary, Glasgow Fibrinolysis is a basic defence mechanism of the blood. The intrinsic activators are, in the main, organism designed to control the deposition of components of the factor XII activation system fibrin in the vascular system and elsewhere. When produced by activation of the intrinsic coagulation thrombin is generated by the coagulation system it mechanism. The activity of these activators is acts on fibrinogen and 'precipitates' it out of regulated by inhibitors, but our knowledge of the solution as a polymer-fibrin. This polymer anti-activators is less clear than our knowledge of deposition is regulated by the fibrinolytic system the anti-plasmins. The activity of vascular activator which acts on the insoluble protein fibrin and, by is also regulated by its high affinity for fibrin. splitting a limited number of peptide bonds, renders it soluble. This solubilisation mechanism which Role of fibrin liquifies the fibrin clot is the fibrinolytic system. Fibrinolysis was first described in a rudimentary Fibrin is central to the whole mechanism of fibrino- form over a century ago. Nowadays the term lysis because the fibrin surface provides a special copyright. fibrinolysis is generally restricted to the 'fibrinoly- milieu with optimum conditions for the reactions of sing' action of the plasminogen-plasmin system, but the plasminogen-plasmin system. Plasminogen and fibrin is of course removed by other means such as plasminogen activator bind selectively to fibrin and phagocytosis and proteolysis by other enzymes. on the fibrin surface plasmin is formed which immediately interacts with the fibrin. This interaction Plasminogen-plasmin occupies the active sites of the plasmin and renders them unavailable for interaction and neutralisation The key component of the fibrinolytic (plasminogen- by the anti-plasmin in the surrounding plasma. plasmin) system is the single-chain glycoprotein Plasmin formed away from the fibrin surface http://jcp.bmj.com/ plasminogen, which is present in plasma and in immediately interacts with the fast anti-plasmin most tissues. From this polypeptide proenzyme the a2-plasmin inhibitor (a2-PI) and is inhibited. This highly proteolytic serine protease plasmin is formed inhibition reaction is somewhat unique in its by limited proteolysis. Plasmin has broad substrate speed of action, being the fastest protein- specificity but in vivo it is rendered relatively selective protein interaction known. This remarkable speed for fibrin by the nature of the molecular biology of of reaction gives an indication of the biological the fibrin, plasminogen, plasmin, and anti-plasmin importance of controlling plasmin activity. In this on September 29, 2021 by guest. Protected interactions. way plasmin-mediated proteolysis is made highly specific for fibrin and is contained within the im- Activators mediate environment of the clot. Like several parts of the coagulation mechanism it is a surface-linked The conversion of plasminogen to plasmin is phenomenon. brought about by plasminogen activators which are Only when excessive amounts of plasmin are in two categories, firstly, extrinsic plasminogen formed or the a2-PI concentration in the plasma is activators which are extrinsic to the plasma and, deficient can the fibrinolytic process extend out secondly, intrinsic plasminogen activators which are into the general circulation and generalised fibrino- intrinsic to the plasma. lysis ensue. The extrinsic activators are the tissue activators, which are firmly fixed in tissues, and the endothelial Fibrinolysis interactions cell activator-vascular activator-which is found in the vascular endothelium and released into the Fibrinolysis is indeed a complex co-operative 1 J Clin Pathol: first published as 10.1136/jcp.33.Suppl_14.1 on 1 January 1980. Downloaded from 2 Davidson and Walker reaction at the fibrin surface of activator, plasmin- suprarenals, thyroid, and ovaries and in lowest ogen, plasmin, and a2-PI which, should it spill over concentration in liver, testes, and spleen. into the surrounding plasma, will be met by a Activator is also found in human milk, tears, massive potential of anti-plasmin activity primarily saliva, cerebrospinal fluid, and bile. A trigger that in the form of a2-PI and secondarily as a2-macro- can initiate fibrinolysis is therefore available globulin (a2-M).' These molecular interactions have throughout most body tissues and body fluids. recently been elucidated for fibrinolysis in blood and The activation of this trigger appears to be the it is assumed that fibrinolysis in tissues and non- result of fibrin deposition, ischaemia, hypoxia, or a vascular spaces follows a similar pattern. combination of these, but our knowledge of this is very scanty. Biological principles of fibrinolysis The extrinsic activation mechanism of fibrinolysis seems to play an important role in keeping body The biological principles which operate the fibrin- cavities and channels free from unwanted fibrin. olytic system are basically the same as those for Thus the urinary tract activator urokinase helps to the coagulation system. An initiating or trigger keep the urinary tract free of fibrin. Various duct mechanism is required to 'turn on' the system and systems like the lacrymal ducts and bile ducts have thereafter serine proteases are generated to achieve plasminogen activator activity which helps to keep the system's biological function. While proteases them patent. Fibrinolysis also plays an important can generate biological functions they can also role in reproduction. It contributes to maintaining destroy them, because proteolysis is an irreversible the patency of reproductive channels and plays a process and proteases are not endowed with repair role in fertilisation. It is also an important component functions. A 'switch off' mechanism or an inhibition in the mechanism of menstrual bleeding. mechanism is, therefore, essential to limit proteolysis. The trigger mechanism for activating fibrinolysis This on-and-off mechanism is controlled by in blood is more complicated. Blood contains some different switches. The on switch is the release, or extrinsic activator in the form of vascular endothelial perhaps activation, of plasminogen activators. The activator which is synthetised in the endothelial copyright. off switch is largely the containment of the reaction cells of the small veins, stored there, and released on by the inhibitor a2-PI supplemented, if necessary, demand. The concentration ofthis activator in blood by a2-M. It is aided by the constraints of a surface has a pronounced diurnal variation, being lowest in mediated reaction, and the run down of available the morning. Its release can be greatly enhanced by surface for the fibrinolytic reaction as fibrin is vascular stasis, ischaemia, and exercise. digested. The on switch of activation is also regulated Blood also contains two types ofintrinsic activators by anti-activators. Unlike the a2-PI mechanism, -one which is associated with activation of factor the mechanism of these anti-activator reactions is XII (the factor XII dependent pathway) and the poorly understood and is still the subject of some other which is independent of factor XII (the factor http://jcp.bmj.com/ controversy. XII independent pathway). It now seems quite In addition to the switch on, switch off mech- clear that when intrinsic coagulation is activated anisms, the fibrinolytic system has an inbuilt fibrinolysis is activated at the same time. There is self-acceleration process. When plasminogen acti- much still to be learned about the various plasmin- vator meets plasminogen, if the conditions are ogen activators and how they become activated. right, plasmin is formed. This plasmin, in addition to attacking fibrin, feeds back into the system and The dynamic hypothesis on September 29, 2021 by guest. Protected greatly accelerates the conversion of plasminogen to plasmin. Many years ago it was proposed that there was a dynamic balance between coagulation and fibrin- Fibrinolysis activation olysis in the vascular system. Thus it was sug- gested that fibrin was being continuously laid down Our knowledge of the trigger mechanism is very from low grade activation of the coagulation system limited, although we know there are several types and continuously cleared by low grade activation of plasminogen activators.2 The tissue type of of the fibrinolytic system. Present evidence to plasminogen activator is firmly fixed in tissues and support this concept is scanty and it must now be can be extracted and studied. It is a stable protein accepted that systemic intravascular fibrin deposition which is probably very closely related to vascular, is normally extremely limited. This, however, does or endothelial, activator and to the urine activator not exclude the possibility of localised fibrin urokinase. It is found in most body tissues and in deposition as an everyday occurrence and as an greatest concentration in tissues such as the uterus, expression of normal 'wear and tear'. J Clin Pathol: first published as 10.1136/jcp.33.Suppl_14.1 on 1 January 1980. Downloaded from Biological role offibrinolysis 3 Clinical problems arise not only from excessive important biological role of a2-PI in
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