Fibrinolysen

Fibrinolysin is the enzyme responsible for dissolving fibrin clots in the blood. Historically, the term referred to the fibrinolytic activity in plasma, and it is now commonly associated with plasmin, the active serine protease in the fibrinolytic system that degrades fibrin and other clot-associated proteins.

Mechanism and components: The fibrinolytic system centers on plasminogen, a zymogen produced mainly by the liver and circulating in blood. Plasminogen is converted to plasmin by plasminogen activators, including tissue-type plasminogen activator (tPA) and urokinase (uPA). Plasmin then cleaves fibrin into soluble degradation products, leading to clot breakdown. Fibrin degradation products, such as D-dimers, are used as clinical markers of fibrinolysis.

Regulation: Fibrinolysis is tightly controlled by inhibitors. Plasminogen activator inhibitors (PAI-1 and PAI-2) limit plasminogen activation, while alpha-2-antiplasmin directly inhibits plasmin. Additional regulators, including antithrombin III, help balance fibrinolysis with coagulation to maintain hemostasis and prevent excessive bleeding or thrombosis.

Clinical significance: Fibrinolysis has important physiological and medical roles. Therapeutically, plasmin generation is induced to dissolve pathological clots in acute myocardial infarction, ischemic stroke, and pulmonary embolism, using agents such as alteplase (rtPA), tenecteplase, or reteplase. Streptokinase was used historically. Adverse effects primarily include bleeding and, in the brain, potential hemorrhagic transformation of an ischemic stroke. Laboratory assessment of fibrinolysis can include D-dimer testing and tests of clot lysis such as the euglobulin lysis time.

Terminology: The term fibrinolysin is largely historical; in modern use, plasmin is regarded as the active fibrinolytic enzyme.