fibrinolysin

Fibrinolysin is an enzyme that dissolves fibrin, the insoluble protein mesh of blood clots. The term is historical; in modern biochemistry the principal fibrinolytic enzyme in blood is plasmin, a serine protease produced from its inactive zymogen plasminogen.

Activation and action: Plasmin is generated at sites of thrombus formation by plasminogen activators, such as tissue-type plasminogen activator (tPA) and urokinase. Streptokinase can also activate plasminogen indirectly by forming a complex that activates plasminogen. Once formed, plasmin cleaves fibrin into soluble degradation products and weakens the clot’s structure. Plasmin also degrades fibrinogen and other proteins, though with lower affinity.

Physiological role and regulation: The fibrinolytic system helps regulate clot resolution after vascular injury. Plasmin activity is tightly controlled by inhibitors including alpha-2 antiplasmin and plasminogen activator inhibitors (PAI-1, PAI-2); thrombin-activatable fibrinolysis inhibitor (TAFI) modulates fibrin’s susceptibility to lysis.

Clinical relevance: Pharmacologic fibrinolysis uses agents that stimulate plasmin generation, such as recombinant tissue plasminogen activator (rtPA), tenecteplase, reteplase, urokinase, and streptokinase. These therapies are used to treat acute myocardial infarction, massive pulmonary embolism, and certain types of stroke, but carry risks of bleeding and have contraindications.

History: Fibrinolysin was the older term for the fibrinolytic system’s components; today plasmin and plasminogen activators are the standard terms used in medicine and research.