Fibrinolyysin

Fibrinolysin, also known as plasmin, is a serine protease that dissolves fibrin clots and is the central enzyme of the fibrinolytic system. It is produced in the bloodstream from its inactive precursor, plasminogen, which is synthesized by the liver and circulates in plasma.

Plasminogen is converted to plasmin by activators such as tissue plasminogen activator (tPA) and urokinase (uPA). Fibrin markedly enhances activation, and the generated plasmin then cleaves fibrin into soluble degradation products, including the D-dimers that are commonly measured in diagnostics. At higher activity, plasmin can degrade fibrinogen and other clotting factors, contributing to systemic bleeding risk.

Regulation occurs primarily through inhibitors such as alpha-2-antiplasmin, which rapidly inactivates free plasmin, and plasminogen activator inhibitors (PAI-1, PAI-2), which limit conversion of plasminogen to plasmin. The activity of plasmin is most pronounced on partially crosslinked fibrin in clots, helping localize fibrinolysis.

Clinical relevance: Endogenous fibrinolysis helps restore blood flow after injury. Pharmacologic fibrinolysis uses plasminogen activators—alteplase (tPA), reteplase, and tenecteplase—as well as older agents such as streptokinase and urokinase—to dissolve pathologic clots. Indications include acute myocardial infarction, ischemic stroke, and pulmonary embolism, typically in combination with antithrombotic therapy. Risks include bleeding and, in some cases, allergic reactions (notably with streptokinase).

Terminology: Fibrinolysin is an older term for plasmin; modern terminology generally uses plasmin and the broader fibrinolytic system.