Plazmin

Plazmin, a spelling variant sometimes used in reference materials, most often refers to plasmin, a principal serine protease of the fibrinolytic system. Plasmin catalyzes the breakdown of fibrin within blood clots, contributing to clot dissolution and the restoration of normal blood flow after vascular injury. The enzyme is formed from its inactive zymogen, plasminogen, which circulates in the blood and binds to fibrin in clots.

Activation occurs when plasminogen is cleaved by plasminogen activators such as tissue-type plasminogen activator (tPA) or urokinase (uPA). This conversion produces active plasmin, which then cleaves fibrin into soluble degradation products. Plasmin also degrades other extracellular matrix components, including fibronectin and laminin, and can modulate platelet interactions indirectly.

Biochemically, plasmin belongs to the peptidase family S1 and operates as a catalytic serine protease. Its activity is tightly regulated by inhibitors to prevent excessive fibrinolysis. Alpha-2-antiplasmin rapidly inhibits free plasmin, while alpha-2-macroglobulin can sequester it. Plasminogen activator inhibitors (PAI-1 and PAI-2) regulate the activation step by limiting the activity of tPA and uPA.

Clinical relevance includes the use of plasminogen activators in thrombolytic therapy to treat pathologic clots in conditions such as myocardial infarction or ischemic stroke. Risks include bleeding due to excessive plasmin activity. Laboratory markers of plasmin activity include D-dimer levels, which reflect fibrin degradation, and changes in fibrinogen during coagulopathies. Plazmin/plasmin thus plays a central role in hemostasis, thrombosis, and tissue remodeling.