plasmiinilla

Plasmiini (plasmin) is a serine protease that plays a central role in fibrinolysis, the body's process for dissolving blood clots. It is produced in an inactive form, plasmin, from the zymogen plasminogen. Activation is driven by plasminogen activators, primarily tissue-type plasminogen activator (tPA) and urokinase-type plasminogen activator (uPA). Bacterial streptokinase can also initiate plasmin formation by forming a complex with plasminogen.

Plasmin's primary action is the breakdown of fibrin within clots, but it also cleaves other proteins such as fibrinogen, fibronectin, laminin, and various components of the extracellular matrix. This broad proteolytic activity supports clot dissolution and tissue remodeling but can contribute to tissue damage if unregulated. Activation and activity are tightly regulated by inhibitors: alpha-2-antiplasmin rapidly neutralizes free plasmin, and plasminogen activator inhibitors (PAI-1 and PAI-2) limit further activation of plasminogen.

Physiologically, plasmin helps restore blood flow after vascular injury and participates in wound healing, angiogenesis, and cellular migration. Pathologically, excessive plasmin activity can lead to bleeding, while insufficient plasmin generation can contribute to thrombosis. Clinically, plasminogen activators are used as thrombolytic agents to treat conditions such as myocardial infarction, ischemic stroke, and pulmonary embolism, though their use carries a risk of major bleeding.

Biochemical features include plasmin’s classification as a trypsin-like serine protease with specificity for peptide bonds adjacent to lysine or arginine residues, and its generation on the surface of forming fibrin, which localizes its activity to the clot.