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a substance that makes another substance active or reactive, induces a chemical reaction, or combines with an enzyme to increase its catalytic activity.
plasminogen activator a substance that activates plasminogen and converts it into plasmin; see t-plasminogen activator and u-plasminogen activator.
tissue plasminogen activator (TPA, t-PA) (t-plasminogen activator) a serine endopeptidase synthesized by endothelial cells, the major physiologic activator of plasminogen; when bound to fibrin clots it catalyzes the conversion of plasminogen to plasmin by hydrolysis of a specific arginine-valine bond. It can be produced by recombinant technology for use in thrombolytic therapy. It acts directly on blood clots and therefore presents a small risk of systemic bleeding; occasionally allergic reactions may occur.
u-plasminogen activator (urinary plasminogen activator) a serine endopeptidase that acts as a plasminogen activator by catalyzing the preferential cleavage of plasminogen at the same arginine-valine bond where t-plasminogen activator cleaves. It is produced in the kidney and excreted in the urine and is used in thrombolytic therapy (when used as a pharmaceutical, it is usually called urokinase). Unlike t-plasminogen activator or prourokinase, it does not require fibrin for activity. Called also urokinase.