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(also plasmin), an enzyme of the hydrolase class present in plasma. Fibrinolysin catalyzes the splitting of fibrin, causing the dissolution of blood clots, or thrombi, in the process called fibrinolysis. Fibrinolysin can also hydrolyze other proteins—for example, casein and fibrinogen—as well as low-molecular peptides and L-arginine and L-lysine esters. The main action of fibrinolysin is to split the peptide bonds formed by arginine and lysine radicals. Fibrinolysin is a globular protein with a molecular weight of 80,000–90,000. A fibrinolysin molecule consists of two polypeptide chains, one light and one heavy, linked by a disulfide bond. The light chain, with a molecular weight of approximately 27,000, contains the active center of fibrinolysin; the heavy chain has a molecular weight of approximately 57,000.
Fibrinolysin is found in blood mainly in the form of the inactive proenzyme profibrinolysin (plasminogen), which is converted to plasmin by the action of enzymatic activators. It is often present in a complex with antiplasmin, an inhibitor. The therapeutic agent fibrinolysin obtained from whole blood or human placental serum is a white powder readily soluble in physiological saline. It is used in the treatment of diseases associated with the formation of thrombi, such as acute thrombophlebitis, thromboembolism of the pulmonary and peripheral arteries, cerebrovascular thrombosis, and myocardial infarction. Intravenous drip infusion of fibrinolysin with heparin is used to dissolve thrombi up to four days after they are formed.
Fibrinolysin activity is measured by the time required for a fibrin clot to dissolve or by the amount of lysed fibrin or casein and is expressed in fibrinolytic or caseinolytic units of activity.
G. V. ANDREENKO