Aprotinin (C284H432N84O79S7) is a serine protease inhibitor that is commonly used in open heart surgery because it minimizes perioperative bleeding and blood product transfusions. It also decreases inflammation generated by cardiopulmonary bypass. During cardiopulmonary bypass, plasmin formation is induced, activating platelets and altering platelet membrane glycoprotein Ib. Glycoprotein Ib is needed for the binding of von Willebrand factor, which is necessary for clotting. Aprotinin is thought to protect platelet activation by inhibiting the formation of plasmin. Aprotinin has not been found to affect the quantity of circulating platelets, instead it plays a role in preventing platelet dysfunction.

Other than plasmin, aprotinin also inhibits other inflammatory processes associated with cardiopulmonary bypass, such as kallikrein formation and neutrophil activation.

Aprotinin has also been shown to have beneficial effects on the heart, lungs, and central nervous system. It is thought to decrease the formation of lung edema and pulmonary vascular resistance, and, with a left ventricle assist device, to improve pulmonary function in heart transplant patients. Recent studies have shown that it may prevent tendon damage in chronic tendon disorders and act as an irritant in prolotherapy to promote healing.

Aprotinin is obtained and purified from bovine lungs, and is available under Bayer's brand name Trasylol. There is a 1-3 in 1000 possibility of anaphylaxis occurring, so most patients are given a test dose prior to incision. Minor allergic reactions give redness and itching near the injection site.

Aprotinin consists of 58 amino acid residues arranged in a single polypeptide chain linked by 3 disulphide bridges. It is a clear, colourless, sterile solution administered intravenously. Each millilitre contains 10,000 Kallikrein Inhibitor Units (1.4mg/mL) and 9 milligrams of sodium chloride dissolved in water. Hydrochloric acid and/or sodium hydroxide is used to keep the pH within 4.5-6.5.

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