Aprotinin is a protein consisting of 58 amino acids, arranged in a single polypeptide chain that is cross-linked by three disulphide bridges. It is found in bovine lymph nodes, lung, parotid gland, spleen, liver, pancreas, seminal vesicles, thyroid gland, kidney, mucous membranes of the trachea and esophagus, ovaries, heart, posterior pituitary and cartilage.

• Extinction coefficient (E1%)(280 nm)=8,3 (water)
• Isoelectric point (pI): 10,5
• Soluble in water and in aqueous buffers of low ionic strengths

It is a competitive serine protease inhibitor that inhibits trypsin, chymotrypsin, kallikrein and plasmin. It forms stable complexes with and blocks the active sites of enzymes. Binding is reversible with most aprotinin-protease complexes dissociating at pH >10 or <3.

Unit definition: One KIU (Kallikrein Inactivating Unit) is identical to the quantity of protease inhibitor that has the ability to inhibit 2 Kallikrein units by 50% under optimal conditions.