Aprotinin is a competitive and reversible inhibitor of esterases and proteases.

• Acid- and heat-stable
• Suppress virus HA cleavage
• Inhibits target proteases at equimolar concentration
• Primary target: chymotrypsin

It inhibits a number of different proteases, including chymotrypsin, coagulation factors involved in the prephase of blood clotting, kallikrein (K_d=1×10⁻⁷ M), plasmin (K_d=2.3×10⁻¹⁰ M), tissue and leukocytic proteinases, and trypsin (K_d=5×10⁻¹⁴ M). It does not inhibit Factor Xa and thrombin. It is used as a serine protease inhibitor during purification of proteins and in studies of zymogen activation systems. It helps to limit the reproduction of human and avian influenza viruses with a single arginine in the HA cleavage site.

Note: 1 KIU = 0.025 antiplasmin units (APU) = 0.0016 trypsin inhibitor units (using porcine trypsin) = 0.0009756 trypsin inhibitor units (using bovine trypsin).

It forms a tight complex with and blocks the active site of the enzymes.