Aprotinin is a reversible inhibitor of serine proteases such as trypsin (Ki = 0.06 pM), chymotrypsin (Ki = 9.5 nM), and kallikrein (Ki = 0.8 nM). It is a much weaker inhibitor of thrombin (Ki = 0.1-0.8 mM) and trypsinogen (Ki = 2 μM). Aprotinin also competitively inhibits nNOS and iNOS with Ki values of 50 and 78 μM, respectively. Aprotinin is widely used as a protein purification tool to prevent proteases present in tissue samples from degrading the protein of interest.
