BPTI (Bovine Pancreatic Trypsin Inhibitor(
 
Bovine pancreatic trypsin inhibitor, BPTI (Kunitz), is also called trypsin kallikrein inactivator, TKI, aprotinin, and Trasylol™. It is the first protein inhibitor of proteinases  to be isolated and characterized by Moses Kunitz as an inhibitor of bovine trypsin. It was independently discovered as a kallikrein inactivator. BPTI was the first protein proteinase inhibitor to be sequenced. Shortly afterward, sequencing the kallikrein inactivator revealed the identity of the two inhibitors. They consist of 58 amino acid residues, crosslinked by three disulfides. Lys15 is at the P1 position of the reactive site. The KI for BPTI-bovine b trypsin interaction, 5 × 10^–14 M, is often—probably incorrectly—referred to as the strongest of the proteinase–protein proteinase inhibitor interactions. BPTI is also extremely stable. Its Tm at neutral pH is 103°C. BPTI was the first protein proteinase inhibitor to have its three-dimensional structure determined both in free form and in complex with bovine b trypsin. It has been—and still is—widely used in Europe as a drug to avoid postsurgical complications. For this use, it is isolated from bovine lungs. BPTI has been the first superbly characterized small protein that was widely available. Therefore, it became the favorite substance of many protein researchers. It was the first protein to have its three-dimensional structure         determined by nuclear magnetic resonance (NMR). It served as the object of numerous molecular dynamics and structure prediction calculations. It was the first protein for which the pattern of closure of disulfide bridges was studied in detail. BPTI (Kunitz) gave its name to a widely studied family of standard-mechanism canonical protein inhibitors of serine proteinases.
 
References
W. Gebhard, H. Tschesche, and H. Fritz (1986) "Bowman–Birk family serine proteinase inhibitors". In Proteinase Inhibitors (A. Barrett and G. Salveson, eds.), Elsevier New York, 375–388.