Because human trypsinogens are prone to autoactivation and because hereditary pancreatitis-associated cationic trypsinogen mutations increase autoactivation, we proposed that autoactivation is a key pathological pathway in human chronic pancreatitis, the hereditary form in particular (Fig. 1). We found that CTRC stimulates autoactivation of cationic trypsinogen through cleaving Protein Tyrosine Kinase inhibitor the Phe18–Asp19 peptide bond in the activation peptide, thereby excising the N-terminal tripeptide

and processing the activation peptide to a shorter form (Fig. 2). This action of CTRC is highly specific, as other human pancreatic chymotrypsins (CTRB1, CTRB2, CTRL1) or elastases (ELA2A, ELA3A, ELA3B) do not digest the trypsinogen activation peptide. The shorter activation peptide is cleaved by trypsin more readily, resulting in approximately threefold increased autoactivation. The structural basis of this phenomenon lies in the disruption of an inhibitory interaction between cationic trypsin

and the trypsinogen activation peptide.50 Thus, Asp218 on cationic trypsin participates in a repulsive electrostatic interaction with the negatively-charged tetra-Asp motif of the activation peptide. This interaction inhibits autoactivation. Once the activation peptide is processed by CTRC, the inhibitory interaction with Asp218 is partially relieved and autoactivation can proceed at a faster rate. Interestingly, Asp218 is unique to human cationic trypsin, suggesting that a similar mechanism of Fostamatinib autoactivation regulation does not exist in other vertebrates. CTRC-mediated stimulation of trypsinogen ADAMTS5 autoactivation might constitute a positive feedback loop in the digestive enzyme activation cascade, which facilitates full activation of trypsinogen in the gut. More importantly, the pancreatitis-associated cationic trypsinogen mutation p.A16V increases the rate of CTRC-mediated processing of the activation peptide fourfold.51 This observation suggests that p.A16V causes accelerated trypsinogen activation by this

indirect mechanism, as opposed to other cationic trypsinogen mutations, which directly stimulate autoactivation. CTRC can trigger degradation of human cationic trypsin by selectively cleaving the Leu81–Glu82 peptide bond within the Ca2+ binding loop (Fig. 2).52 Degradation and inactivation of cationic trypsin is then achieved through tryptic (autolytic) cleavage of the Arg122–Val123 peptide bond. The peptide segment between Glu82 and Arg122 is not stabilized by disulfide bonds, and it becomes detached from the enzyme. Because the catalytically important Asp107 amino-acid residue (Asp102 in classic chymotrypsin numbering) is located within this sequence, loss of trypsin activity can be explained by disruption of the catalytic triad.