Responsible for initiating activation of pancreatic proteolytic proenzymes (trypsin, chymotrypsin and carboxypeptidase A). It catalyzes the conversion of trypsinogen to trypsin which in turn activates other proenzymes including chymotrypsinogen, procarboxypeptidases, and proelastases. Activation of trypsinogen by selective cleavage of 6-Lys-|-Ile-7 bond. Heterotrimer of a catalytic (light) chain, a multidomain (heavy) chain, and a mini chain. The chains are derived from a single precursor that is cleaved by a trypsin-like protease. The mini chain may be cleaved by elastase. Belongs to the peptidase S1 family.