Trypsin acts to hydrolyze peptides into their smaller building blocks, namely amino acids. These peptides are the result of the enzyme pepsin breaking down the proteins in the stomach. This is necessary for the uptake of protein in the food. Although peptides are smaller than proteins, they are still too big to be absorbed through the lining of the ileum. Trypsin catalyzes the hydrolysis of peptide bonds. Trypsin is remarkably similar in chemical composition and in structure to the other chief pancreatic proteinase, chymotrypsin.
Trypsin is secreted by the pancreas in the form of inactive zymogen and trypsinogen. It is then released into the small intestine where the enzyme enteropeptidase activates it into trypsin by proteolytic cleavage. The resulting trypsins themselves activate more trypsinogens, so only a small amount of enteropeptidase is necessary to start the reaction. This activation mechanism is common for most serine proteases, and serves to prevent autodigestion of the pancreas.