An enzyme that digests proteins (see endopeptidase; protease). It is secreted in an inactive form (trypsinogen) by the pancreas into the duodenum. There, trypsinogen is acted on by an enzyme (enterokinase) produced in the brush border of the duodenum to yield trypsin. The active enzyme plays an important role in the digestion of proteins in the anterior portion of the small intestine. It also activates other proteases in the pancreatic juice (see carboxypeptidase; chymotrypsin).