How the pancreatic enzyme become activated in the lumen of intestine.

The enzyme the pancreas secretes digest fat, polysaccharides, proteins and nucleic acids to fatty acids, sugar, amino acids, and nucleotides respectively. The proteolytic enzymes are secreted in inactive forms (zymogens), and then activated in the duodenum by other enzymes. A key step in this activation is mediated by enterokinase, which is embedded in the luminal plasma membranes of the intestinal epithelial cells. Enterokinase is a proteolytic enzyme that splits off a peptide from pancreatic trypsinogen, forming the active enzyme trypsin. Trypsin is also a proteolytic enzyme; once activated, it activates the other pancreatic zymogens by splitting off peptide fragments. This activating function is in addition to the role of trypsin in digesting ingested protein. The non-proteolytic enzymes secreted by the pancreas (amylase, lipase) are released in fully active form. This mechanism is needed for protection of pancreatic cells from autodigestion which the secretion of zymogens prevents.