The serine protease enterokinase is the physiological activator of trypsinogen and has a specificity for the sequence (Asp) 4-Lys-Ile. The enzyme consists of two subunits linked by a disulfide bond. The heavy chain achors enterokinase in the intestinal brush border membrane and the light chain is the catalytic subunit, which has the same mechanism of action as trypsin and chymotrypsin. Many properties of enterokinase resemble blood-clotting enzymes, suggesting that enterokinase lies on the same phylogenetic branch as the blood-clotting proteins.

Enterokinase is an essential enzyme for normal mammalian digestion. The enzyme is the physiological activator of trypsinogen, and the trypsin produced activates other zymogens forming a mixture of proteolytic enzymes 1. In the absence of enterokinase, intestinal digestion of foodstuffs is impaired. Newborn infants with a congenital deficiency of the enzyme must have a pancreatic extract or an amino acid mixture supplied in the diet for growth and normal health.