As partially digested food enters the small intestine (beginning in the duodenum), it mixes with bile and pancreatic juices. Bicarbonate in these secretions raises the luminal pH, which deactivates the stomach enzyme pepsin. The pancreas secretes inactive protein-digesting enzymes, known as zymogens, including trypsinogen, chymotrypsinogen, proelastase, and procarboxypeptidase. On the brush border surface of the intestinal cells, the enzyme enteropeptidase cleaves trypsinogen to form active trypsin. Trypsin then initiates an activation cascade, converting the remaining pancreatic zymogens into their active protease forms (chymotrypsin, elastase, and carboxypeptidase). These active proteases then further cleave dietary proteins into smaller peptides and amino acids.