Protein digestion continues in the small intestine.
Further protein digestion occurs in the small intestine (Figure 16.4). The passage of gastric juice into the small intestine stimulates intestinal cells to release two peptide hormones, secretin and cholecystokinin, into the blood. Secretin stimulates the pancreas to secrete sodium bicarbonate into the small intestine to neutralize the acidic gastric juice. Cholecystokinin acts together with secretin to stimulate the pancreas to release trypsinogen, chymotrypsinogen, procarboxypeptidase A, and procarboxypeptidase B into the small intestine. Like pepsinogen, these zymogens must be activated before they can participate in protein hydrolysis.