Pepsinogen is synthesized in chief cells found in the lower reaches of gastric glands. Contact with HCl chops off a few amino acids and converts pepsinogen to pepsin. Pepsin is an endoprotease that breaks peptide bonds that are next to aromatic amino acids. Infants of mammals also release the enzyme rennin which especially helps to break down milk protein. This ability is lost as the infant matures.
Pancreatic juice is of two types.
Secretin causes a HCO3- rich secretion that is poor in enzymes. CCK on the other hand causes a rich mixture of proenzymes to be released. In this way, conversion normally does not take place until the proenzymes or zymogen reach the gut lumen. Chymotrypsinogen is converted to chymotrypsin by trypsin. Bile , synthesized by the liver, contains HCO3-, bile salts and bile pigments.