Pepsin

Glands present in the mucous membrane lining of the stomach produce and store an inactive protein named pepsinogen. Impulses from the vagus nerve and the hormonal secretions of the hormones gastrin and secretin promote the release of pepsinogen into the stomach, where it is mixed with hydrochloric acid and quickly converted to the active enzyme pepsin. The digestive potency of pepsin is highest at the acidic pH of normal gastric juice. In the intestine, the gastric acids are then neutralized, and pepsin is no longer effective.

Pepsin, the proteolytic enzyme of the stomach is normally responsible for less than 20% of the protein digestion occuring the gastrointestinal tract. It is an endopeptidase enzyme that metabolizes proteins to peptides. It preferentially hydrolyzes peptide linkages where one of the amino acids is aromatic. Pepsin, like other protease enzymes, is produced from an inactive precursor, pepsinogen, which is stored in granule form in the chief cells of the stomach and are released by a process called exocytosis.

In the digestive tract, pepsin activity only contributes to the partial breakdown of proteins into smaller units called peptides, which then either are absorbed from the intestine into the bloodstream or are broken down further by pancreatic enzymes.