Pepsin is a digestive protease (EC 188.8.131.52) released by the chief cells in the stomach that functions to degrade food proteins into peptides.
According to American Heritage Dictionary, pepsin derives from the Greek word pepsis, meaning digestion (peptein: to digest).
Pepsin was discovered by Theodor Schwann in 1836. It was the first animal enzyme to be discovered.
Pepsin are expressed as a pro-form zymogen, pepsinogen, whose primary structure has an additional 44 amino acids.
In the stomach, chief cells release pepsinogen. This zymogen is activated by hydrochloric acid (HCl), which is released from parietal cells in the stomach lining. The hormone gastrin and the vagus nerve trigger the release of both pepsinogen and HCl from the stomach lining when food is ingested. HCl creates an acidic environment which allows pepsinogen to unfold and cleave itself in an autocatalytic fashion, thereby generating pepsin (the active form). Pepsin may then cleave the dietary proteins into smaller peptides. These peptides may be further digested by other proteases and eventually absorbed by the body.
Pepsin is stored as pepsinogen so it will only be released when needed, and does not digest the body's own proteins in the stomach's lining.