Pepsin was first recognized in 1836 by the German physiologist Theodor Schwann

Pepsin was first recognized in 1836 by the German physiologist Theodor Schwann. In 1929 its crystallization and protein nature were reported by American biochemist John Howard Northrop of the Rockefeller Institute for Medical Research. (Northrop later received a share of the 1946 Nobel Prize for Chemistry for his work in successfully purifying and crystallizing enzymes.)

Glands in the mucous-membrane lining of the stomach make and store an inactive protein called pepsinogen. Impulses from the vagus nerve and the hormonal secretions of gastrin and secretin stimulate the release of pepsinogen into the stomach, where it is mixed with hydrochloric acid and rapidly converted to the active enzyme pepsin. The digestive power of pepsin is greatest at the acidity of normal gastric juice (pH 1.5–2.5). In the intestine the gastric acids are neutralized (pH 7), and pepsin is no longer effective.

In the digestive tract pepsin effects only partial degradation 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.