The inactive molecule pepsinogen is cleaved in a strongly acidic environment to give off the active enzyme, pepsin. Pepsinogen is produced in the stomach by chief cells.
The other main cell type found in the stomach is the parietal cell. This cell is specialized to release hydrochloric acid, which is the catalyst for the conversion of pepsinogen to pepsin. Both the chief and parietal cells are stimulated by the molecule gastrin, which is made in the blood when peptides are detected in the small intestines. Gastrin is regulated by a negative feedback mechanism, and levels decrease when high amounts of acid are detected in the stomach. This prevents excessive acid production and keeps the stomach at an optimal pH around 2.
Pepsinogen, as an inactive precursor molecule, is known as a zymogen or a pro-enzyme. The active peptide pepsin is a protease that, along with hydrochloric acid, breaks down food in the stomach into small peptides. This step is important in the digestion process because the nutrients found in food are better absorbed in the small intestines when they have been adequately broken down.
In cases where too much hydrochloric acid is produced, heartburn or reflux can occur. Several classes of medications have been formulated to counteract this, including histamine-2 blockers and proton pump inhibitors.