pepsinogen

Pepsinogen is synthesized and stored in zymogen granules within chief cells located in the gastric glands of the fundus and body of the stomach. Its secretion is stimulated by vagal (ACh/M3) input, gastrin, and secretin, and is often co-regulated with acid secretion from parietal cells. Upon entering the highly acidic gastric lumen (pH <5, optimal ~2), pepsinogen undergoes autocatalytic cleavage, removing an N-terminal peptide to expose the active site and form pepsin. Once formed, pepsin can activate additional pepsinogen molecules, creating a self-amplifying cascade. Pepsin is an aspartic protease that cleaves peptide bonds adjacent to aromatic amino acids (phenylalanine, tyrosine, tryptophan), initiating protein digestion into smaller peptides that are further processed by pancreatic proteases in the duodenum. Pepsin is irreversibly denatured and inactivated at pH >6, such as when gastric contents enter the duodenum and are neutralized by bicarbonate. Clinically, serum pepsinogen levels (particularly the ratio of pepsinogen I to II) are used as biomarkers for atrophic gastritis and gastric cancer risk, since chronic atrophic gastritis (e.g., from H. pylori infection or autoimmune gastritis) leads to loss of chief cells and decreased pepsinogen I secretion. Zollinger-Ellison syndrome and other acid-hypersecretory states can increase pepsinogen release indirectly through excess gastrin stimulation. Understanding pepsinogen activation is also relevant to peptic ulcer disease, as excess pepsin activity can contribute to mucosal injury when protective mechanisms (mucus-bicarbonate barrier) are impaired.