Enzymes like pepsin are created in the form of pepsinogen, an inactive zymogen. It also has the function of stimulating pepsinogen secretion by chief cells. Northrop also isolated and crystallized pepsinogen (the precursor to pepsin), trypsin, chymotrypsin, and carboxypeptidase. Pepsin in its secreted form is inactive (pepsinogen). Gastric juice contains hydrochloric acid, pepsinogen and mucus in a healthy adult. The analysis of sera for increased plasma pepsinogen levels is a useful diagnostic aid. Leptin has been found in the pepsinogen granules of chief cells. Affected glands lack differentiated acid-producing parietal and pepsinogen producing chief cells. Gastric chief cells produce pepsinogen, which is activated by the acid to form pepsin. Gastric juice in the stomach also contains pepsinogen. In the stomach, chief cells release pepsinogen. Pepsin cleaves the 44 amino acids from pepsinogen to create more pepsin. I (pepsinogen A) is a protein that in humans is encoded by the PGA3 gene. Pepsinogen levels in serum may serve as a biomarker for atrophic gastritis and gastric cancer. Pepsinogen produces pepsin, which breaks the peptide bonds found in amino acids.