Pepsinogen is the precursor form of pepsin an enzyme released by the chief cells in the stomach and that degrades food proteins into peptides. Pepsin is one of three principal protein-degrading, or proteolytic, enzymes in the digestive system. During the process of digestion, these enzymes, each of which is specialized in severing links between particular types of amino acids, collaborate to break down dietary proteins into their components, i.e., peptides and amino acids, which can be readily absorbed by the intestinal lining. Description: Human stomach.