In 1836, scientist Theodor Schwann figured out that a specific substance in stomach juices could break down egg white. According to a 2012 article in "Annals of Gastroenterology," the isolation of this substance followed from the discovery of hydrochloric acid, another major constituent of stomach juices. Schwann named his discovery "pepsin," which turned out to be an enzyme that breaks down proteins in food.
Pepsin is produced initially as its inactive precursor pepsinogen. This molecule is produced by the chief cells in the body of the stomach, although some is also produced in mucus cells of the stomach glands. At meal time, a variety of stimuli lead to increased release of pepsinogen from these cells, which brings the precursor into contact with hydrochloric acid in the stomach.
Exposure to stomach acid triggers pepsinogen to change its three-dimensional arrangement, known as its conformation, which in turn allows the enzyme to cut or cleave itself to become the active form of pepsin. As well as requiring acid to activate, pepsin needs an acidic environment of less than pH 5 to work, as its enzymatic activity is virtually nonexistent in less acidic environments. According to the 2012 "Annals of Gastroenterology" article, despite pepsin's potential to also attack proteins of the stomach cells if the protective mucus layer of the stomach lining is damaged, scientists have not identified a specific role for pepsin in the development of stomach ulcers.
Food is composed of varying amounts of protein, fats or carbohydrates, and the pepsin enzyme targets the protein portion of a meal. It has a proteolytic function, which means that it slices up large proteins into smaller polypeptides in preparation for absorption in the intestines. Proteins are made up of long strings of amino acids held together by peptide bonds, and pepsin cuts proteins at certain amino acids, such as glutamic acid, leucine or aspartic acid. Later during digestion, these polypeptide fragments will be further broken down by additional enzymes, such as endopeptidases and exopeptidases, so they can easily cross the intestinal wall into the body. Pepsin's preference for certain amino acids, though, means that some proteins entering the stomach escape cleavage by pepsin. The persistence of these unbroken proteins can lead to disease in certain people. This is exemplified by celiac disease, in which whole gluten proteins of wheat enter the intestines and trigger inflammation.
Regulation of Secretion
Pepsinogen production and release are regulated in part by the nervous system and also by endocrine hormones and the amount of acid in the stomach. People with medical conditions that reduce stomach acid secretion, such as the autoimmune condition achlorhydria, have a reduced pepsin level and are less able to digest proteins efficiently. Significant reduction in stomach acid can also occur with Helicobacter pylori infection of the stomach, following certain gastric surgeries or as a result of medications like proton pump inhibitors.