Digestive Enzyme - Stomach

Stomach

The enzymes that are secreted in the stomach are called gastric enzymes. The stomach plays a major role in digestion, both in a mechanical sense by mixing and crushing the food, and also in an enzymatic sense, by digesting it. The following are the enzymes produced by the stomach and their respective function:

• Pepsinogen is the main gastric enzyme. It is produced by the stomach cells called "chief cells" in its inactive form pepsinogen, which is a zymogen. Pepsinogen is then activated by the stomach acid into its active form, pepsin. Pepsin breaks down the protein in the food into smaller particles, such as peptide fragments and amino acids. Protein digestion, therefore, first starts in the stomach, unlike carbohydrate and lipids, which start their digestion in the mouth.
• Hydrochloric acid (HCl): This is in essence positively charged hydrogen atoms (H), or in lay-terms stomach acid, and is produced by the cells of the stomach called parietal cells. HCl mainly functions to denature the proteins ingested, to destroy any bacteria or virus that remains in the food, and also to activate pepsinogen into pepsin.
• Intrinsic factor (IF): Intrinsic factor is produced by the parietal cells of the stomach. Vitamin B12 (Vit. B12) is an important vitamin that requires assistance for absorption in terminal ileum. Initially in the saliva, haptocorrin secreted by salivary glands binds Vit. B, creating a Vit B12-Haptocorrin complex. The purpose of this complex is to protect Vitamin B12 from hydrochloric acid produced in the stomach. Once the stomach content exits the stomach into the duodenum, haptocorrin is cleaved with pancreatic enzymes, releasing the intact vitamin B12. Intrinsic factor (IF) produced by the parietal cells then binds Vitamin B12, creating a Vit. B12-IF complex. This complex is then absorbed at the terminal portion of the ileum.
• Mucin: The stomach has a priority to destroy the bacteria and viruses using its highly acidic environment but also has a duty to protect its own lining from its acid. The way that the stomach achieves this is by secreting mucin and bicarbonate via its mucous cells, and also by having a rapid cell turn-over.
• Gastrin: This is an important hormone produced by the "G cells" of the stomach. G cells produce gastrin in response to stomach stretching occurring after food enters it, and also after stomach exposure to protein. Gastrin is an endocrine hormone and therefore enters the bloodstream and eventually returns to the stomach where it stimulates parietal cells to produce hydrochloric acid (HCl) and Intrinsic factor (IF).

Of note is the division of function between the cells covering the stomach. There are four types of cells in the stomach:

• Parietal cells: Produce hydrochloric acid and intrinsic factor.
• Gastric chief cells: Produce pepsinogen. Chief cells are mainly found in the body of stomach, which is the middle or superior anatomic portion of the stomach.
• Mucous neck and pit cells: Produce mucin and bicarbonate to create a "neutral zone" to protect the stomach lining from the acid or irritants in the stomach chyme.
• G cells: Produce the hormone gastrin in response to distention of the stomach mucosa or protein, and stimulate parietal cells production of their secretion. G cells are located in the antrum of the stomach, which is the most inferior region of the stomach.

Secretion by the previous cells is controlled by the enteric nervous system. Distention in the stomach or innervation by the vagus nerve (via the parasympathetic division of the autonomic nervous system) activates the ENS, in turn leading to the release of acetylcholine. Once present, acetylcholine activates G cells and parietal cells.