The stomach is one of the important organs of digestion. It receives food and beverages that have been swallowed, after being shunted from the throat to the stomach through the esophagus. The stomach has a muscular wall which can crush food into smaller particles (mechanical digestion). The inner lining of the stomach wall also gas cells that secrete different substances, many of which are important for chemical digestion.
What are stomach enzymes?
Stomach enzymes are chemicals that help to breakdown foods into simpler nutrients as part of the digestive process. Digestion does not start in the stomach but a significant part of the digestive process occurs in the stomach. The digestion then continues in the small intestine. Digestion allows for the body to absorb nutrients from the gastrointestinal tract (gut).
These digestive enzymes are also secreted in the mouth, from the pancreas and in the small intestine. Although some types of enzymes may overlap in their function, all enzymes from each part of the digestive tract are needed for efficient digestion. The stomach enzymes are therefore crucial for proper nourishment. When there are problems with these enzymes, then nutritional deficiencies can arise.
Glands in the Stomach
The glands at different locations of the stomach wall may secrete different substances. These glands are therefore termed according to its location – cardia glands (in the cardia of the stomach), fundic glands (in the fundus of the stomach) and pyloric glands (in the pylorus ofthe stomach).
The secretions from these glands are as follows:
- Cardia glands – mainly mucus.
- Fundic glands (oxyntic glands) – pepsinogen, intrinsic factor and gastric acid.
- Pyloric glands – gastrin.
The two enzyme-producing cells in the stomach are the chief cells and parietal cells. Chief cells secrete pepsin while parietal cells secrete gastric acid and intrinsic factor.
Read more on gastric acid.
List and Names of Stomach Enzymes
Although gastric acid is secreted by the stomach wall, it is not considered to be an enzyme. Acid indiscriminately breaks down substances, which are not resistant to it. Mucus is also secreted by the stomach wall and one of the main functions of this mucus is to protect the lining of the stomach wall. Gastrin, a hormone, is also secreted by cells of the pyloric glands. These other substances are important for digestion despite not being enzymes.
Enzymes speed up and/or facilitate chemical reactions and are usually specific for certain substances.The two main types of stomach enzymes include:
- Pepsin (active form) secreted in the inactive form, pepsinogen.
- Intrinsic factor (IF)
Read more on digestive enzymes.
Functions of Stomach Enzymes
The function of stomach enzymes is to break down foods into simpler nutrients, which can then be absorbed or further digested within the gut.
Pepsin is the active form of pepsinogen which is secreted by the chief cells in the stomach wall. Pepsinogen is converted to pepsin by the action of stomach acid. Pepsin digest protein. It breaks down large protein chains (polypeptides) into smaller proteins (dipeptides and peptides).
Pepsin is most active when it is in an acidic environment. Coupled with the fact that it is secreted in an inactive form means that it does not damage or digest the tissue of wall. The mucus barrier that separates the stomach contents from the stomach wall also prevents auto-digestion.
Although the stomach acid is effective in destroying bacteria that may be consumed with food of beverages, pepsin may also assist in this regard to some extent.
Intrinsic factor is a glycoprotein that is secreted by the parietal cells in the stomach wall. It is an enzyme-like substance which is responsible for vitamin B12 absorption. However, intrinsic factor only acts within the small intestine despite being secreted by the stomach wall.
Vitamin B12 that is released from food in the stomach is bound by specific binding proteins, and not intrinsic factor. When this bound vitamin B12 enters the small intestine, enzymes from the pancreas releases the vitamin B12 from the binding proteins.
Vitamin B12 then becomes bound to intrinsic factor and is eventually released in the latter portions of the small intestine where is then absorbed into the bloodstream.
Problems with Stomach Enzymes
Diseases or disorders that affect the quantity and activity of the stomach enzymes are mainly due to two factors: damage of the glands or problems with stomach pH. Although gastric acid is not an enzyme, it is needed for stomach enzyme activation, particularly for converting pepsinogen to its active form of pepsin.
Any damage or disease of the stomach wall can affect the glands and cells responsible for stomach enzyme production and secretion. Despite the stomach lining’s ability to regenerate in a short period of time, this does not happen in some conditions, like atrophic gastritis. In this condition there is destruction of the glandular cells due to chronic inflammation.
Antibodies may be produced by the immune system that target the parietal cells of the stomach lining, thereby damaging or destroying these cells. It can affect intrinsic factor as well as stomach acid. Another consequence is that loss of acid-producing cells means that there is less stomach acid in the stomach. Activation of enzymes like pepsin is therefore impaired because acid is required for this activation.
Stomach pH Problems
The stomach juices are acidic (low pH) due to the presence of gastric acid. There are various ways in which the stomach pH can be disrupted, mainly due to the loss of acid-producing cells, disturbances in the secretory function of the acid-producing cells or the use of medication that hampers acid production. As mentioned, this acidity is necessary for the activation of pepsin. Therefore pepsin levels are low since pepsinogen is not activated.
In chronic H.pylori infection, the chemicals responsible for promoting inflammation can impair the secretory function of the acid-producing cells, although it may not be damaged or destroyed. Acid-suppressing drugs like proton pump inhibitors (PPIs), which are commonly used to treat conditions like gastritis and peptic ulcers, can also affect stomach pH due to the lower than normal levels of stomach acid.