Pepsin

Pepsin is an enzyme found in the gastric juices of the stomach that helps with the digestion of proteins. Pepsin is inactive until it reaches the acidic environment of the stomach. Once there, it acts as one of the primary enzymes that break down the chemical bonds of proteins so they can be absorbed in the intestines. Pepsin was discovered in the nineteenth century, and it was the first enzyme to be categorized by scientists. In addition to its role in digestion, pepsin can also be used in industrial applications, such as making of cheese and the treatment of leather.

Background

Proteins are large organic molecules made up of chains of hundreds of thousands of amino acids held together by chemical bonds. There are twenty types of amino acids that can form a protein. The specific combination of amino acids determines the protein's function. Proteins can act as disease-fighting antibodies, a weapon unleashed by the immune system to target invading organisms. They can also act as biological messengers, transmitting chemical signals among the body's cells, tissues, or organs. Proteins help support cells and carry atoms and other molecules throughout the body. They are also a component in some foods, such as eggs, milk, fish, chicken, and pork.

Another main function of protein is to act as an enzyme, a biological catalyst that produces chemical reactions within the body. Enzymes can speed up the natural processes of the body, break apart organic compounds, or help in the formation of new molecules by "translating" the genetic information found in DNA. Proteins that function as enzymes are usually categorized with the suffix -ase. The main part of the enzyme's name generally refers to type of chemical reaction it activates. Pepsin, for example, is a protease, an enzyme designed to digest proteins.

Overview

In 1836, German physiologist Theodor Schwann determined that a substance in gastric juices was actively digesting protein-rich nutrients in the stomach. Although he could not identify the substance, he recognized its distinct chemical properties and its effect on digestion. Schwann called the substance pepsin after the Greek word pepsis, meaning "digestion." It was the first specific substance ever associated with causing chemical changes within the body. At the time, scientists referred to the substances as ferments; however, the term enzyme was adopted in 1876. The chemical properties of pepsin were not fully understood until 1930, when American chemist John Northrop crystalized it in a laboratory. Northrop eventually received a share of the Nobel Prize in Chemistry for the discovery.

Pepsin is produced in the mucous-lined walls of the stomach. Because the body's cells are made up of protein, pepsin is released in an inactive form known as pepsinogen to keep the pepsin from digesting its birth cell. This form comes with an extra set of forty-four amino acids that block its active properties. When pepsinogen comes into contact with the stomach's gastric juices, these extra amino acids fall off, and the pepsin is activated. Gastric juices are a mixture of water, hydrochloric acid, enzymes, mucus, and other substances. Pepsin is only effective within a specific range of pH levels—a measure of acidity. The lower the pH number, the more acidic the environment. Pepsin functions between a pH range of 1 to 3; the stomach's pH level is about 1.5 to 3.5.

Like all proteins, the amino acids in the protein from food are held together by chemical bonds. The acid of the stomach rearranges the molecular structure of the food to allow the pepsin to access these chemical bonds. Normally, the stomach's natural environment is enough to break down the protein from food; however, the process would be extremely slow. Pepsin speeds up digestion by breaking apart the bonds and creating smaller amino acid fragments called peptides or polypeptides.

Pepsin focuses on severing the bonds in larger amino acid chains. Other enzymes, such as chymotrypsin and trypsin, break down smaller segments of amino acids. After several hours of digestion, the food mixture transfers to the small intestine, where the peptides are further broken down into their respective amino acids. These are then absorbed into the bloodstream.

If the environment of the stomach is negatively altered, pepsin can result in the erosion of the stomach lining or the upper part of the small intestine. This condition can cause an open sore known as a peptic ulcer. One of the most common causes of peptic ulcers is the presence of a bacterium called Helicobacter pylori, which can prevent the body from producing protective mucus to guard against pepsin. Other causes include the overuse of aspirin, stress, or alcohol. Peptic ulcers can also result from radiation therapy.

The digestive effects of pepsin can be reproduced outside the stomach and are used in several commercial applications. Pepsin is a component of rennet, a collection of enzymes found in the stomachs of calves, lambs, or young goats. Rennet is used in the process of making cheese by separating milk into solid components called curds and a liquid form called whey. Pepsin is also used to produce soy protein and gelatin. In the leather industry, animal hides are sometimes placed in a solution containing pepsin to remove any unwanted hair and fat. The process, called bating, also softens the leather. In the nineteenth century, pepsin was mixed with the plant extract senna to create a popular laxative. It was also used in several other products where it was seen as a digestive aid. Despite long-standing rumors, pepsin was never an ingredient in the soft drink Pepsi-Cola. Pepsi was given its name in 1898 as a marketing ploy to tout its supposed ability to cure dyspepsia, or indigestion.

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