Free radicals are produced as a normal part of cellular metabolism. Such compounds include peroxides — oxidizing compounds that can damage DNA, lipids, and proteins. This oxidative stress can contribute to diseases like atherosclerosis and to the aging process. Glutathione peroxidase (GPx) is a type of enzyme that serves as a cellular antioxidant. It reduces the peroxide group to a relatively un-reactive alcohol group, using glutathione as the reducing agent, and thus protects the cell from oxidative damage.
When a compound is oxidized, it gives up an electron. With oxygen-containing compounds, this results in an unstable compound that will take an electron from a nearby substance to stabilize itself, potentially causing great damage to cellular components. Peroxides are compounds made up of two oxygen molecules connected by a single bond that are highly oxidizing. These substances can be reduced and made less harmful by enzymes known as peroxidases. This activity is an important antioxidant detoxification mechanism for virtually all organisms that live in an atmosphere of oxygen, including microorganisms.
Peroxidases of the type known as horseradish peroxidase have an iron-containing heme group at their active site. Instead, glutathione peroxidase has the metal selenium at its active site. This is one of the reasons selenium is needed in the human diet. This type of peroxidase uses the sulfur-containing compound glutathione to reduce the peroxides and detoxify them.
Glutathione peroxidase comprises an enzyme family. Thus, there are a number of closely related proteins produced by different human genes. The various enzymes appear to have separate functions. They are produced in disparate tissues and vary in the types of reactions mediated by the enzymes.
Eight different types have been identified in humans as of 2010, all encoded by separate genes. The primary form appears to be GPx1. It is the most common of these enzymes and is found in the cytosol of most mammalian tissues. This form of glutathione peroxidase is most effective at reducing hydrogen peroxide to produce water as the end product.
Hydrogen peroxide is a compound frequently found in cells that easily degrades to free radicals. It is produced as a result of the activity of the immune system of animals. This compound is toxic, however, and there is a need for the widespread presence of an enzyme to detoxify it. Fortunately, GPx1 is commonly present to fill such a role by reducing the hydrogen peroxide to water. Some ethnic groups have an altered version of this enzyme that improves their resistance to malaria.
Another type of glutathione peroxidase that is an important antioxidant enzyme is GPx4. Lipids can be oxidized to form hydroperoxides, a particularly damaging form of peroxide. These reactive lipids can damage the lipids around them, leading to potential conditions such as inflammation and heart disease. Also known as phospholipid hydroperoxide, GPx4 reduces these damaged lipids from peroxides to alcohols. This form of glutathione peroxidase is also widely present in cells, but is expressed at a lower level than GPx1.