Glutathione peroxidase is an important antioxidant in mammals, including humans. Glutathione peroxidase is important because it is involved in the reduction of hydrogen peroxide to water, and also reduces lipid hydroperoxides. In this respect glutathione peroxidase plays an important role in modulating cellular oxidative stress and inhibiting the disease process. Glutathione peroxidase is a actually a family of enzymes with a number of subforms. Glutathione peroxidase 1 is the most abundant mammalian form found in the cytoplasm of cells, and possibly the most important because it is able to reduce hydrogen peroxide to water. In contrast glutathione peroxidase 2 and 3 are extracellular enzymes and so give their antioxidant activity on the exterior of cells. Glutathione 2 is active mainly in the intestine, whereas glutathione peroxidase 3 is active mainly in plasma.
Glutathione peroxidase activity is dependent on the mineral selenium, which bonds to the enzyme and gives it shape and form. Without selenium glutathione peroxidase cannot function. Selenium is an inorganic element present in the soil and to pass from the soil to the cells of man it must be consumed in food. Historically, plants were a rich source of selenium, but extensive over farming of the soils of the world have left many depleted in the minerals required for human health, including selenium. More recently, meat has become the main source of dietary selenium, as selenium is supplemented to livestock and accumulates in their tissues in enzymes such as glutathione peroxidase. A dietary deficiency of selenium causes a drop in the activity of the glutathione peroxidase activity in man, and this can lead to increases in oxidative stress in intra- and extracellular environments.
The benefits of dietary selenium on cellular glutathione peroxidase has been demonstrated in studies that have supplemented human subjects with the mineral. For example, in one study1 subjects who received 200 µg selenium in the form of sodium selenite showed increased levels of selenium and glutathione peroxidase activity in whole blood and plasma when compared to a placebo or vitamin E supplement. Tissue biopsies of the subjects taking selenium supplements also showed higher levels of glutathione peroxidase compared to the other groups. The researchers also reported that there was a correlation between the tissue concentrations of selenium in liver and muscle samples and the glutathione peroxidase activity in these tissues. Because low selenium status is associated with an increased risk of cancer, and selenium sources of the diet are not reliable, supplementation of selenium is recommended.
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