The three most common and significant antioxidant enzymes include glutathione peroxidase, catalase, and superoxide dismutase. The four remaining antioxidant enzymes are glutathione reductase, thioredoxin reductase, heme oxygenase, and biliverdin reductase. Some people confuse antioxidants with antioxidant enzymes. Antioxidants help repair damage done by free radicals in the body and the resulting oxidation. Enzymes, however, attempt to stop damage before it occurs by triggering chemical reactions that rid the body of free radicals and dangerous oxygen in the form of oxides.
Glutathione peroxidase and glutathione reductase make up the glutathione system of antioxidant enzymes that specifically protects the body against peroxides, especially hydrogen peroxide. These two enzymes are particularly active in the brain, which is the organ most susceptible to oxidation by free radicals. There are four different types of the peroxidase enzyme. Cytosolic glutathione peroxidase (cGPx) primarily helps the brain, thyroid, and heart. Plasma glutathione peroxidase (pGPx) protects the plasma of the blood from peroxides.
Gastrointestinal glutathione peroxidase (GIGPx), which is made in the kidneys, works in the liver and gastrointestinal tract. It attacks peroxides generated by food consumption. Phospholipid hydroperoxide glutathione peroxidase (PHGPx) works on peroxides attached to fats in sexual organs, the cerebral region, and in plasma membranes. Glutathione reductase strips oxygen away from any oxidized glutathione.
Like the glutathione antioxidant enzymes, catalase also destroys hydrogen peroxide. Thioredoxin reductase is different. Its job is to rid the body of protein disulfides, which are the main triggers for harmful oxidation. By eliminating protein disulfides, thioredoxin reductase can reduce the overall amount of oxygen in the body. This enzyme is dependent on selenium and cannot work properly unless the body has sufficient stores of this mineral.
Superoxide dismutase separates free radicals into two materials: oxygen atoms and hydrogen peroxide molecules that can then be destroyed by other antioxidant enzymes. Dismutase enzymes work in one of two areas of the cell: the cytoplasm or the mitochondria. Heme oxygenase reduces heme in the blood to carbon dioxide, iron, and biliverdin. Oxygen inside biliverdin is stripped away by the enzyme biliverdin reductase. This chemical reaction actually produces the antioxidant known as bilirubin.
Oxygen is normally seen as desirable. Breathed into the lungs, it is beneficial. At the cellular level, however, oxygen causes cells to breakdown, age, and die. Antioxidant enzymes seek to stop that cycle of destruction.