We know that all life requires oxygen, but sometimes oxygen can produce radicals and high-energy (singlet) oxygen. Because of this, most animals have developed many lines of defense against free radical formation to preserve their very existence and to lessen the chance of abnormal cell (cancer) development.
There are many mechanisms that prevent or decrease the occurrence of radicals and the formation of hydroperoxide and thereby decrease the amount of cell membrane damage. The most obvious one is the protective protein coat that lines the surface of the cell membrane. This protective coat prevents oxygen from directly reacting with lipids in the membrane, which otherwise would produce many free radicals. In this situation the protein coat is an antioxidant, which protects the body from the formation of free radicals by preventing oxidation.
A second defensive mechanism involves the protective enzymes that float around in all cell membranes. These enzymes also act as antioxidants, preventing radical formation and also converting existing hydroperoxyl radicals into stable oxygen and hydrogen peroxide. They eliminate the danger of radicals and prevent radical damage to the cell membrane. These protective enzymes are small proteins that swim in the membrane fluid around polyunsaturated fats and hence are in close proximity to them. They include superoxide dismutase catalase, and selenium-containing glutathione peroxidase.
There is a very narrow but equal space between polyunsaturated fat molecules. One vitamin E molecule just fits snugly between them. This location and closeness to the polyunsaturated fats is extremely important. Vitamin E is also an antioxidant and is the third mechanism that inhibits radical formation and thereby prevents their destructive damage. Vitamin E competes with the polyunsaturated fat for free radicals that are formed when polyunsaturated fats react with oxygen. This means that if there are more vitamin E molecules than polyunsaturated fat molecules, the radicals will be taken out of the system and neutralized by vitamin E. The more polyunsaturated fats you eat, the more vitamin E you will require. On the other hand, if the number of vitamin E molecules is low, then the radicals that are formed will not be neutralized and will proceed to cause membrane damage. Vitamin E also destroys hydroperoxides and therefore prevents more radical formation. Vitamin E and selenium also protect vitamin A because vitamin A is a polyunsaturated compound. W.L. Wattenberg has reviewed lipid antioxidants and demonstrated their role in preventing cancer. The process of aging is also most probably related to hydroperoxide action on lipids. Aging is thought to be due to the process of oxidation.
Free radicals are unstable chemical substances with high energy. Normal enzymes can produce free radicals, which can form more radicals and singlet oxygen (very-high-energy oxygen). Oxygen reacting with lipids to free its stored energy may also produce hydroperoxides. All of these will cause tissue damage and may lead to the development of cancer. As already discussed, the body does have defenses to protect itself against these, including the protective-protein coat of the cell membrane, protective enzymes (catalase, superoxide dismutase, and selenium-containing glutathione peroxidase), and vitamin E.