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antioxidant,substance that prevents or slows the breakdown of another substance by oxygen. Synthetic and natural antioxidants are used to slow the deterioration of gasoline and rubber, and such antioxidants as vitamin C (ascorbic acid), butylated hydroxytoluene (BHT), and butylated hydroxyanisole (BHA) are added to foods (see food additivesfood additives,
substances added to foods by manufacturers to prevent spoilage or to enhance appearance, taste, texture, or nutritive value. By quantity, the most common food additives are flavorings, which include spices, vinegar, synthetic flavors, and, in the greatest
..... Click the link for more information. ) to prevent them from becoming rancid or from discoloring.
In the body, nutrients such as beta-carotene (a vitamin A precursor), vitamin C, vitamin E, and selenium have been found to act as antioxidants. They act by scavenging free radicalsfree radical,
in chemistry, a molecule or atom that contains an unpaired electron but is neither positively nor negatively charged. Free radicals are usually highly reactive and unstable. They are produced by homolytic cleavage of a covalent bond (see chemical bond); i.e.
..... Click the link for more information. , molecules with one or more unpaired electrons, which rapidly react with other molecules, starting chain reactions in a process called oxidation. Free radicals are a normal product of metabolism; the body produces its own antioxidants (e.g., the enzyme superoxide dismutase) to keep them in balance. However, stress, aging, and environmental sources such as polluted air and cigarette smoke can add to the number of free radicals in the body, creating an imbalance. The highly reactive free radicals can damage healthy DNA and have been linked to changes that accompany aging (such as age-related macular degeneration, a leading cause of blindness in older people) and with disease processes that lead to cancer, heart disease, and stroke.
Studies have suggested that the antioxidants that occur naturally in fresh fruits and vegetables have a protective effect. For example, vitamin E and beta-carotene appear to protect cell membranes; vitamin C removes free radicals from inside the cell. There is still some question as to whether antioxidants in the form of dietary supplements counteract the effects of increased numbers of free radicals in the body. Some scientists believe that regular consumption of such supplements interferes with the body's own production of antioxidants.
A substance that, when present at a lower concentration than that of the oxidizable substrate, significantly inhibits or delays oxidative processes, while being itself oxidized. In primary antioxidants, such as polyphenols, this antioxidative activity is implemented by the donation of an electron or hydrogen atom to a radical derivative, and in secondary antioxidants by the removal of an oxidative catalyst and the consequent prevention of the initiation of oxidation.
Antioxidants have diverse applications. They are used to prevent degradation in polymers, weakening in rubber and plastics, autoxidation and gum formation in gasoline, and discoloration of synthetic and natural pigments. They are used in foods, beverages, and cosmetic products to inhibit deterioration and spoilage. Interest is increasing in the application of antioxidants to medicine relating to human diseases attributed to oxidative stress.
The autoxidation process is shown in reactions (1), (2), and (3). Lipids,
When lipid autoxidation occurs in food, it can cause deterioration, rancidity, bad odor, spoilage, reduction in nutritional value, and possibly the formation of toxic by-products. Oxidation stress in a lipid membrane in a biological system can alter its structure, affect its fluidity, and change its function, causing disease.
An antioxidant can eliminate potential initiators of oxidation and thus prevent reaction (1). It can also stop the process by donating an electron and reducing one of the radicals in reaction (2) or (3), thus halting the propagation steps. A primary antioxidant can be effective if it is able to donate an electron (or hydrogen atom) rapidly to a lipid radical and itself become more stable then the original radical. The ease of electron donation depends on the molecular structure of the antioxidant, which dictates the stability of the new radical. Many naturally occurring polyphenols, such as flavonoids, anthocyanins, and saponins, which can be found in wine, fruit, grain, vegetables, and almost all herbs and spices, are effective antioxidants that operate by this mechanism.
A secondary antioxidant can prevent reaction (1) from taking place by absorbing ultraviolet light, scavenging oxygen, chelating transition metals, or inhibiting enzymes involved in the formation of reactive oxygen species, for example, NADPH oxidase and xanthine oxidase (reducing molecular oxygen to superoxide and hydrogen peroxide), dopamine-β-hydroxylase, and lipoxygenases. The common principle of action in the above examples is the removal of the component acting as the catalyst that initiates and stimulates the free-radical chain reaction. See Enzyme
Among antioxidants, the synthetic compounds butylated hydroxyanisole (BHA), propyl gallate, ethoxyquin, and diphenylamine are commonly used as food additives. Quercetin belongs to a large natural group of antioxidants, the flavonoid family, with more than 6000 known members, many acting through both mechanisms described above. Ascorbic acid is an important water-soluble plasma antioxidant; it and the tocopherols, the main lipid soluble antioxidants, represent the antioxidants in biological systems. β-Carotene belongs to the carotenoid family, which includes lycopene, the red pigment in tomatoes; the family is known to be very effective in reacting with singlet oxygen (1O2), a highly energetic species of molecular oxygen. See Ascorbic acid, Carotenoid, Flavonoid