Antioxidant 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.
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-,B-hydroxylase, and lipoxygenases. 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. ,B-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 (102), a highly energetic species of molecular oxygen.
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