Antioxidants

What are the key antioxidants in the body? See Table 27.2. Apparently, the key natural anti-oxidant is the end product of purine metabolism, uric acid. Uric acid for most people is probably the main antioxidant representing from 75 to 95% of antioxidant protection in the blood. Vitamin C or ascorbic acid is an important water-soluble antioxidant and the various tocopherols (vitamin E) are excellent fat-soluble antioxidants, particularly for membranes of cells. There are certain proteins that can act like antioxidants, and the tripeptide glutathiones (reduced form) are the main intercellular endogenous/exogenous antioxidants (or, at least, are the ones usually mentioned). There are a variety of enzymes that function as antioxidants, e.g., the superoxide dismutases, glutathione peroxidase, and catalase, and various peroxidases metabolize different oxygen-based oxidants.

Luminescence assays can be used to find new antioxidants that have the potential to increase the antioxidant load above and beyond that which exists from the antioxidants previously mentioned. This is because the light produced from the oxidation of luminol occurs as the result of attack by known, physiologically relevant oxidants such as peroxynitrite and hypochlorite/hydrogen peroxide. By using the light-producing reaction between the known oxidants, e.g., peroxynitrite or hypochlo-rite, and luminol to produce oxidation-based blue light, we can identify antioxidants; antioxidants inhibit this light. Using this simple chemical system, we have found many polyphenolic compounds that are strong antioxidants. These antioxidants originate in plants, e.g., green or black tea, grapes, blueberries, cranberries, etc. A variety of research has shown that these polyphenolic substances are protective, possibly as anticancer substances. A major reason that this may occur can be seen in the attack of peroxynitrite or hypochlorite/hydrogen peroxide on the amino acid tyrosine alone or as a polymerized amino acid in proteins. Peroxynitrite nitrates or oxidizes tyrosine; hypochlorite oxidizes or chlorinates tyrosine. Therefore, the polyphenols act as bystander molecules that can be readily attacked by oxidants but that destroy the ability of the oxidants to attack anything else because the ability of chemical oxidation has been spent on attacking phenols. We present this work in Chapters 28 through 35.

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