A number of other compounds that are not nutrients but are formed in the body as normal metabolites also provide protection against radical damage. Such compounds include uric acid (the end-product of the metabolism of the purines) and the coenzyme ubiquinone (section 18.104.22.168). The latter is sometimes marketed as vitamin Q, but it can be synthesized in the body, and there is no evidence that it is a dietary essential or that an increase above the amount that is normally present in tissues confers any benefit.
In addition to these protective nutrients and normal metabolites, a wide variety of compounds that are naturally present in plant foods also have antioxidant action. These are mainly polyphenols, which are capable of forming stable radicals that persist long enough to undergo reaction to non-radical products. However, they are also potentially capable of undergoing non-enzymic redox cycling, leading to increased formation of oxygen radicals.
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