To limit cellular damage caused by excessive AOS levels, plants have evolved a broad variety of nonenzymatic and enzymatic protection mechanisms that efficiently scavenge AOS (19). The best-known nonenzymatic antioxidants are ascorbate, glutathione, a-tocopherol, and carotenoids. They are present in relatively high concentrations within plant cells. For a detailed overview on these components, the reader is referred to Alscher and Hess (20).
Because hydroxyl radicals are too reactive to be directly controlled, aerobic organisms prefer to eliminate the less reactive precursor forms, such as superoxide and H202, and hence prevent the formation of hydroxyl radicals. Superoxide dismutases scavenge superoxide radicals, whereas cata-lases and peroxidases remove H202. Catalases consume the bulk of H202. Ascorbate peroxidases remove H202 that is not accessible for catalase, because of their higher affinity and their diverse subcellular locations. Other enzymes that are involved in the removal of AOS are monodehydroascorbate reductase, dehydroascorbate reductase, glutathione reductase, and glutathione peroxidase. However, these enzymes will not be described in detail in this chapter. For an overview of these enzymes the reader is referred to Noctor and Foyer (21).
Superoxide dismutases (SOD; superoxide:superoxide oxidoreductase; EC 184.108.40.206) can be considered key enzymes of the antioxidative stress defense mechanism (22). They directly determine the cellular concentrations of 02 and H202 because they dismutate 02 into 02 and H202.
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