Apart from a few exceptions, SODs are present in all aerobic organisms and in all subcellular compartments that have to deal with AOS. They are classified according to their metal cofactor as isozymes containing Cu/Zn, Fe, and manganese (Mn). The FeSOD and MnSOD proteins are structurally similar (23), whereas the Cu/ZnSOD family is structurally unrelated (24). The SODs catalyze the disproportionation of superoxide through an oxidation-reduction cycle of the prosthetic Cu, Mn, or Fe cofactor. Experimentally, the three SOD types can easily be distinguished via in situ gel staining (25). Incubating the gels with KCN or H202 allows discrimination between the different classes. The Cu/ZnSOD is characterized as being sensitive to both H202 and KCN; FeSOD is sensitive only to H202, whereas MnSOD is resistant to both inhibitors. However, an FeSOD from rice was shown to be resistant to both inhibitors (26). Besides their differential sensitivity toward inhibitors, these enzymes also have a distinctive subcellular distribution. The amount and relative abundance of SOD isozymes vary within each organism. Developmental control and environmental stresses that generate AOS (ultraviolet, ozone, air pollutants, low temperatures, salt stress, drought, heat shock, pathogen infections, etc.) can induce plant SOD activities (22). The MnSOD is found in the mitochondria of all eukaryotic cells, including plants; Cu/ZnSODs are found in the cytosol, peroxisomes, and chloroplasts of higher plants. Multiple Cu/ZnSOD isoforms are also present in the extracellular fluids of Scots pine (27,28). Until now, FeSODs were found only in prokaryotes and in the chloroplasts of plants.

B. Ascorbate Peroxidase

Peroxidases are ubiquitous enzymes found in plants. Besides the peroxidases, whose oxidation products play mainly physiological roles (lignification, cross-linking of cell wall matrices), a second class is also part of the AOS defense system. Ascorbate peroxidases (APXs; EC destroy harmful H202 via the ascorbate-glutathione pathway in chloroplasts and cytosol of plants, algae, and some cyanobacteria (21). Also, APX activity has been identified in insects and purified from bovine eye tissue (29,30). The ascorbate-glutathione pathway provides protection against oxidative stress by a series of coupled redox reactions, particularly in photosynthetic tissues (21) but also in mitochondria and peroxisomes (31). The APX eliminates H202 by using ascorbate as an electron donor in an oxidation-reduction reaction. Ascorbate is then oxidized to monodehydroascorbate (MDHA).

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