Salin, M. L. 1988. Toxic oxygen species and protective systems of the chloroplast. -Physiol. Plant. 72: 681-689.As a consequence of living in an environment enriched in oxygen, which they themselves at least partially generate, photosynthetic organisms are exposed to large fluxes of oxyradicals and reactive oxygen species. Among these are superoxide, hydrogen peroxide, hydroxyl radical and singlet oxygen. These highly reactive intermediates pose the threat of toxicity unless neutralized by scavenger substrates or enzymes. The production of oxyradicals and intermediates by chloroplasts as well as the means of protection are discussed in this review.

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Superoxide dismutases (EC 1.15.1.1) in vascular plants representing different evolutionary levels were characterized using polyacrylamide gel electrophoresis. The three forms of the enzyme were distinguished from each other based on the following criteria: a) the Cu-Zn enzyme is sensitive to cyanide wherease the Fe and Mn enzymes are not; and b) the Cu-Zn and Fe enzymes are inhibited by H202 whereas the Mn enzyme is H202-resistant. Of the 43 plant families investigated, the Fe-containing superoxide dismutase was found in three families: Gingkoaceae, Nymphaceae, and Cruciferae.yotic algae lack the Cu-Zn SOD but contain either the Mn and/ or Fe enzymes (3,20). Land plants contain large amounts of the Cu-Zn enzyme plus an additional cyanide-resistant SOD which was assumed to be the Mn protein (3,11,17). The Fe enzyme was previously thought to be restricted to procaryotic organisms but has recently been purified from Euglena gracilis (14,18) MATERIALS AND METHODS Mature leaves from native plants and cultivars were collected locally. Etiolated seedlings were grown from seed in a darkened chamber and harvested 7 to 10 days after sowing. Leaves or etiolated seedlings were ground in a Waring Blendor in ice-cold media consisting of 50 mm K-phosphate (pH 7.0), and 0.1% Triton X-100. The homogenate was squeezed through four layers of cheesecloth and centrifuged at 12,000g for 10 min. Leaf extracts were dialyzed overnight against 10 mm K-phosphate (pH 7.0), to remove small mol wt compounds which interfered with the activity stain for SOD on polyacrylamide gels.Electrophoresis was performed on 7.5% acrylamide gels (9). The photochemical method of Beauchamp and Fridovich (6) was used to localize SOD activity on gels. The three types of SOD were distinguished by their different sensitivity to inhibitors (5,8). Inhibitors (1 mim KCN or 2 mi H202) were added to the staining solution prior to addition to the gels. Gels were incubated in the staining solution for 45 min before exposure to light. Densitometric scans of gels were made with a Perkin-Elmer model 552-0800 integrating gel scanner. Spectrophotometric measurements of SOD activity were made by the method of McCord and Fridovich (21). Assays for SOD were routinely done in the presence of 10 ylM KCN to inhibit Cyt oxidase. This low cyanide concentration was insufficient to inhibit the Cu-Zn enzyme. RESULTSThe Cu-Zn enzyme could easily be distinguished from the other two isozymes by its sensitivity to cyanide. Asada et al.

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