Active O2 Species Research Articles

High Light Intensity Enhances Chlorosis and Necrosis in Leaves of Zinc, Potassium, and Magnesium Deficient Bean ( Phaseolus vulgaris) Plants

Summary The effect of varied light intensities (80μEm2s-1 to 600μEm-2s-1) on chlorosis and necrosis in leaves of zinc (Zn), potassium (K) and magnesium (Mg) deficient bean (Phaseolus vulgaris L. cv. Prelude) plants was studied in water culture experiments with different Zn, K and Mg supplies. With increasing light intensity plant dry weight increased in plants with sufficient nutrient supply, but not in Zn and K deficient plants. Chlorophyll concentrations declined with increasing light intensity, particularly in plants deficient in either Zn, K or Mg. At high light intensity, severe symptoms of chlorosis and necrosis occurred in the deficient plants, although light intensity was without significant effect on the concentrations of these mineral elements in the leaves. In the deficient plants exposed to high light intensity, partial shading of the leaf blades either prevented or at least drastically delayed development of chlorosis and necrosis in the shaded areas. In leaves of Zn deficient plants the concentrations of carbohydrates (reducing sugars, sucrose, starch) increased with increasing light intensity, particularly in primary leaves. In contrast, root concentrations of carbohydrates were much lower in Zn deficient plants exposed to high light intensity. Resupply of Zn to Zn deficient plants for up to 96 h remarkably decreased concentrations of sucrose in the older leaves. The effects of increasing light intensity on the severity of chlorosis and necrosis are discussed in relation to photooxidation of thylakoid constituents by activated O2 species. Elevated levels of these toxic O2 species are to be expected as a result of impairment in the photosynthetic carbon turnover and electron transfer in leaves of plants deficient in Zn, K or Mg.

Selenium deficiency, endurance exercise capacity, and antioxidant status in rats.

Increased O2 metabolism imposed by physical exercise is likely to augment the production of active O2 species that have been shown to react with lipids, proteins, and DNA. Antioxidants and antioxidant enzymes, such as the selenium enzyme glutathione peroxidase, minimize or prevent such potentially toxic reactions. This study shows that selenium deficiency decreases glutathione peroxidase activity in liver and muscle (less than 80%, P less than 0.001), increases total glutathione in liver, muscle, and plasma (P less than 0.05) and increases muscle cytochrome oxidase activity, and ubiquinone content (P less than 0.05) but has no effect on endurance capacity. Exercise to exhaustion resulted in a significant (P less than 0.001) elevation of total and oxidized glutathione (GSSG) and a significant (P less than 0.05) decrease of vitamin E in plasma of control and selenium-deficient rats. Acute exercise also increased tissue GSSG levels in both control and selenium-deficient groups of rats. Hence, despite a large depletion of selenium-deficient glutathione peroxidase, pronounced oxidation of glutathione to GSSG can be produced by the increased oxidative metabolism during physical exercise. The results suggest that the residual glutathione peroxidase activity is sufficient to detoxify hydroperoxides in exercising selenium-deficient animals and to prevent the impairment of endurance capacity.

Paraquat and NADPH-dependent lipid peroxidation in lung microsomes.

Since there exists some controversy in the literature as to whether paraquat augments microsomal lipid peroxidation via superoxide anion (O2-), the role of paraquat and active oxygen species in NADPH-dependent lung microsomal lipid peroxidation was investigated. Incubation of buffered aerobic mixture of bovine lung microsome and NADPH, in the presence or absence of exogenously added iron, resulted in a progressive formation of lipid peroxides whose accumulation could be followed at 535 nm as malondialdehyde. Paraquat strongly inhibited this lipid peroxidation. Thus, malondialdehyde formation was 50% inhibited by 4 X 10(-5) M paraquat in the reaction mixture. The malondialdehyde color development by lipid peroxides was not affected by this concentration of paraquat. Lipid peroxidation was also strongly inhibited by singlet oxygen scavengers, e.g. dimethylfuran and diphenylfuran, and by catalase. Hydroxyl radical scavengers, e.g. mannitol, benzoate, and ethanol, had little effect in malondialdehyde production. Superoxide dismutase, which removes O2- efficiently, did not inhibit malondialdehyde production by lung microsomes and rather enhanced its formation. A scheme in which paraquat and active O2 species may be involved with microsomal lipid peroxidation is presented.

Photooxidative Damage in Photosynthetic Activities of Chromatium vinosum

The capacity of photosynthetic CO(2) fixation in the anaerobic purple-sulfur bacterium, Chromatium vinosum is markedly impaired by strong illumination (9 x 10(4) lux) in the presence of 100% O(2). In the absence of HCO(3) (-), decline in activity occurred gradually, with about 40% of the initial activity remaining after a 1-hour incubation. The addition of 50 millimolar HCO(3) (-) to the incubation medium resulted in a measurable delay (about 30 minutes) of the inactivation process. Ribulose-1,5-bisphosphate carboxylase activity and light-dependent O(2) uptake (electron flow) or crude extracts prepared after pretreatment of the bacterial cells with O(2) and light were not affected but the photophosphorylation capacity of either bacterial cells or chromatophores was drastically reduced. The inhibition of photophos-phorylation in the chromatophore preparations was significantly reduced by the addition of either an O(2) (-) scavenger, Tiron, or an (1)O(2) scavenger, alpha-tocopherol. These results suggest that the active O(2) species, O(2) (-) or (1)O(2), might take part in the observed inactivation.The pretreatment of the bacteria with O(2) and light inhibited CO(2) assimilation through the Calvin-Benson cycle, while relatively stimulating the formation of aspartate and glutamate. It also inhibited the conversion of glycolate to glycine, resulting in a sustained extracellular excretion of glycolate. The inactivation of photosynthetic CO(2) fixation by intact cells was enhanced by low temperature, KCN, or methylviologen addition during the pretreatment with O(2) and light. The mechanism(s) of O(2)-dependent photoinactivation of photosynthetic activities in Chromatium are discussed in relation to the possible role of photorespiration as a means of producing CO(2) in the photosynthetic system.