Chloroplast, hydrogen peroxide

Smirnoff, J. Colombe, S.V. (1988). Drought influences the activity of enzymes of the chloroplast hydrogen peroxide scavenging system. Journal of Experimental Botany, 39, 1097-1109. 

IV. Superoxide dismutase (EC 1.15.1.1) Within a cell the superoxide dismutases (SODs) constitute the first line of defense against ROS. Superoxide radical (02) is produced where an electron transport chain is present, as in mitochondria and chloroplasts, but 02 activation may occur in other subcellular locations such as glyoxysomes, peroxisomes, apoplast and the cytosol. Thus SODs are present in all these cellular locations, converting superoxide into hydrogen peroxide and water (i.e. copper/zinc SODs are typically found in the nuclei and cytosol of eukaryotic cells). 

Cyclic photophosphorylation is also a highly energetic reaction. The bipyridyliums, paraquat and diquat (Figure 2.2), divert the electron flow of cyclic photophosphorylation (photosystem I). The capture of an electron from the chlorophyll reduces the herbicide and the reduced herbicide reacts with oxygen to form superoxide. Superoxide produces hydrogen peroxide within the chloroplast and these two compounds interact to form hydroxyl radicals in the presence of an iron catalyst. Hydroxyl radicals are very damaging and lead to the destruction of the cellular components leading to rapid plant death. 

Ascorbate is synthesized in large amounts in plants, it can reach 20 to 300 mmol per L in chloroplasts, where its function is mainly to remove hydrogen peroxide formed during photosynthesis. Ascorbate-deficient mutant plants are especially sensitive to ozone- and ultraviolet-induced stress (Smirnoff, 2000). 

Gillham, D. J., and Dodge, A. D., 1986, Hydrogen-peroxide-scavenging systems within pea chloroplasts, Planta 167 2469251. 

Hossain, M. A., and Asada, K., 1984, Inactivation of ascorbate peroxidase in spinach chloroplasts on dark addition of hydrogen peroxide its protection by ascorbate, Plant Cell Physiol. 25 128591295. 

Jablonski, P. P., and Anderson, J. W., 1982, Light-dependent reduction of hydrogen peroxide by ruptured pea chloroplasts. Plant Physiol. 69 1407nl413. 

In solution, bipyridiums almost completely dissociate into ions, and in chloroplasts, during photosynthesis, the positive ion is reduced to a stable flee radical. In the presence of oxygen, the free radicals are reorgaitized to the original ion and hydrogen peroxide, which destroys the plant tissue. 

Many herbicides act by damaging the photosynthetic chain. Atrazine damages tlie quinone binding area on PSII paraquat takes electrons from ferredoxin and generates (lethal) hydrogen peroxide. Uncouplers like DNP (obviously) work in chloroplasts as well as mitochondria. 

Generation of Superoxide Radicals and Hydrogen Peroxide in Chloroplasts. Isolated, illuminated chloroplast thylakoids slowly take up oxygen in the absence of added electron acceptors. This phenomenon was first observed by Mehler (7) and is often known as the "Mehler reaction. The reaction appears to result from the reduction of O2 to the... 

Hydrogen peroxide has a second deleterious effect on the chloroplast—it slowly inactivates chloroplast superoxide dismutase (46). If this enzyme becomes incapable of rapidly removing 02% then the remaining 02" can react with hydrogen peroxide to form the hydroxyl radical, -OH (23,40). Hydroxyl radicals are the most reactive species known to chemistry they will attack and damage almost every molecule... 

It is therefore imperative that the chloroplast has some mechanism for disposing of excess hydrogen peroxide. 

Therefore, illuminated chloroplasts must be protected against hydrogen peroxide and singlet oxygen. Ascorbic acid plays a key role in such protection, as summarized in the next section. 

Ascorbic Acid for Removing Hydrogen Peroxide in Chloroplasts. In 1976, it was suggested (70) that hydrogen peroxide could be removed in illuminated chloroplasts in vivo by a nonenzymic reaction with ascorbic acid ... 

The direct anodic oxidation of cytochrome c at a bipyridyl-modified electrode has already been incorporated in enzyme electrodes for lactate, carbon monoxide, and hydrogen peroxide. Here, cytochrome c is reduced by cytochrome b2, CO oxidoreductase, or horseradish peroxidase and anodically reoxidized. Cytochrome c has also been applied to couple mitochondria and chloroplasts to redox electrodes (Albery et al. 1987). Although no practically applicable sensor has been constructed as yet, this principle offers a new avenue to the determination of inhibitors of photosynthesis or respiration (Cardosi and Turner, 1987). 

The formation of hydrogen peroxide has been demonstrated by Davenport (1963) and Davenport and Dodge (1969) in a chloroplast suspension treated with diquat. Reduction and oxidation of bipyridylium herbicides are cyclically repeated in the plant, so that catalytic quantities are sufficient to kill the plant. 

Nakano Y and Asada K (1981) Hydrogen peroxide is scavenged by ascorbate-specific peroxidase in spinach chloroplasts. Plant Cell Physiol 22 867-880... 

Gillham DJ and Dodge AD (1987) Chloroplast superoxide and hydrogen peroxide scavenging systems from pea leaves Seasonal variations. Plant Sci 50 105-109... 

Some of the hydrogen peroxide formed may react nonenzymatically, decarboxylating glyoxylate to formate and CO2, but most is probably destroyed by peroxidases or catalase. The latter enzyme is lacking in chloroplasts, one reason why oxidation of glycolate must occur in the peroxisomes. 

---