Ascorbate peroxidase enzyme

This postulated reaction cycle was greatly strengthened by the discovery of high activities of an ascorbate peroxidase enzyme in spinach chloroplasts (71), which catalyzes Reaction 12. Earlier attempts (78, 79)... 

Observations Table 1 shows the activity of the antioxidant enzymes of tomato roots after 72 h of exposure of allelochemical stress caused by S. deppei. Catalase (CAT) activity increases by 1.5 fold Ascorbate Peroxidase (APX) decreases 2.3 fold Glutathione reductase (GR) activity does not change with the treatment and Superoxide dismutase (SOD) decreases 1.3 fold. 

In contrast, antioxidant enzymes can efficiently counteract all UV-induced ROS (Aguilera et al. 2002). These enzymes are represented by superoxide dismutase (SOD), catalase and glutathione peroxidase as well as those involved in the ascorbate-glutathione cycle, such as ascorbate peroxidase, mono-dehydroascorbate reductase, dehydroascorbate reductase and glutathione reductase. One of the most important classes of antioxidant enzymes is the SOD family, which eliminate noxious superoxide radical anions. Different metalloforms of SOD exist (Fe, Mn, CuZn and Ni), which due to their intracellular localisation protect different cellular proteins (Lesser and Stochaj 1990). 

In plants, ascorbate is required as a substrate for the enzyme ascorbate peroxidase, which converts H202 to water. The peroxide is generated from the 02 produced in photosynthesis, an unavoidable consequence of generating 02 in a compartment laden with powerful oxidation-reduction systems (Chapter 19). Ascorbate is a also a precursor of oxalate and tartrate in plants, and is involved in the hydroxylation of Pro residues in cell wall proteins called extensins. Ascorbate is found in all subcellular compartments of plants, at concentrations of 2 to 25 mM—which is why plants are such good sources of vitamin C. 

However, the average number of turnovers an enzyme molecule is able to perform before deactivation varies substantially among preparations two turnovers for ascorbate peroxidase [50], 238 for horseradish peroxidase A2 [28], 265 for horseradish peroxidase C [51], and 81,000 for Zo peroxidase [28]. 

Ascorbate peroxidase is a heme-eontaining enzyme that catalyses the ascorbate-dependent reduction of potentially damaging hydrogen peroxide in plants and algae, Eq. (1),... 

Chen, G.-X., and Asada, K., 1989, Ascorbate peroxidase in tea leaves occurrence of two isozymes and the differences in their enzymic and molecular properties, Plant Cell Physiol. 30 987n998. 

Mandelman, D., Jamal, J., and Poulos, T. L., 1998a, Identification of two-electron transfer site in ascorbate peroxidase using chemical modification, enzyme kinetics, and crystallography, Rioc/iemutjy 37 17610nl7617. 

Mathews, M. C., Summers, C. B., and Felton, G. W., 1997,Ascorbate peroxidase a novel antioxidant enzyme in insects. Arch. Insect Biochem. Physiol. 34 57n68. 

Ushimara, T., Maki, Y., Sano, S., Koshiba, K., Asada, K., and Tsuji, H., 1997, Induction of enzymes involved in the ascorbate-dependent antioxidative systems, namely ascorbate peroxidase, monodehydroascorbate reductase and dehydroascorbate reductase, after exposure to air of rice Oryza sativa) seedlings germinated under water, Plant Cell Physiol. 38 541n549. 

Enzymes involved I, ascorbate peroxidase and 2, glutathione reductase. Reactions 3 and 4 are nonenzymatic. Reaction 3 proceeds rapidly at pH 8, the pH of the stroma in illuminated chloroplasts. 

Antioxidants, quenching molecules or radical scavenging enzymes such as superoxide dismutase (SOD), ascorbate peroxidase and catalase react with and neutralize the effects of the highly toxic ROS or hydroxyl radicals (HO ) pro-... 

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