Dissecting the Superoxide Dismutase-Ascorbate-Glutathione-Pathway in Chloroplasts by Metabolic Modeling. Computer Simulations as a Step Towards Flux Analysis

• Published in: Plant physiology (Bethesda) Vol. 126; no. 1; pp. 445 - 462
• Main Author: Polle, Andrea
• Format: Journal Article
• Language: English
• Published: Rockville, MD American Society of Plant Physiologists 01.05.2001; American Society of Plant Biologists
• Subjects: Adaptation to environment and cultivation conditions; Agronomy. Soil science and plant productions; Antioxidants; Ascorbic Acid - metabolism; Biological and medical sciences; Chloroplasts; Chloroplasts - enzymology; Chloroplasts - metabolism; Computer Simulation; Environmental Stress and Adaptation; Enzymes; Fundamental and applied biological sciences. Psychology; Genetics and breeding of economic plants; Glutathione - metabolism; Hydrogen peroxide; Leaves; Mathematical constants; Metabolism; Metabolism. Physicochemical requirements; Modeling; Oxidation; Oxidative stress; Oxidizing agents; Plant physiology and development; Plants; Reactive Oxygen Species; Redox reactions; Superoxide Dismutase - metabolism; Superoxides; Varietal selection. Specialized plant breeding, plant breeding aims; Ascorbic acid; Glutathione dehydrogenase (ascorbate); Computer simulation; Enzyme; Glutathione reductase (NADPH); Metabolic cycle; L-Ascorbate peroxidase; Detoxication; Superoxide dismutase; Antioxidant; Metabolism; Stress; Monodehydroascorbate reductase (NADH); Peroxidases; Genetic improvement; Oxidoreductases; Simulation model; Chloroplast; Adaptation; Glutathione
• ISSN: 0032-0889; 1532-2548
• DOI: 10.1104/pp.126.1.445

The present study introduces metabolic modeling as a new tool to analyze the network of redox reactions composing the superoxide dismutase-ascorbate (Asc)-glutathione (GSH) cycle. Based on previously determined concentrations of antioxidants and defense enzymes in chloroplasts, kinetic properties of antioxidative enzymes, and nonenzymatic rate constants of antioxidants with reactive oxygen, models were constructed to simulate oxidative stress and calculate changes in concentrations and fluxes of oxidants and antioxidants. Simulated oxidative stress in chloroplasts did not result in a significant accumulation of $\text{O}_{2}{}^{.-}$ and H2O2 when the supply with reductant was sufficient. Model results suggest that the coupling between Asc- and GSH-related redox systems was weak because monodehydroascorbate radical reductase prevented dehydroascorbate (DHA) formation efficiently. DHA reductase activity was dispensable. Glutathione reductase was mainly required for the recycling of GSH oxidized in nonenzymatic reactions. In the absence of monodehydroascorbate radical reductase and DHA reductase, glutathione reductase and GSH were capable to maintain the Asc pool more than 99% reduced. This suggests that measured DHA/Asc ratios do not reflect a redox balance related to the Asc-GSH-cycle. Decreases in Asc peroxidase resulted in marked H2O2 accumulation without significant effects on the redox balance of Asc/DHA or GSH/GSSG. Simulated loss of SOD resulted in higher H2O2 production rates, thereby affecting all subsequent steps of the Asc-GSH-cycle. In conclusion, modeling approaches contribute to the theoretical understanding of the functioning of antioxidant systems by pointing out questions that need to be validated and provide additional information that is useful to develop breeding strategies for higher stress resistance in plants.