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Plant Physiol, May 2001, Vol. 126, pp. 445-462
Dissecting the Superoxide Dismutase-Ascorbate-Glutathione-Pathway
in Chloroplasts by Metabolic Modeling. Computer Simulations as a Step
towards Flux Analysis
Andrea
Polle*
Georg-August Universitaet, Forstbotanisches Institut, Abteilung I,
Forstbotanik und Baumphysiologie, Buesgenweg 2, 37077 Goettingen,
Germany
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 O2. 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.
*
Corresponding author; e-mail apolle{at}gwdg.de; fax
49-551-392705.
© 2001 American Society of Plant Physiologists
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