Abstract
Despite the importance of superoxide dismutases (SODs) in the plant antioxidant defence system little is known about their regulation by post-translational modifications. Here, we investigated the in vitro effects of nitric oxide derivatives on the seven SOD isoforms of Arabidopsis thaliana. S-nitrosoglutathione, which causes S-nitrosylation of cysteine residues, did not influence SOD activities. By contrast, peroxynitrite inhibited the mitochondrial manganese SOD1 (MSD1), peroxisomal copper/zinc SOD3 (CSD3), and chloroplastic iron SOD3 (FSD3), but no other SODs. MSD1 was inhibited by up to 90% but CSD3 and FSD3 only by a maximum of 30%. Down-regulation of these SOD isoforms correlated with tyrosine (Tyr) nitration and both could be prevented by the peroxynitrite scavenger urate. Site-directed mutagenesis revealed that-amongst the 10 Tyr residues present in MSD1-Tyr63 was the main target responsible for nitration and inactivation of the enzyme. Tyr63 is located nearby the active centre at a distance of only 5.26 Å indicating that nitration could affect accessibility of the substrate binding pocket. The corresponding Tyr34 of human manganese SOD is also nitrated, suggesting that this might be an evolutionarily conserved mechanism for regulation of manganese SODs.
Highlights
In plant cells the reactive oxygen species (ROS) superoxide (O2–) arises as a potentially harmful by-product of photosynthetic and respiratory electron transport chains
superoxide dismutase (SOD) are important enzymes of the antioxidant system and several enzyme activities of this system are affected by nitric oxide (NO)
Under inflammatory conditions human manganese SOD (MnSOD) is site- nitrated at Tyr34, which results in inhibition of SOD activity and disturbance of mitochondrial redox homeostasis (Radi, 2013; Yamakura et al, 1998)
Summary
In plant cells the reactive oxygen species (ROS) superoxide (O2–) arises as a potentially harmful by-product of photosynthetic and respiratory electron transport chains. It can be enzymatically produced by various oxidases to serve as a signal or intermediate in general metabolism, development, and stress responses (Mittler et al, 2011). Independent of origin and function, O2– levels are carefully controlled by the antioxidant system (Foyer and Noctor, 2009). O2– is either scavenged by antioxidants such as reduced ascorbate and glutathione or is efficiently converted to hydrogen peroxide (H2O2) by superoxide dismutase (SOD; O2– + 2 H+→H2O2 + O2). By controlling O2– (and indirectly H2O2) levels SODs are important regulators of cellular redox homeostasis and signalling
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