Fig. 2.

Model of NO and H₂O₂ metabolism in plant peroxisomes.L-Arginine nitric oxide synthase (NOS)-like activity generates NO which can react with reduced glutathione (GSH) in the presence of O₂ to form S-nitrosoglutathione (GSNO). This metabolite can interact with SH-containing proteins by a process of S-nitrosylation affecting their function. NO can also react with superoxide radicals (O₂^·-) to generate peroxynitrate (ONOO^-) which can mediate a process of tyrosine nitration of proteins. Alternatively, either NO or GSNO can be released to the cytosol to participate in signaling cascades, although the S-nitrosoglutathione reductase (GSNOR) could modulate the GSNO release. Hydrogen peroxide (H₂O₂) is produced by different biochemical pathways. Additionally, the superoxide radical (O₂^·-) generated by some enzymes such as xanthine oxidase (XOD) which is part of purine metabolism can dismutate into H₂O₂ by the enzyme superoxide dismutase (SOD). The level of peroxisomal H₂O₂ is controlled either by catalase located in the matrix or by the membrane bound ascorbate peroxidase (APX), and its release to the cytosol could be through putative aquaporins.