Nitro-fatty acids-mediated nitroalkylation modulates fine-tuning catalase antioxidant function during salinity stress in plants

Nitro-fatty acids (NO2-FAs) are novel molecules resulting from the interaction of unsaturated fatty acids and nitric oxide (NO) or NO-related molecules. In plants, it has recently been described that NO2-FAs trigger a powerful antioxidant and defense response against stressful situations, the induction of the heat-shock response (HSR), and they exert their signaling function mainly through a reversible post-translational modification called nitroalkylation. Catalase (CAT) is a key antioxidant enzyme for the control of the hydrogen peroxide (H2O2) levels generated by environmental oxidative stress. The data presented in this study provide novel information on the role of NO2-FAs in modulating the antioxidant activity of catalase 2 (CAT2) during salinity stress in Arabidopsis thaliana. Initially, in vitro treatment with nitro-linolenic acid (NO2-Ln) down-regulated Arabidopsis CAT2 activity, as a consequence of the nitroalkylation of His 156 and His 248, evolutionarily conserved residues with key functional implications for the quaternary structure and hence CAT2 activity. Any effect of NO2-Ln on the heme group or S-nitrosylation of CAT2 was excluded. To further our knowledge of the regulatory mechanism of this antioxidant enzyme by nitroalkylation, the functional modulation of CAT by NO2-FAs was analyzed in 5-day-old Arabidopsis cell suspension cultures subjected to salinity stress. In this situation, the oxidative stress generated caused the nitroalkylation of these residues to disappear through the cleavage of NO2 -Ln binding to CAT2, thus restoring CAT2 catalytic activity. Thus, during salinity stress, CAT2 enzymatic activity increased without changes in protein levels. These results highlight the amino acid targets that are susceptible to nitroalkylation and the modulatory effect of this post-translational modification on CAT2 enzymatic activity in vitro and in vivo. These findings underline the regulatory role of nitroalkylation in CAT2 functionality, which is strongly influenced by the redox state thus becoming a new key control mechanism of this antioxidant enzyme in abiotic stress cell response processes.