Peroxisomal H2O2-generating sulfite oxidase (SOX) from pepper fruits is negatively modulated by NO and H2S.

Nitric oxide and hydrogen sulfide are signal molecules that can exert regulatory functions in diverse plant processes including fruit ripening. Sulfite oxidase (SOX) is a peroxisomal enzyme that catalyzes the oxidation of sulfite (SO32-) to sulfate (SO42-) with the concomitant generation of H2O2. SOX requires the molybdenum cofactor (Moco) and it has been proposed that SOX functions as a mechanism of protection against sulfite toxicity. Based on the analysis of the pepper genome and fruit transcriptome (RNA-seq), a single gene encoding for a SOX, was identified in chromosome 2. The CaSOX gene expression analysis during fruit ripening, from green immature (G) to red ripe (R) indicates that its expression increased. In-gel analysis using non-denaturing PAGE of a 50-75% (NH4)2SO4 protein fraction allowed the detection of its SOX activity in green pepper fruits. In vitro assay of the SOX from pepper fruits showed that the SOX activity is differently regulated by NO and H2S. Mass spectrometric analysis of the nitrated recombinant pepper SOX enables us to corroborate that this enzyme undergoes inhibition by nitration in Tyr10. Protein modeling analysis also reveals that Cys70 and Cys163 are susceptible targets for S-nitrosation and persulfidation. These findings suggest that NO and H2S could function upstream of the peroxisomal H2O2-generating SOX, highlighting the intricate network of signaling molecules within this subcellular compartment.