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

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 (SO) to sulfate (SO) with the concomitant generation of HO. 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% (NH)SO 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 HS. 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 HS could function upstream of the peroxisomal HO-generating SOX, highlighting the intricate network of signaling molecules within this subcellular compartment.