Peroxisomal Sulfite Oxidase (SOX), an alternative source of NO in higher plants which is upregulated by H₂S

Nitric oxide (^•NO) is a free radical that is endogenously produced in plant cells, though its enzymatic synthesis remains a subject of ongoing debate. Plant peroxisomes, subcellular compartments with active nitro-oxidative metabolism, play a role in various metabolic pathways. Sulfite oxidase (SOX), a peroxisomal enzyme requiring the molybdenum cofactor (MoCo), catalyzes the oxidation of sulfite (SO₃^2-) to sulfate (SO₄^2-), along with the concomitant production of H₂O₂. Using reconstituted recombinant SOX from pepper (Capsicum annuum L.) fruit, it was shown that this enzyme has the capacity to generate ^•NO using nitrite (NO₂^-) as a substrate and NADH as an electron donor which was detected by electron paramagnetic resonance (EPR) spectroscopy coupled with the spin-trapping method. Furthermore, this ^•NO generation was upregulated in the presence of hydrogen sulfide (H₂S) but was downregulated by H₂O₂ which highlights the relationship between H₂O₂, ^•NO, and H₂S. This data opens new avenues for understanding the enzymatic sources of ^•NO in higher plants, particularly within peroxisomes.