The gas nitric oxide (NO) is synthesized in the cytosol primarily from L-arginine by nitric oxide synthase (Nos). Alternatively, NO can be produced from the reduction of nitrate via the nitrate-nitrite-nitric oxide pathway; however, this pathway only occurs under certain conditions, such as hypoxia. NO acts as a signaling molecule: it binds and activates soluble guanylate cyclase resulting in the production of cyclic guanosine monophosphate (cGMP). NO is also involved in: (i) S-nitrosylation of proteins, a reversible post-translational modification in which a nitrosyl group is covalently attached to the thiol group of a L-cysteine residue, and (ii) S-nitrosylation of glutathione, generating S-nitrosoglutathione (GSNO), which acts as a carrier and reservoir of NO. (Adapted from FBrf0258786.)