Nitrogen Oxides and Hydroxyguanidines: Formation of Donors of Nitric and Nitrous Oxides and Possible Relevance to Nitrous Oxide Formation by Nitric Oxide Synthase

Abstract The involvement of nitric oxide in numerous biological functions has led to the intense study of nitric oxide (NO) generation by the nitric oxide synthases (NOS) responsible. In addition to NO, nitric oxide synthases produce N G -hydroxy- l -arginine, superoxide anion and, indirectly, NOx species such as peroxynitrite and, possibly, nitrous oxide (N 2 O). Consequently, the interactions of N G -hydroxy- l -arginine with NO and other oxides of nitrogen (NOx) are of considerable interest. N G -Hydroxy- l -arginine and other monosubstituted hydroxyguanidines react with aqueous aerobic NO, peroxynitrite, and various NOx and nitrosating agents to form compounds that subsequently release NO and N 2 O. Spectrometric data indicate that the nitrosation product of N G -hydroxy- l -arginine is of the same N -nitroso- N -hydroxy/diazeniumdiolate (formerly “NONOate”) structure as previously found for the nitrosation products of other model hydroxyguanidines. These decompose in aqueous solution in a pH-dependent manner to yield mainly NO and ureas at low pH, N 2 O and cyanamides at basic pH, and what appear to be primary nitrosamines/nitrosoimines. Studies on purified iNOS using a mass spectrometer with a gas-permeable membrane inlet identified both NO and N 2 O (or 15 NO and 15 N 15 NO with 15 N-labeled l -arginine as substrate) as products of NOS activity. These experiments suggest that much more NO than N 2 O is produced under the conditions studied and that N 2 O formation can be rationalized via the reaction of NOx species with N G -hydroxy- l -arginine.