OXIDATION OF NITRIC-OXIDE IN AQUEOUS-SOLUTION TO NITRITE BUT NOT NITRATE - COMPARISON WITH ENZYMATICALLY FORMED NITRIC-OXIDE FROM L-ARGININE

Nitric oxide (NO) in oxygen-containing aqueous solution has a short ha lf-life that is often attributed to a rapid oxidation to both NO2- and NO3-. The chemical fate of NO in aqueous solution is often assumed to be the same as that in air, where NO is oxidized to NO2 followed by d imerization to N2O4. Water then reacts with N2O4 to form both NO2- and NO3-. We report here that NO in aqueous solution containing oxygen is oxidized primarily to NO2- with little or no formation of NO3-. In th e presence of oxyhemoglobin or oxymyoglobin, however, NO and NO2- were oxidized completely to NO3-. Methemoglobin was inactive in this regar d. The unpurified cytosolic fraction from rat cerebellum, which contai ns constitutive NO synthase activity, catalyzed the conversion of L-ar ginine primarily to NO3- (NO2-/NO3- ratio = 0.25). After chromatograph y on DEAE-Sephacel or affinity chromatography using 2',5'-ADP-Sepharos e 4B, active fractions containing NO synthase activity catalyzed the c onversion Of L-arginine primarily to NO2- (NO2-/NO3- ratio = 5.6) or o nly to NO2-, respectively. Unpurified cytosol from activated rat alveo lar macrophages catalyzed the conversion of L-arginine to NO2- without formation of NO3-. Addition of 30 muM oxyhemoglobin to all enzyme rea ction mixtures resulted in the formation primarily of NO3- (NO2-/NO3- ratio = 0.09 to 0.20). Cyanide ion, which displaces NO2- from its bind ing sites on oxyhemoglobin, inhibited the formation of NO3-, thereby a llowing NO2-to accumulate. These observations indicate clearly that th e primary decomposition product of NO in aerobic aqueous solution is N O2- and that further oxidation to NO3- requires the presence of additi onal oxidizing species such as oxyhemoproteins.