Lj. Ignarro et al., OXIDATION OF NITRIC-OXIDE IN AQUEOUS-SOLUTION TO NITRITE BUT NOT NITRATE - COMPARISON WITH ENZYMATICALLY FORMED NITRIC-OXIDE FROM L-ARGININE, Proceedings of the National Academy of Sciences of the United Statesof America, 90(17), 1993, pp. 8103-8107
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.