FORMATION OF FREE NITRIC-OXIDE FROM L-ARGININE BY NITRIC-OXIDE SYNTHASE - DIRECT ENHANCEMENT OF GENERATION BY SUPEROXIDE-DISMUTASE

Although nitric oxide (NO) appears to be on of the oxidation products of L-arginine catalyzed by NO synthase (NOS; EC 1.14.13.39), past stud ies on the measurement of NO in cell-free, enzymatic assays have not b een based on the direct detection of the free NO molecule. Instead, as says have relied on indirect measurements of the stable NO oxidation p roducts nitrite and nitrate and on indirect actions of NO such as guan ylate cyclase activation and oxyhemoglobin oxidation. Utilizing a spec ific chemiluminescence assay, we report here that the gaseous product of L-arginine oxidation, catalyzed by both inducible macrophage and co nstitutive neuronal NOS, is indistinguishable from authentic NO on the basis of their physicochemical properties. NO gas formation by NOS wa s dependent on L-arginine, NADPH, and oxygen and inhibited by N-G-meth yl-L-arginine and cyanide anion. Superoxide dismutase (SOD) caused a m arked, concentration-dependent increase in the production of free NO b y mechanisms that were unrelated to the dismutation of superoxide anio n or activation of NOS. These observations indicate that free NO is fo rmed as a result of NOS-catalyzed L-arginine oxidation and that SOD en hances the generation of NO without directly affecting NO itself. SOD appears to elicit a novel biological action, perhaps accelerating the conversion of an intermediate in the L-arginine-NO pathway such as nit roxyl (HNO) to NO.