Inhibition of neuronal nitric oxide synthase by 6-nitrocatecholamines, putative reaction products of nitric oxide with catecholamines under oxidative stress conditions.

6-Nitrodopamine and 6-nitronoradrenaline (6-nitronorepinephrine), putative products of the nitric oxide (NO)-dependent nitration of dopamine and noradrenaline, are reported to be reversible, competitive inhibitors of neuronal nitric oxide synthase (nNOS) with Ki values of 45 and 52 μM resp. The nitrocatecholamines inhibited H2O2 prodn. in the absence of L-arginine and tetrahydrobiopterin (BH4) (the IC50 values for 6-nitrodopamine and 6-nitronoradrenaline were 85 and 55 μM resp.) but without affecting cytochrome c redn. The apparent Ki values for nitrocatecholamine inhibition of enzyme activation by BH4 were 18 μM for 6-nitrodopamine and 40 μM for 6-nitronoradrenaline. Both nitrocatecholamines antagonized the dimerization of nNOS induced by BH4 and by L-arginine, the effect being reversed by BH4 (more than 10 μM) and L-arginine (e.g., 100 μM). Overall, these results suggest that nitrocatecholamines interfere with nNOS activity by binding to the enzyme in the proximity of the substrate and BH4-binding sites near the heme group.