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 K i values of 45 and 52 μ M respectively. The nitrocatecholamines inhibited H 2 O 2 production in the absence of l-arginine and tetrahydrobiopterin(BH 4 ) (the IC 50 values for 6-nitrodopamine and 6-nitronoradrenaline were 85 and 55 μ M respectively) but without affecting cytochrome c reduction. The apparent K i values for nitrocatecholamine inhibition of enzyme activation by BH 4 were 18 μ M for 6-nitrodopamine and 40 μ M for 6-nitronoradrenaline. Both nitrocatecholamines antagonized the dimerization of nNOS induced by BH 4 and by l-arginine, the effect being reversed by BH 4 (more than 10 μ M) and l-arginine (e.g. 100 μ M). Overall, these results suggest that nitrocatecholamines interfere with nNOS activity by binding to the enzyme in the proximity of the substrate and BH 4 -binding sites near the haem group.