Oxidation and Nitration of Catecholamines by Nitrogen Oxides Derived from Nitric Oxide

The reactivity of catecholamines with nitrogen oxides formed from NO in aerated solutions, nitrite, and peroxynitrite was evaluated. Dopamine and norepinephrine in aerobic buffer (pH 7.4) were almost completely converted to their 6-nitro-derivatives by nitric oxide (NO) at room temperature, while epinephrine was nitrated and above all oxidized. The products obtained from each catecholamine treated with sodium nitrite at pH 4-7 were compared to those produced by NO at pH 7.4. Peroxynitrite, which can nitrate tyrosinyl residues, did not produce nitro-derivatives, only oxidized ones. The physiological relevance, particularly for the vascular and nervous system, is discussed. Catecholamine oxidation reactions could be relevant to physiological conditions and also explain neurotoxicity in Parkinson's disease and aging. Nitration reactions, requiring such high NO concentrations, do not seem possible to occur directly under normal physiological conditions, but could take place in acidic vesicules where nitrite, catecholamines, and their nitrated products could accumulate. Finally, the ability of dopamine to increase 2',5'-cyclic adenosine monophosphate (cAMP) formation in cultured striatal neurons was blocked by its nitration by NO or its nitrogen oxide derivatives.