Nitrite- and peroxide-dependent oxidation pathways of dopamine: 6-nitrodopamine and 6-hydroxydopamine formation as potential contributory mechanisms of oxidative stress- and nitric oxide-induced neurotoxicity in neuronal degeneration

In the presence of nitrite ions (NO2-) in phosphate buffer (pH 7.4) and at 37 degrees C, dopamine was oxidized by a variety of hydrogen peroxide (H2O2)-dependent enzymatic and chemical systems to give, in addition to black melanin-like pigments via 5,6-dihydroxyindoles, small amounts of the potent neurotoxin 6-hydroxydopamine (1) and of 6-nitrodopamine (2), a putative reaction product of dopamine with NO-derived species. Treatment of 0.5 or 1 mM dopamine with horseradish peroxidase (HRP) or lactoperoxidase (LPO) in the presence of 1 or 2 mM H2O2 with NO2- at a concentration of 0.5-10 mM resulted in the formation of 1 and 2 in up to 8 and 2 mu M yields, respectively, depending on the substrate concentration and the NO2H2O2 ratio. Nitration and hydroxylation of 0.1 mM dopamine was observed with 1 mM NO2- using HRP and the D-glucose/glucose oxidase system to generate H2O2 in situ. In the presence of NO2--, Fe2+-, or Fe2+/EDTA-promoted oxidations of dopamine with H2O2 also led to the formation of 1 and 2, the apparent product ratios varying with peroxide concentration and the partitioning of the metal between EDTA and catecholamine chelates. In the presence of NO2-, Fe2+-promoted autoxidation of dopamine gave 2 but no detectable 1. When injected into the brains of laboratory rats, 2 caused sporadic behavioral changes, indicating that it could elicit a neurotoxic response, albeit to a lower extent than 1. Model experiments using tyrosinase as an oxidizing system and mechanistic considerations suggested that formation of 2 does not involve reactive nitrogen radicals but results mainly from nucleophilic attack of NO2- to dopamine quinone. Generation of 1, on the other hand, may be derives from different H2O2-dependent pathways. Collectively, these results outline a complex interplay of NO2-- and peroxide-dependent oxidation pathways of dopamine, which may contribute to impair dopaminergic neurotransmission and induce cytotoxic processes in neurodegenerative disorders.