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 me lanin-like pigments via 5,6-dihydroxyindoles, small amounts of the potent n eurotoxin 6-hydroxydopamine (1) and of 6-nitrodopamine (2), a putative reac tion 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 result ed 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 a nd hydroxylation of 0.1 mM dopamine was observed with 1 mM NO2- using HRP a nd the D-glucose/glucose oxidase system to generate H2O2 in situ. In the pr esence of NO2--, Fe2+-, or Fe2+/EDTA-promoted oxidations of dopamine with H 2O2 also led to the formation of 1 and 2, the apparent product ratios varyi ng with peroxide concentration and the partitioning of the metal between ED TA and catecholamine chelates. In the presence of NO2-, Fe2+-promoted autox idation of dopamine gave 2 but no detectable 1. When injected into the brai ns of laboratory rats, 2 caused sporadic behavioral changes, indicating tha t it could elicit a neurotoxic response, albeit to a lower extent than 1. M odel experiments using tyrosinase as an oxidizing system and mechanistic co nsiderations suggested that formation of 2 does not involve reactive nitrog en 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 int erplay of NO2-- and peroxide-dependent oxidation pathways of dopamine, whic h may contribute to impair dopaminergic neurotransmission and induce cytoto xic processes in neurodegenerative disorders.