Oxidative Conversion of 6-Nitrocatecholamines to Nitrosating Products: A Possible Contributory Factor in Nitric Oxide and Catecholamine Neurotoxicity Associated with Oxidative Stress and Acidosis

Oxidn. of 6-nitrodopamine (1) and 6-nitronorepinephrine (2), as well as of the model compds. 4-nitrocatechol and 4-methyl-5-nitrocatechol, with horseradish peroxidase (HRP)/H2O2, lactoperoxidase (LPO)/H2O2, Fe2+/H2O2, Fe2+-EDTA/H2O2 (Fenton reagent), HRP or Fe2+/EDTA in combination with D-glucose-glucose oxidase, or Fe2+/O2, resulted in the smooth formation of yellowish-brown pigments pos. to the Griess assay. In the case of 1, formation of the Griess pos. pigment (GPP-1) promoted by HRP/H2O2 proceeded through the intermediacy of two main dimeric species that could be isolated and identified as 3 and the isomer 4, featuring the 4-nitro-6,7-dihydroxyindole system linked to a unit of 1 through ether bonds. Spectroscopic (FAB-MS, 1H NMR) and chem. anal. of GPP-1 indicated a mixt. of oligomeric species related to 3 and 4 in which oxidative modification of the nitrocatechol moiety of 1 led to the generation of reactive nitro groups supposedly linked to sp3 hybridized carbons. In the pH range 3-6, GPP-1 induced concn.- and pH-dependent nitrosation of 2,3-diaminonaphthalene, but very poor (up to 2%) nitration of 600 μM tyrosine. At pH 7.4, 1 exerted significant toxicity to PC12 cells, while GPP-1 proved virtually innocuous. By contrast, when assayed on Lactobacillus bulgaricus cells at pH 3.5, 1 was inactive whereas GGP-1 caused about 70% inhibition of cell growth. Overall, these results hint at novel pH-dependent mechanisms of nitrocatecholamine-induced cytotoxicity of possible relevance to ischemia- or inflammation-induced catecholaminergic neuron damage.