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Aberrant oxidation of norepinephrine (1) via the transient o-quinone has been implicated as a critical pathogenetic mechanism underlying the degeneration of noradrenergic cell bodies in the locus coeruleus in Parkinson's disease, the degeneration of noradrenergic nerve terminals in Alzheimer's disease and following transient cerebral ischemia, and the onset and progression of idiopathic vitiligo. An oxidative pathway of 1 is also believed to account for the slow deposition of neuromelanin in pigmented neurons of the locus coeruleus. Remarkably, after extensive investigations spanning over several decades, there is still a lack of knowledge of the oxidation chemistry of 1 beyond the classic cyclization route leading to aminochrome and lutin intermediates. We report herein that oxidation of 1 in the 50-500 microM concentration range with H2O2-dependent oxidizing agents, such as the Fenton reagent (Fe2+-EDTA/H2O2) and the horseradish peroxidase (HRP)/H2O2 system, leads not only to the known cyclization products, such as noradrenochrome and 5,6-dihydroxyindole (3), but also to a significant proportion of chain breakdown products, including 3,4-dihydroxybenzaldehyde, 3,4-dihydroxybenzoic acid, 3,4-dihydroxymandelic acid, and 3,4-dihydroxyphenylglyoxylic acid, which has never been described among the oxidation products or metabolites of 1. Analysis of the brown melanin-like pigment obtained by oxidation of 1 with HRP/H2O2 gave pyrrole-2,3-dicarboxylic acid and pyrrole-2,3,5-tricarboxylic acid, diagnostic markers of 3-derived units in eumelanins. Comparison with reference pigments prepared by similar oxidation of dopamine and 3 indicated that in the case of 1 oxidative polymerization of indole units through the 2-position contributes only to a minor extent to melanin formation. Overall, the results of this study provide a complete characterization of the oxidative chain fission pathways of 1, highlight 3,4-dihydroxyphenylglyoxylic acid as a novel possible metabolic product of this catecholamine, and yield an insight into norepinephrine-melanin, a putative component of locus coeruleus neuromelanin.

This paper reports a systematic characterization of the products formed by oxidation of 17 beta-estradiol (1) with tyrosinase/O-2 at low concentrations of physiological relevance. With the substrate at 1-10 nM concentration, the main reaction products included, beside the catechol estrogens 2-hydroxyestradiol (2) and 4-hydroxyestradiol (3), 6-oxo-2-hydroxyestradiol (4), 9,11-dehydro-2-hydroxyestradiol (6), 6,7-dehydro-2-hydroxyestradiol (7), and 9,11-dehydro-4-hydroxyestradiol (10). At higher estradiol concentrations, e.g., 1 mu M, 6,7,8,9-dehydro-2-hydroxyestradiol (5) and the dimeric products 8 and 9 were also formed. The origin of these products from oxidative routes of 2 and 3 was established. Overall, the results of this study disclose novel aspects of the reactivity of 1 with the tyrosinase/O-2 system and provide the first inventory of the oxidation products of catechol estrogen quinones.

The nature of the products formed by reaction of all-trans retinoic acid (1) and its major metabolite, 5,6-epoxyretinoic acid (2), with the Fenton reagent was investigated. Oxidation of 1 in a vigorously stirred biphasic medium (0.1 M phosphate buffer, pH 7.4/ethyl acetate 5:1 v/v) with Fe2+/EDTA complex (2 mol equiv) and a 10-fold excess of H2O2 proceeded smoothly to give a very complex mixture of products. Repeated TLC fractionation of the reaction mixture after methylation allowed isolation of the main products which were identified as 2 methyl ester, (7E)-7,8-epoxyretinoic acid methyl ester (6), all-(E)-2,6-dimethyl-8-(2,6,6-trimethyl-2-cyclohexen-1-ylidene)-2,4,6-octatrienal (11), the novel (9E)-5,6,9,10-diepoxyretinoic acid methyl ester (7), and (9Z)-5,6,9,10-diepoxyretinoic acid methyl ester (8) (1:1 mixture of syn/anti isomers each), 5,6-epoxy-beta-ionone (9), 5,6-epoxy-beta-ionylideneacetaldehyde (10), and trace amounts of beta-ionone (12), beta-ionylideneacetaldehyde (13), and 4-oxoretinoic acid (3) methyl ester. When the oxidation was carried out with the substrate and the Fenton reagent at concentrations as low as 10 microM, the main detectable products were 2 methyl ester, 11, and 7/8. Under similar conditions, the epoxide 2 gave mainly products 7-10. A less efficient conversion of 1 and 2 but similar product patterns were observed with other oxidizing systems such as peroxidase/H2O2 and 13-hydroperoxyoctadecadienoic acid in the presence of Fe(II). Besides providing the first detailed insight into the products formed by reaction of a retinoid with the Fenton reagent, the results of this study disclose a novel nonenzymatic route from 1 to the epoxide 2 and offer an improved chemical basis to inquire into the mechanism of the antiinflammatory, antimutagenic, and cancer chemopreventive action of these retinoids.

The structural modifications of the unsaturated fatty acid components of triglycerides in extra virgin olive oil (EVOO) following exposure to nitrite ions in acidic media were determined by two-dimensional (2D) NMR spectroscopy, aided by (15)N labeling and GC analysis, allowing investigation of the matrix without fractionation steps. In the presence of excess nitrite ions in a 1% sulfuric acid/oil biphasic system, extensive double bond isomerization of the oleic/linoleic acid components of triglycerides was observed associated with nitration/oxidation processes. Structurally modified species were identified as E/Z-nitroalkene, 1,2-nitrohydroxy, and 3-nitro-1-alkene(1,5-diene) derivatives based on (1)H, (13)C, and (15)N 2D NMR analysis in comparison with model compounds. Minor constituents of EVOO, including phenolic compounds and tocopherols, were also substantially modified by nitrite-derived nitrating species, even under milder reaction conditions relevant to those occurring in the gastric compartments. Novel nitrated derivatives of tyrosol, hydroxytyrosol, and oleuropein (6-8) were identified by LC/MS analysis of the polar fraction of EVOO and by comparison with synthetic samples. Overall, these results provide the first systematic description at the chemical level of the consequences of exposing EVOO to nitrite ions at acidic pH and offer an improved basis for further investigations in the field of toxic nitrosation/nitration reactions and dietary antinitrosating agents.