The characteristic absorption and photochemical properties of pheomelanins are generally attributed to "benzothiazine" structural units derived biogenetically from 5-S-cysteinyldopa. This notion, however, conveys little or no information about the structural chromophores responsible for the photoreactivity of pheomelanins. At pH 7.4, natural and synthetic pheomelanins show a defined maximum around 305 nm, which is not affected by reductive treatment with sodium borohydride, and a monotonic decrease in the absorption in the range 350-550 nm. These features are not compatible with a significant proportion of structural units related to 2H-1,4-benzothiazine and 2H-1,4-benzothiazine-3-carboxylic acid, the early borohydride-reducible pheomelanin precursors featuring absorption maxima above 340 nm. Rather, these features would better accommodate a contribution by the nonreducible 3-oxo-3,4-dihydrobenzothiazine (lambdamax 299 nm) and benzothiazole (lambdamax 303 nm) structural motifs, which are generated in the later stages of pheomelanogenesis in vitro. This conclusion is supported by a detailed liquid chromatography/UV and mass spectrometry monitoring of the species formed in the oxidative conversion of 5-S-cysteinyldopa to pheomelanin, and would point to a critical reassessment of the commonly reported "benzothiazine" chromophore in terms of more specific and substantiated structural units, like those formed during the later stages of pheomelanin synthesis in vitro.
With a view to elucidating the structural requisites for effective antinitrosating properties in plant polyphenolics and their metabolites, we have undertaken a comparative investigation of the nitrite scavenging effects of representative catechol derivatives of dietary relevance in the 2,3-diaminonaphthalene (DAN) nitrosation and tyrosine nitration assays. Compounds tested included caffeic acid (1), chlorogenic acid (2), piceatannol (3), hydroxytyrosol (4), and the corresponding S-glutathionyl conjugates 5-8, which were prepared using either tyrosinase (5 and 6) or a novel, o-iodoxybenzoic acid (IBX)-based oxygenation/ conjugation methodology (7b and 8). In the DAN nitrosation assay at pH 4.0, the rank order of inhibitory activities was found to be 5-S-glutathionylpiceatannol (7b) > 3 > 1 > 2 > 2-S-glutathionylcaffeic acid (5) > 2-S-glutathionylchlorogenic acid (6) > 4 approximately 5-S-glutathionylhydroxytyrosol (8). Quite unexpectedly, in the tyrosine nitration assay in 0.5 M HCl, 2 was the most efficient inhibitor followed by 1 > 4 > 3 > 7b approximately 5 > 8 > 6. Under the assay conditions, the glutathionyl conjugates were usually consumed at faster rates than the parent catechols (decomposition rates: 3 > 1 > 4 > 2). The 2,2-diphenyl-1-picrylhydrazyl radical (DPPH) assay indicated that the most effective hydrogen donors were 4 > 7b > 1 approximately 3. Overall, these results indicated that catechol compounds and their glutathionyl conjugates may exhibit profoundly different inhibitory properties depending on the specific conditions of the assay, including especially pH, and that their antinitrosating properties do not correlate tout-court with their hydrogen donor capacity. The glutathionyl-piceatannol conjugate 7b was found to be one of the most potent inhibitors in the physiologically relevant DAN assay and may provide a new structural lead for the design of effective antinitrosating agents based on dietary polyphenolic compounds.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.