NO scavenging by 3-hydroxyanthranilic acid and 3-hydroxykynurenine: N-nitrosation leads via oxadiazoles to o-quinone diazides

The tryptophan metabolites kynurenine, 3-hydroxykynurenine, anthranilic, 3-hydroxyanthranilic and 3-methoxyanthranilic acids were compared with regard to diazotation by -NO or NO+, using three different donors, nitrite at pH 5, PAPA-NONOate at pH 7.4 and NO+SbF6- at pH 2.0. With all three sources of NO species, 3-hydroxykynurenine and 3-hydroxyanthranilic acid were readily nitrosated, thereby forming an intensely yellow compound. Nitrosation of the non-hydroxylated analogs did not lead to colored products within the period of observation. Competition experiments, using PAPA-NONOate as NO donor, showed that 3-hydroxyanthranilic acid is a more potent NO scavenger than N-acetylcysteine. Nitrosation of 3-hydroxykynurenine and 3-hydroxyanthranilic acid leads, presumably via a nitrosamine intermediate, to a diazonium ion, which forms an oxadiazole tautomerizing to a yellow o-quinone diazide. While the diazonium-derived clumone diazide is apparently the sole product detected directly after incubation of 3-hydroxyanthranilic acid, additional substances are formed from 3-hydroxykynurenine. Contrary to rapid carbenium ion formation from diazonium ions of non-hydroxylated anilines, nitrogen is practically not released from oxadiazoles/quinone diazides at moderate temperatures. Since carbenium ions are known to cause adduct formation with other biomolecules, and since non-hydroxylated anilines and their aminophenol analogs differ in their reactions following diazotation, these findings should be of relevance for the relative toxicity of anilines. (c) 2008 Elsevier Inc. All rights reserved.