o-Aminophenol-Type Tryptophan Metabolites: 3-Hydroxykynurenine, 3-Hydroxy-anthranilic Acid, and Their Role in Living Organisms

Among tryptophan metabolites, 3-hydroxykynurenine and 3-hydroxyanthranilic acid are unique because of their o-aminophenolic structures. Therefore, they share a typical chemical behaviour and are easily (auto)oxidised to a number of interesting derivatives such as ommatins and phenoxazinones. Moreover, their (auto) oxidation usually proceeds via a radical chain mechanism, in which both semiquinonoid species and intermediates in dioxygen reduction are involved. They can also undergo enzymic oxidation. For these reasons, 3-hydroxykynurenine and 3-hydroxyanthranilic acid participate in carcinogenesis, neurotoxicity, delignification by white-rot fungi, eye lens senescence, melanogenesis. They also show pro-oxidant as well as anti-oxidant activities.