Indole-3-pyruvic and -propionic acids, kynurenic acid, and related metabolites as luminophores and free-radical scavengers.

Chemiluminescence associated with oxidation by free radicals was investigated in an alkaline, hemin-catalysed hydrogen peroxide system, using the following tryptophan metabolites as radical scavengers: indole-3-pyruvic, indole-3-propionic, kynurenic, xanthurenic and quinaldic acids and 4-hydroxy-quinoline. Light emission from oxidation of the indolic compounds was only partially inhibited by the hydroxyl-radical scavenger DMSO, but strongly suppressed by the superoxide-anion scavenger Tiron, whereas chemi-luminescence generated from kynurenic acid was strongly inhibited by either of these compounds. Light emission from oxidation of kynurenic acid lasts for a surprisingly long period of time: At 0.4 mM and 20 degrees C, luminescence increased for 5 hours and continued at a high rate for more than a day. Comparison of structural analogues indicated that the 4-hydroxyl and carboxyl groups of kynurenic acid are essential for effective light emission, and that an additional 8-hydroxyl residue leading to an intramolecular hydrogen bond diminishes the reaction rate.