Photochemical properties of kynurenine pathway metabolites and indoleamines

Photochemical damages to the biological system may occur through photodynamic action in the presence of photosensitive molecules. Photodynamic action contains the following processes; 1) photosensitisation and/or 2) electron transfer, in which singlet oxygen and superoxide radical production for each in the presence of oxygen molecules. We have studied those processes after the absorption of light by kynurenine pathway metabolites and indoleamine derivatives. We found that kynurenine and 3-hydroxykynurenine generate superoxide radical after electron transfer from their excited state molecules to oxygen molecules, and superoxide makes reduction reaction. On the other hand, it was found that kynurenic acid, melatonin, 5-methoxytryptamine and 5-methoxytryptophol work as photosensitisers with the detection of singlet oxygen production by using the N, N-dimethyl-4-nitrosoaniline bleaching method, while xanthurenic acid, serotonin and N-acetylserotonin generate no detectable amount of singlet oxygen. We have determined the photochemical quantum yields of singlet oxygen production for those photosensitisers, in which quantum yields are not so high except kynurenic acid (f3 = 0.101). In view of the multiple roles played by their metabolites in various systems, these results are relevant to taking into consideration of their photoeffect in the presence of light.