Figure 1.

The scheme of tryptophan metabolism along the kynurenine pathway (KP). The majority of available serum tryptophan enters the KP. IDOs or TDO convert tryptophan into L-kynurenine, which is further catabolized by kynurenine aminotransferases to kynurenic acid, kynurenine-3-monooxygenase to 3-OH-L-kynurenine, or kynureninase to anthranilic acid. Kynurenine aminotransferases may convert 3-OH-L-kynurenine to xanthurenic acid or kynureninase to form 3-OH-anthranilic acid. Further, 3-OH-anthranilic acid is converted into quinolinic or picolinic acid by a series of enzymatic conversions.