Table 4: Functions of the kynurenine pathway.

Function Mediator (mechanism)

Detoxification of tryptophan TDO (Glucocorticoid induction)
Control of plasma tryptophan availability TDO (Trp flux and oxidation)
Control of liver haem biosynthesis TDO (Utilisation of the regulatory haem pool)
Modulation of the immune system Kyn, KA, 3-HK, 3-HAA, QA, PA (Cytokine induction of IDO)
Modulation of carbohydrate metabolism XA, PA (Binding of insulin and Zn); QA (inhibition of PEPCK)
Pellagra prevention QA, Nicotinic acid, nicotinamide (NAD + synthesis)
NAD + synthesis QA, nicotinic acid, nicotinamide (De novo and Salvage pathways)

Abbreviations used: 3-HAA (3-hydroxyanthranilic acid), 3-HK (3-hydroxykynurenine), IDO (indoleamine 2,3-dioxygenase), KA (kynurenic acid), Kyn (kynurenine), NAD + (oxidized nicotinamide-adenine dinucleotide), PEPCK (phosphoenolpyruvate carboxykinase), PA (picolinic acid), QA (quinolinic acid), TDO (tryptophan 2,3-dioxygenase), Trp (tryptophan), XA (xanthurenic acid).