Figure 1: Schematic overview of the kynurenine pathway. It is estimated that only 1% of dietary tryptophan (TRP) can be converted into serotonin (5-HT). The remaining 99% of TRP is metabolized via the kynurenine (KYN) pathway. Tryptophan hydroxylase (TH), 5-hydroxy TRP (5-HTP), N-acetylserotonin (NAS), indoleamine 2,3-dioxygenase 1 and 2 (IDO1/2), tryptophan 2,3-dioxygenase (TDO), kynurenine 3-monooxygenase (KMO), kynurenine aminotransferase (KAT I, II, III), kynurenic acid (KA), anthranilic acid (AA), 3-hydroxykynurenine (3-HK), xanthurenic acid (XA), 3-hydroxyanthranilic acid (3-HAA), 3-hydroxyanthranilic acid oxidase (3-HAO), quinolinic acid (QUIN), quinolinic-acid phosphoribosyl transferase (QPRT), nicotinic acid mononucleotide (NaMN), nicotinamide mononucleotide adenylyltransferase (NMNAT), nicotinamide adenine dinucleotide (NAD).