Figure 1.

Degradation of tryptophan in kynurenine pathway. L-Trp is first converted into N-formylkynurenine, which is mediated by IDO or TDO, and then into L-kynurenine (3). L-Kynurenine is mainly metabolized into 3-hydroxykynurenine by KMO and then further converted into 3-hydroxyanthranilic acid by KYNU. 3-hydroxyanthranilic acid is catabolized into quinolinic acid by HAAO and then processed into end product NAD+ by QPRT. The catabolism of 3-hydroxykynurenine into xanthurenic acid by KATs and the metabolism of L-kynurenine into anthranilic acid and kynurenic acid are the minor L-kynurenine metabolism pathways. IDO, indoleamine 2,3-dioxygenase; TDO, tryptophan 2,3-dioxygenase; AFMID, kynurenine formamidase (arylformamidase); KMO, kynurenine-3-monooxygenase; KYNU, kynureninase; HAAO, 3-hydroxyanthranilate 3,4-dioxygenase; NAD+, nicotinamide adenine dinucleotide; QPRT, quinolinic aacid phosphoribosyltransferase; KATs, kynurenine aminotransferases.