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. 2020 Feb 28;11:313. doi: 10.3389/fimmu.2020.00313

Figure 1.

Figure 1

Main pathways of TRP degradation, including relevant enzymes [modified after (2, 3)]. Black arrows mark enzymatic reactions and dashed arrows include more than one catalytic reaction step. Metabolites analyzed in this study are shaded gray. The catabolism of TRP by the enzymes TDO, IDO1/IDO2 represents the rate-limiting step of the kynurenine pathway in which 95% of dietary TRP is oxidized. One percentage of dietary tryptophan for the synthesis of serotonin. Apart from neuromodulatory properties (not shown), KYNA is an agonist of the broadly expressed receptors GPR35 and AhR. Furthermore, KYNA functions as an ROS scavenger. KYNA production. In addition to the presented canonical pathway of KYNA formation, alternative routes of KYNA production, promoted by the presence of ROS, are described (4). AADC, aromatic L-amino acid decarboxylase; AANAT, N-acetyltransferase; AhR, hydrocarbon receptor; AMO, anthranilate 3-monooxygenase; FOR, formamidase; GPR35: G-protein-coupled receptor 35; HAAO, 3-hydroxyanthranilic acid oxidase; HADO, 3-hydroxyanthranilic acid 3,4-dioxygenase; IDO, indolamine 2,3-dioxygenase; KAT, kynurenine aminotransferase; KMO, kynurenine 3-monooxygenase; KYNU, kynureninase; TDO, tryptophan 2,3-dioxygenase; TPH, tryptophan hydroxylase; ROS, reactive oxygen species.