Figure 2.

Tryptophan Metabolism Pathway. Tryptophan is metabolized by two main pathways: the serotonin pathway (~1% of tryptophan metabolism) and the kynurenine pathway (~95% of tryptophan metabolism). The rate limiting enzymes IDO1/2 can be induced with pro-inflammatory cytokines, resulting in the production of more kynurenine, which is an agonist for AHR. AHR can also regulate the expression of IDO1/2, TDO2, KYNU, and KMO. The black arrows indicate enzymatic reactions, the red arrows indicate positive (+) regulation and the blue arrows indicate negative (−) regulation. DDC: dopa decarboxylase; HAAO: 3-hydroxyanthranilate 3,4-dioxygenase; IDO: indoleamine 2,3-dioxygenase; KATs: kynurenine amino transferases; KMO: Kynurenine 3-Monooxygenase; KYNU: Kynureninase; MAOA/B: monoamine oxidase A/B; TDO2: tryptophan 2,3-dioxygenase; TPH1/2: tryptophan hydroxylase.