Endotoxin-Induced Tryptophan Degradation along the Kynurenine Pathway: The Role of Indolamine 2,3-Dioxygenase and Aryl Hydrocarbon Receptor-Mediated Immunosuppressive Effects in Endotoxin Tolerance and Cancer and Its Implications for Immunoparalysis

Figure 1

Main pathways of TRP degradation including enzymes and their potential stimuli (modified after [1, 4, 16, 35]). Superscripted numbers indicate described biological effects of TRP metabolites. Black arrows mark enzymatic reactions and dashed arrows include more than one catalytic reaction step. Enzymes: FOR = formamidase, IDO = indolamine 2,3-dioxygenase; TDO = tryptophan 2,3-dioxygenase, TPH = tryptophan hydroxylase; KAT = kynurenine aminotransferase, KMO = kynurenine 3-monooxygenase; KYNU = kynureninase, HADO = 3-hydroxyanthranilic acid dioxygenase; HAAO = 3-hydroxyanthranilic acid oxidase, AMO = anthranilate 3-monooxygenase, and AADC = aromatic L-amino acid decarboxylase; AANAT = N-acetyltransferase; HIOMT = hydroxy-O-methyltransferase.