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. 2022 Jun 1;13(3):732–752. doi: 10.14336/AD.2021.1105

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Copyright: © 2022 Yu et al.

this is an open access article distributed under the terms of the creative commons attribution license, which permits unrestricted use, distribution and reproduction in any medium provided that the original work is properly attributed.

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Figure 1. — Schematic representation of the effect of tryptophan metabolism in age-related tissue fibrosis through AhR. Tryptophan is mainly metabolized through three pathways in the host and gut microbiota: the kynurenine pathway, serotonin pathway and indole pathway. The endogenous ligands for AhR that are surrounded in blue, including FICZ, kynurenine, kynurenic acid, xanthurenic acid, cinnabarinic acid, 5-HTP, serotonin, 5-HIAA, indole-3-pyruvate, indole-3-lactic acid, indole-3-propionic acid, indole acetic acid, 3-methylindole, tryptamine, indole-3-aldehyde, indole and indoxyl sulfate, may modulate fibrosis progression after binding with AhR. AAA-ATs: aromatic amino acid aminotransferases; AA-DC: aromatic amino acid decarboxylase; AA-NAT: arylalkylamine N-acetyltransferase; CYP2E1: cytochrome P450 2E1; FICZ: 6-formylindolo[3,2-b]carbazole; HAAO: 3-hydroxyanthranilic acid oxygenase; 5-HIAA: 5-hydroxy- indole-3-acetic acid; HIOMT: hydroxyindole O-methyltransferase; 5-HTP: 5-hydroxy-tryptophan; 5-MTP: 5-methoxy-tryptophan; IDO: indoleamine 2,3-dioxygenase; KAT: kynurenine aminotransferase; KMO: kynurenine 3-monooxygenase; KYNU: kynureninase; SULT1A1: sulfotransferase 1A1; TDO: tryptophan 2,3-dioxygenase; THP: tryptophan hydroxylase; Try: tryptophan.