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. 2022 Sep 12;16:987697. doi: 10.3389/fnbeh.2022.987697

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Copyright © 2022 Lukić, Ivković, Mitić and Adžić.

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

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Schematic overview of tryptophan metabolic pathways in the host and its commensal bacteria. Black arrows represent host metabolism. Blue arrows represent microbial metabolism. Serotonin pathway – TPH, tryptophan hydroxylase; 5-HTP, 5-hydroxytryptophan; AAAD, aromatic amino acid decarboxylase; AANAT, aralkylamine N-acetyltransferase; ASMT, acetylserotonin O-methyltransferase; kynurenine pathway – IDO, indolamine 2,3-dioxygenase; TDO, tryptophan 2,3-dioxygenase; KAT, kynurenine aminotransferase; KYNA, kynurenine acid; KMO, kynurenine 3-monooxygenase; KYNU, kynureninase; 3-HK, 3-Hyroxykynurenine; 3-HAA, 3-hydroxyanthranilic acid; QUIN, quinolinic acid; NAD+, nicotinamide adenine dinucleotide; indole pathway: ARAT, aromatic amino acid aminotransferase; IAM, indole-3-acetamide; IAA, indole acetic acid; IAld, indole-3-aldehyde; TNA, tryptophanase; CYP450, cytochrom P450 enzymes; SULT, sulfotransferase; IS, indoxil-3-sulfate; TMO, tryptophan 2-monooxygenase; IPYA, indole-3-pyruvate; ILA, indole-3-lactic acid; IA, indole acrylic acid; IPA, indole-3-propionic acid.