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. 2022 Apr 22;54(10):1339–1356. doi: 10.1007/s00726-022-03161-5

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Fig. 1 — Metabolic pathways of tryptophan. A Kynurenine pathway in host immune cells and intestinal epithelial cells. B Indole pyruvate pathway performed by gut microbiota. C Serotonin pathway performed by enteroendocrine cells. Key enzymes of the pathways are displayed in green: ArAT aromatic amino acid transaminase, IDO1 indolamine-2,3-dioxygenase-1, TNA tryptophanase, TpH1 tryptophan hydroxylase 1, TrpD tryptophan dehydrogenase