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. 2019 Jul 24;2:271. doi: 10.1038/s42003-019-0520-5

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© The Author(s) 2019

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Fig. 1 — The kynurenine pathway of tryptophan metabolism. Enzymes are indicated in italics. The neurotoxic metabolites 3-HK and QUIN are highlighted in red and the neuroprotective metabolite KYNA in green. The prodrug 1b is transported across the blood–brain barrier via riboflavin transporters, where it is converted, and thereby releases 1. TDO, tryptophan 2,3-dioxygenase. IDO, indoleamine-2,3-dioxygenase