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. 2021 May 20;11:674720. doi: 10.3389/fonc.2021.674720

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Copyright © 2021 Fultang, Gneo, De Santo and Mussai

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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Tryptophan metabolism in cancer. Tryptophan is oxidized by the heme-dependent endocellular enzymes tyrptophan 2,3 dioxygenase (TDO) or indoleamine 2,3 dioxygenase (IDO) to produce N-formyl kynurenine. N-formyl kynurenine is hydrolyzed by kynurenine formidase (AFMID) to kynurenine. Kynurenine may be further transaminated to kynurenic acid by kynurenine-oxoglutrate transaminase (KYAT) or oxidized to 3-hydroxy kynurenine by kynurenine 3-monooxygenase (KMO). Kynureninase (KYNU) catalyzes the cleavage of either kynurenine or 3-hydroxy kynurenine to anthranilic acid which is toxic to T and NK cells and promotes the recruitment of MDSCs.