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. Author manuscript; available in PMC: 2023 May 15.
Published in final edited form as: Curr Opin HIV AIDS. 2022 Dec 28;18(2):102–110. doi: 10.1097/COH.0000000000000782

Figure 1. The Kynurenine Pathway.

Figure 1

In the kynurenine pathway, TRP is metabolized to kynurenine (KYN) by tryptophan 2,3-dioxygenase (TDO), found mostly in hepatic tissue or indoleamine 2,3-dioxygenase (IDO), an enzyme found in most tissues that is stimulated by steroid hormones, cytokines and growth factors. Kynurenine is then either metabolized to kynurenic acid (KA) or to 3-hydroxykynurenine (3-HK). Under basal conditions, most of kynurenine is metabolized to KA, a N-methyl-D-aspartate (NMDA) and α7-nicotinic acetylcholine (α7nACh) receptor antagonist. However, inflammation, oxidative stress and pro-inflammatory cytokines will shift kynurenine metabolism to 3-HK. Further metabolism through this pathway results in quinolinic acid (QA), a metabolite that is a NMDA receptor agonist and an oxidative stressor. The KP produces several other biologically active metabolites, including the redox cofactors oxidized NAD+. NAD is a common mediator of various biological processes, including energy metabolism, mitochondrial function, calcium homeostasis and generation of oxidative stress. TRP metabolites like serotonin, quinolinic acid, and kynurenic acid have important implications in neurotransmission, growth and inflammation. IDO, indoleamine 2,3-dioxygenase; NAD, nicotinamide adenine dinucleotide.