Kynurenine pathway: the pathway in the body

This page shows the biochemical pathway behind the laboratory value Kynurenine pathway: which stations follow one another, which enzymes carry out each step and which cofactors they use. Every statement has a source. The page describes general textbook knowledge and says nothing about any individual person.

In brief

The kynurenine pathway is the main breakdown route of the amino acid tryptophan and includes kynurenine, kynurenic acid, 3-hydroxykynurenine and quinolinic acid. It opens the ring of tryptophan and ultimately yields NAD⁺.

The pathway step by step

1. Tryptophan → Kynurenine IDO, TDO · Haem (iron), Oxygen The enzymes IDO and TDO open the ring of tryptophan; kynurenine forms via a short-lived intermediate. The kynurenine-to-tryptophan ratio describes how active this step currently is. Source 1, 2
2. Kynurenine → 3-Hydroxykynurenine KMO · FAD, NADPH Kynurenine 3-monooxygenase (KMO) attaches an OH group to kynurenine. The enzyme sits in the outer membrane of the mitochondria and works with FAD and NADPH. Source 3, 1
3. 3-Hydroxykynurenine → 3-OH-anthranilic acid Kynureninase · Vitamin B6 (PLP) Kynureninase cleaves the side chain from 3-hydroxykynurenine, forming 3-hydroxyanthranilic acid. This enzyme also works with pyridoxal phosphate, the active form of vitamin B6. Source 1
4. 3-OH-anthranilic acid → Quinolinic acid 3-HAO · Iron An iron-containing enzyme opens the ring; the intermediate ACMS rearranges spontaneously into quinolinic acid. In the nervous system, quinolinic acid binds to NMDA receptors. Source 1
5. Quinolinic acid → NAD⁺ QPRT · PRPP The enzyme QPRT converts quinolinic acid further; NAD⁺ forms over two steps. This route from tryptophan provides NAD⁺ in addition to the route that starts from niacin (vitamin B3). Source 4, 1

Cofactors in this pathway

• Vitamin B6 — As pyridoxal phosphate, cofactor of kynureninase and the kynurenine aminotransferases Source 1, 5In the ORY catalogue as a laboratory value: Vitamin B6
• Iron — Haem iron in IDO and TDO; another iron enzyme opens the ring of 3-hydroxyanthranilic acid Source 2, 1In the ORY catalogue as a laboratory value: Eisen
• Vitamin B2 (FAD) — As FAD, cofactor of kynurenine 3-monooxygenase (KMO) Source 3
• NADPH — Supplies KMO with the electrons for adding the OH group Source 3
• PRPP — Supplies QPRT with the sugar-phosphate part on the way to NAD⁺ Source 4

What acts on this pathway

• Interferon-γ — The messenger interferon-γ stimulates many cells to form IDO; more tryptophan then flows along this route. In the liver, cortisol acts mainly on the related enzyme TDO. Source 2, 1

Sources

1. Badawy AA. Kynurenine Pathway of Tryptophan Metabolism: Regulatory and Functional Aspects. Int J Tryptophan Res 2017 · PubMed 28469468
2. King NJ, Thomas SR. Molecules in focus: indoleamine 2,3-dioxygenase. Int J Biochem Cell Biol 2007 · PubMed 17320464
3. Smith JR, Jamie JF, Guillemin GJ. Kynurenine-3-monooxygenase: a review of structure, mechanism, and inhibitors. Drug Discov Today 2016 · PubMed 26589832
4. Magni G, Amici A, Emanuelli M et al. Enzymology of NAD+ synthesis. Adv Enzymol Relat Areas Mol Biol 1999 · PubMed 10218108
5. Rossi F, Miggiano R, Ferraris DM et al. The Synthesis of Kynurenic Acid in Mammals: An Updated Kynurenine Aminotransferase Structural KATalogue. Front Mol Biosci 2019 · PubMed 30873412

Related pathways

• NAD⁺ — tryptophan
• Serotonin and melatonin — tryptophan
• Carnitine — Vitamin B6, Eisen
• Dopamine, noradrenaline, adrenaline — Vitamin B6, Eisen
• Spermidine — Vitamin B6, Eisen

As of 2026-09-16. Draft written by Claude to schema v2; sources checked in PubMed; expert review pending
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