Kynurenin-3-Monooxygenase: the pathway in the body

Kynurenin-3-Monooxygenase is part of the pathway “Kynurenine pathway”. This page shows the whole pathway; the station of Kynurenin-3-Monooxygenase is highlighted.

Where this laboratory value sits: Kynurenine — first intermediate. 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

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

Whole pathway: Kynurenine pathway

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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