Fig. 1. Tryptophan metabolism along the kynurenine pathway. Tryptophan is an essential amino acid wholly derived from the diet. It is metabolized into active molecules, namely the neurotransmitter serotonin and the hormone melatonin. However, greater than 95% of tryptophan is metabolized along the kynurenine pathway to neuroactive metabolites like quinolinic acid and kynurenic acid. Here key metabolites and enzymes of the kynurenine pathway are illustrated. Refer to the text for a full description of the role of kynurenine pathway in neuropsychiatric disorders. IDO, indoleamine 2,3-dioxygenase; TDO, tryptophan dioxygenase; KAT, kynurenine aminotransferase; ACMSD, amino-β-carboxymuconate-semialdehyde-decarboxylase; QPRT, quinolinate phosphoribosyltransferase; NAD+, nicotinamide adenine dinucleotide; KMO, kynurenine 3-monooxygenase; 3-HAO, 3-hydroxyanthranilic acid oxidase.
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