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. 2020 May 13;12(5):1403. doi: 10.3390/nu12051403

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© 2020 by the author.

Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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Tryptophan degradation pathway. (1) Tryptophan 2,3-dioxygenase/Indoleamine 2,3-dioxygenase (2) formamidase, (3) kynurenine 3-monoxygenase, (4) kynureninase, (5) kynurenine aminotransferase, (6) 3-hydroxyanthranilic acid oxygenase, (7) 2-amino-3-carboxymuconate-6-semialdehyde decarboxylase, (8) nonenzymatic reaction, (9) quinolinate phosphoribosyltransferase, (10) nicotinic acid (nicotinamide) mononucleotide adenylyltransferase, (11) NAD⁺ synthetase, (12) NAD⁺ degrading enzyme, (13) nicotinamide phosphoribosyltransferase, (14) nicotinamide methyltransferase, (15) 2-Py-forming N¹-methylnicotinamide oxidase, and (16) 4-Py-forming N¹-methylnicotinamide oxidase. Abbreviations: NAD⁺: nicotinamide adenine dinucleotide; 2 Py: N¹-methyl-2-pyridone-5-carboxamide; and 4 Py: N¹-methyl-4-pyridone-3-carboxamide.