Skip to main content
. 2022 Aug 21;11(16):2607. doi: 10.3390/cells11162607

Figure 2.

Figure 2

The tryptophan (Trp)–kynurenine (KYN) metabolic system and the subcellular location of the enzymes. More than 95% of L-Trp enters the KYN system producing multifarious biomolecules. The main enzymes of the KYN system are tryptophan 2,3-dioxygenase (TDO), indoleamine 2,3-dioxygenases (IDOs), kynurenine formamidase (KFA), kynurenine 3-monooxygenase (KMO), kynurenine aminotransferases (KATs), and kynureninase (KYNU). Most of the enzymes are located in the cytosol. However, KMO is located in the outer membrane of mitochondria; KAT II and KAT III are in the inner membrane of mitochondria; and KAT IV is in the matrix of mitochondria and in the plasma membrane. The main metabolites are L-KYN, kynurenic acid (KYNA), 3-hydroxy-L-kynurenine (3-HK), quinolinic acid (QUIN), and nicotinamide adenine dinucleotide (NAD+) which exhibit a wide range of biological properties and the metabolites freely cross the mitochondrial membranes. AA: anthranilic acid; ACMSD: amino-β-carboxymuconate-semialdehyde-decarboxylase; CA: cinnabarinic acid, 3-HAA: 3-Hydroxyanthranilic acid, 3-HA0: 3-hydroxyanthranilate oxidase; PIC: picolinic acid; XA: xanthurenic acid.