Fig. 1
From: Targeting the IDO1 pathway in cancer: from bench to bedside
Overview of the IDO metabolic pathway. Approximately 95% of L-tryptophan (Trp) is catabolized into kynurenine (Kyn) through three rate-limiting enzymes: tryptophan 2,3-dioxygenase (TDO) in the liver and indoleamine 2, 3-dioxygenase 1/2 (IDO1/2) in peripheral tissues. Kyn is converted to 3-hydroxykynurenine (3-HK) by kynurenine 3-monooxygenase (KMO), to anthranilic acid (AA) by kynureninase (KYNase), or to kynurenic acid (KYNA) by kynurenine aminotransferase (KAT). Next, catalyzed by KYNase, 3-HK is converted to 3-hydroxyanthranilic acid (3-HAA), which is further converted to quinolinic acid (QA), picolinic acid, nicotinamide adenine dinucleotide (NAD+), and other molecules