Brain-derived neurotrophic factor and inflammation in depression: Pathogenic partners in crime?

Figure 3 Inflammation shifts kynurenine metabolism pathway towards oxidative stress-associated metabolites. The enzyme indolamine-2,3-dioxygenase (IDO) converts tryptophan to kynurenine. Kynurenine aminotransferase converts kynurenine to kynureninic acid in astrocytes while kynurenine monooxygenase (KMO) metabolizes kynurenine to 3-hyroxykynurenine (3-HK) in microglia. 3-HK is metabolized by kynureninase to 3-hydroxyanthranilic acid and 3-hydroxyanthranilic acid dioxygenase to quinolinic acid. Inflammation up-regulates the enzymes IDO and KMO, resulting in increased levels of KMO-dependent metabolites associated with oxidative stress and depression. IDO: Indolamine-2,3-dioxygenase; KAT: Kynurenine aminotransferase; KMO: Kynurenine monooxygenase; 3-HK: 3-hyroxykynurenine; KYNU: Kynureninase; HAAO: 3-hydroxyanthranilic acid dioxygenase; QA: Quinolinic acid.