FIG. 4. The kynurenine pathway of tryptophan metabolism. Tryptophan is the biochemical precursor for the production of serotonin. Activation of the enzyme indoleamine 2, 3 dioxygenase (IDO) by pro-inflammatory cytokines (PIC) and reactive oxidative species (ROS) metabolizes tryptophan into kynurenine (KYN) which can then be metabolized by kynurenine aminotransferase (KAT) into the neuroprotective kynurenic acid or by kynurenine mono-oxygenase (KMO) into the potentially neurotoxic 3-hydroxykynurenine and subsequently to quinolinic acid (QA).