Figure 2. The essential amino acid tryptophan is converted either into the neurotransmitter serotonin, or into the neuroactive metabolite kynurenine, which is further degraded to quinolinic acid. The rate-limiting enzyme in the kynurenine pathway, indoleamine2,3-dioxygenase (IDO), and kynurenine hydroxylase (KYNHydrox) are activated by the cytokine interferon gamma (IFN-γ).