The essential amino acid L-tryptophan (TRP) plays a critical role as a constituent of human proteins and serves as the original substrate for two biosynthetic pathways: the serotonin pathway, which has mood regulating capabilities and the kynurenine pathway, which is the major route for the catabolism of TRP in mammals [1]. At the first and rate-limiting step in the kynurenine pathway, indoleamine 2,3-dioxygenase (IDO) and tryptophan-2,3-dioxygenase (TDO) initiate the oxidation of the TRP [1-3]. While TDO is predominantly located in liver, IDO is widely expressed in extrahepatic tissues including brain and is stimulated by pro-inflammatory cytokines among which interferon-γ is one of the most potent inducers [4,5]. Since it is localized in both the peripheral organs and brain, and regulated by cytokines, IDO may be a link between the immune system and TRP metabolism in the brain [6].
Elevated Expression of Brain Indoleamine 2,3-Dioxygenase is Associated with Early Mouse Development
