Figures & data
Figure 1. Tryptophan metabolism through the kynurenine pathway. Tryptophan is the least abundant of the essential amino acids brought into the body as dietary proteins or through dietary supplements. Tryptophan can be converted into the neurotransmitter serotonin or degraded through a variety of pathways. Degradation of tryptophan thru the kynurenine pathway is most common and begins with conversion of tryptophan to N-formyl kynurenine, the rate-limiting reaction of the catabolic pathway. This can be accomplished by the action of tryptophan 2,3-deoxygenase (TDO) or of indoleamine 2,3-deoxygenase (IDO). TDO is present in the liver while IDO is present in many tissues, including the placenta. IDO is induced by interferon gamma and inhibited by nitric oxide (NO). IDO can use superoxide (O2.-) as a substrate or as a co-factor. Catabolism of N-formylkynurenine results in the production of kynurenine, 3-HKyn, 3-HAA and several other metabolites. 3-HAA and 3-HKyn can function as peroxyl scavengers.
