The kynurenine pathway generates multiple tryptophan metabolites called collectively kynurenines and leads to formation of the enzyme cofactor nicotinamide adenine dinucleotide. The first step in this pathway is tryptophan degradation, initiated by the rate-limiting enzymes indoleamine 2,3-dioxygenase, or tryptophan 2,3-dioxygenase, depending on the tissue. The balanced kynurenine metabolism, w...