Fig. 1

Schematic overview of the kynurenine pathway. The first and rate-limiting step is catalyzed by tryptophan 2,3-dioxygenase (TDO2) or by indoleamine 2,3-dioxygenase (IDO)1 and 2. N-formyl kynurenine is then converted by kynurenine formamidase to l-kynurenine before entering different possible branches, depending on cell-type or environmental context, to form various metabolites, which can exhibit immunological, antioxidant, or neurological activities. Pathways represented by two arrows involve several metabolites and enzymatic reactions. Nicotinamide adenine dinucleotide (NAD+)