Table 1.

The enzymes of the tryptophan-kynurenine pathway and related diseases.

Enzymes	Substrates	Products	Diseases
TDO	Tryptophan	L-kynurenine	Human brain tumors [34] Other tumor types [35,36]
IDO	Tryptophan	L-kynurenine	Tumors [36,37] Neurological and neurodegenerative diseases [25] Depression [38] Systemic lupus erythematosus [33] Sjörgen’s syndrome [31,32]
KAT	L-kynurenine 3-hydroxy-L-kynurenine	Kynurenic acid Xanthurenic acid	Multiple sclerosis [39] Parkinson’s disease [40] Alzheimer’s disease [41] Huntington’s disease [42,43,44] Epilepsy [25] Amyotrophic lateral sclerosis [45] Schizophrenia [46,47,48]
KMO	L-kynurenine	3-hydroxy-L-kynurenine	Bipolar disorder [49] Autoimmunity related diseases [50]