IDO, indole- 2,3-dioxygenase; TDO, tryptophan-2,3- dioxygenase; AFMID, Arylformamidase; KAT, kynurenine aminotransferase; KYNU, kynurenase; HD, Huntington’s disease; PD, Parkinson’s disease; AD, Alzheimer’s disease; SCZ, Schizophrenia. Neurotoxic metabolites 3-hydroxykynurenine and quinolinic acid are highlighted in red. Neuroprotective kynurenic acid is highlighted in green. Their relation with neuro disorders is labelled in the figure.