NPPA and neurodegenerative disease: Generally, the expression level of KP enzyme and metabolites in different cells and tissues is strictly regulated, but the abnormality of Trp metabolism and the accumulation of metabolites to varying degrees are involved in a wide range of disease processes, including tumors, neurodegenerative diseases, and self-immunity and mental illness, etc. Moreover, dysfunction of KP enzymes can trigger or facilitate diseases of CNS (41).