3-Hydroxykynurenine - 13 C 3 , 15 N (3-Hydroxy-DL-kynurenine- 13 C 3 , 15 N) is the 13 C and 15 N labeled 3-Hydroxykynurenine. 3-Hydroxykynurenine , a metabolite of tryptophan, is a potential endogenous neurotoxin whose increased levels have been described in several neurodegenerative disorders. 3-Hydroxykynurenine induces neuronal apoptosis.