A Review of Chromatographic Methods for Bioactive Tryptophan Metabolites, Kynurenine, Kynurenic Acid, Quinolinic Acid, and Others, in Biological Fluids

Kynurenine (KYN) is synthesized from an essential amino acid, tryptophan, by tryptophan 2,3-dioxygenase or indoleamine 2,3-dioxygenase via N-formyl-KYN in vivo. Subsequently, KYN acts as a precursor of some neuroactive metabolites such as kynurenic acid, quinolinic acid, and an important enzyme co-factor, nicotine adenine dinucleotide. These metabolites of tryptophan are a part of the 'kynurenine pathway.' In addition, KYN functions as an endogenous ligand for the aryl hydrocarbon receptor, which acts as a transcription factor. The levels of tryptophan metabolites are important for the assessment of the stage of neurological disorders, and therefore have garnered significant interest for clinical diagnosis. In this review, the detection of kynurenine, kynurenic acid, quinolinic acid, and other tryptophan metabolites performed via chromatographic methods such as HPLC using UV absorbance, fluorescence, and chromatographic-mass spectrometric detection is summarized.