L-Kynurenine-d4 is intended for use as an internal standard for the quantification of kynurenine by GC- or LC-MS. L-Kynurenine is an active metabolite of tryptophan . It is formed from tryptophan via tryptophan 2,3-dioxygenase (TDO) or indoleamine 2,3-dioxygenase (IDO). L-Kynurenine binds to the aryl hydrocarbon receptor (AhR; Kd = ~4 µM) and increases the activity of ethoxyresorufin-O-deethylase (EROD) in H-4-II-E rat hepatoma cells (EC50 = 12.3 µM). It decreases intracellular concentrations of NAD+ in, and increases lactate dehydrogenase (LDH) activity in the supernatant of, primary human astrocytes and neurons in a concentration-dependent manner. L-Kynurenine in combination with IFN-γ reduces lung injury induced by A. fumigatus infection in the p47phox-/- mouse model of chronic granulomatous disease.