Biochem/physiol Actions Key intermediate in the breakdown pathway of tryptophan. L-Kynurenine is a key intermediate in the breakdown of L-tryptophan and the formation of nicotinamide adenine dinucleotide (NAD+) via the kynurenine pathway. L-kynurenine is involved in a variety of neurological processes and diseases. L-kynurenine is a substrate for kynureninase/kynurenine hydrolase; kynurenine 3-monooxygenase and kynurenine-oxoglutarate transaminase.