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The essential amino acid tryptophan is a constituent of proteins and is also a substrate for two important biosynthetic pathways: the generation of the neurotransmitter serotonin (5-hydroxytryptamine) and the formation of nicotinamide adenine dinucleotides via different kynurenine derivatives.

L-kynurenine is formed in the mammalian brain (40%) and is taken up from the periphery (60%), indicating that it can be transported across the blood-brain-barrier.

It can be converted by the inducible enzyme indoleamine 2,3-dioxygenase (IDO) into two other important compounds: the neuroprotective kynurenic acid and the neurotoxic quinolinic acid. The kynurenine pathway is also closely linked to the immune system, as different cytokines do influence the activity of the key enzymes.

An unbalanced tryptophan-kynurenine pathway is associated with different kind of diseases such as mental and psychotic disorders (e.g. depressions; Alzheimer´s disease; Huntington´s chorea), stress, and inflammatory neurological diseases (amyotrophic lateral sclerosis; ALS).

LDN has developed an Enzyme-linked Immunosorbent Assay (ELISA) which is useful for measuring Kynurenine levels under different healthy and pathological conditions.