Kynurenine is a breakdown product (catabolite) of the essential amino acid tryptophan used in the production of niacin - an organic compound and essential human nutrient found in food. Kynurenine is made by the enzyme indoleamine 2,3-dioxygenase (IDO) which is found in many tissues in response to activation of the immune systen, and also by the enzyme tryptophan dioxygenase, which is found in the liver. Kynurenine is further converted to kynurenic acid, or "Fibrostop 2" (FS2), carry out many functions in the body, including dilating blood vessels during inflammation and regulating the immune response.