The kynurenine pathway of tryptophan metabolism is found in bacteria, fungi and animals.  The first step in all cases is the reaction of tryptophan with O2 catalyzed by a hemoprotein, tryptophan 2,3-dioxygenase and indoleamine 2,3-dioxygenase.  The initial product, N-formylkynurenine, is deformylated to give the eponymous kynurenine.  In bacteria, kynurenine is cleaved to anthranilic acid and alanine, while in eucaryotes, kynurenine is hydroxylated by kynurenine monooxygenase before reaction with kynureninase.  Anthranilic acid is converted to catechol by bacteria, which is metabolized by pyrocatechase and the muconate pathway.  In eucaryotes, 3-hydroxyanthranilate is metabolized by an oxygenase to 2-amino-3-carboxy-cis,cis-muconic semialdehyde.  This intermediate can be decarboxylated, leading to catabolism, or it can cyclize spontaneously to quinolinate.  Quinolinate is converted to the essential coenzyme, NAD(P)+.  Quinolinate is also an agonist at the metabotropic NMDA glutamate receptor, and is neurotoxic at elevated concentrations.