HAAO is a monomeric cytosolic protein which is a member of intramolecular dioxygenases family containing nonheme ferrous iron. The HAAO protein catalyzes the synthesis of quinolinic acid（QUIN) from 3-hydroxyanthranilic acid. QUIN is an excitotoxin whose toxicity is facilitated by its ability to activate glutamate N-methyl-D-aspartate receptors. Amplified cerebral levels of QUIN may contribute to the pathogenesis of neurologic and inflammatory disorders. HAAO is widely distributed in peripheral organs， such as liver and kidney， and is also present in low amounts in the central nervous system.
