A protohemoprotein. Requires ascorbic acid and methylene blue for activity. This enzyme has broader substrate specificity than EC 184.108.40.206, tryptophan 2,3-dioxygenase . It is induced in response to pathological conditions and host-defense mechanisms and its distribution in mammals is not confined to the liver . While the enzyme is more active with D-tryptophan than L-tryptophan, its only known function to date is in the metabolism of L-tryptophan [2,6]. Superoxide radicals can replace O2 as oxygen donor [4,7].