A protohemoprotein. In mammals, the enzyme appears to be located only in the liver. This enzyme, together with EC 1.13.11.52, indoleamine 2,3-dioxygenase, catalyses the first and rate-limiting step in the kynurenine pathway, the major pathway of tryptophan metabolism [5]. The enzyme is specific for tryptophan as substrate, but is far more active with L-tryptophan than with D-tryptophan [2].
