Indoleamine 2,3-dioxygenase (IDO) is a monomeric cytosolic enzyme that is distributed in many human tissues including the lens, brain, lung, kidney, spleen, and in macrophages. 1 It is a heme-containing enzyme that uses superoxide or oxygen to cleave the 2,3-double bond of the pyrrole ring of l-tryptophan to yield N-formylkynurenine. This is the first and rate-determining step of the kynurenine pathway (KP), which is the major metabolic path for the breakdown ...