Catalyzes the first and rate-limiting step in the kynurenine pathway of tryptophan catabolism (PubMed:17499941, PubMed:17499941). Involved in immune regulation (PubMed:25477879). L-tryptophan + O2 = N-formyl-L-kynurenine Binds 1 heme group per subunit. Activity is inhibited by D-1MT (1-methyl-D-tryptophan) and MTH-trp (methylthiohydantoin-DL-tryptophan) but not L-1MT (1-methyl-L-tryptophan). Do not accept D-tryptophan as substrate. Amino-acid degradation; L-tryptophan degradation via kynurenine pathway; L-kynurenine from L-tryptophan: step 1/2. Expressed mainly in antigen-presenting immune cells, liver, kidney, brain, and placenta (PubMed:25691885, PubMed:17671174). Highly expressed in kidney, followed by epididymis and liver (at protein level) (PubMed:17499941). Detected in the tails of the spermatozoa in the testis and in the kidney tubules (at protein level) (PubMed:17499941). Constitutively expressed in brain. Knockout mice have no apparent defects in embryonic development or hematopoietic differentiation and have wild-type profiles for kynurenine in blood serum and for immune cells in spleen, lymph nodes, peritoneum, thymus and bone marrow. Knockout mice exhibit defects in IDO-mediated T-cell regulation and inflammatory responses (PubMed:25691885). They exhibit defects in allergic or autoimmune responses (PubMed:24402311). Ido1 and Ido2 are 2 distinct enzymes which catalyze the same reaction. Ido2 Km for tryptophan is much higher than that of Ido1. Ido2 may play a role as a negative regulator of Ido1 by competing for heme-binding with Ido1. Low efficiency Ido2 enzymes have been conserved throughout vertebrate evolution, whereas higher efficiency Ido1 enzymes are dispensable in many lower vertebrate lineages. Ido1 may have arisen by gene duplication of a more ancient proto-IDO gene before the divergence of marsupial and eutherian (placental) mammals. Belongs to the indoleamine 2,3-dioxygenase family. Truncated N-terminus.