Catalyzes the first and rate limiting step of the catabolism of the essential amino acid tryptophan along the kynurenine pathway. Involved in the peripheral immune tolerance, contributing to maintain homeostasis by preventing autoimmunity or immunopathology that would result from uncontrolled and overreacting immune responses. Tryptophan shortage inhibits T lymphocytes division and accumulation of tryptophan catabolites induces T-cell apoptosis and differentiation of regulatory T-cells. Acts as a suppressor of anti-tumor immunity. Limits the growth of intracellular pathogens by depriving tryptophan. Protects the fetus from maternal immune rejection. D-tryptophan + O2 = N-formyl-D-kynurenine L-tryptophan + O2 = N-formyl-L-kynurenine Binds 1 heme group per subunit. Activity is inhibited by and MTH-trp (methylthiohydantoin-DL-tryptophan), modestly inhibited by L-1MT (1-methyl-L-tryptophan) but not D-1MT (1-methyl-D-tryptophan). Monomer. By IFNG/IFN-gamma in most cells. Ido1 and Ido2 are 2 distinct enzymes which catalyze the same reaction. Ido2 affinity for tryptophan is much lower 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.