Indoleamine-pyrrole 2,3-dioxygenase (IDO or INDOEC1.13.11.52) is a heme-containing enzyme physiologically expressed in a number of tissues and cells, such as the small intestine, lungs, female genital tract or placenta.[5] In humans is encoded by the IDO1gene.[6] IDO is involved in tryptophan metabolism. It is one of three enzymes that catalyze the first and rate-limiting step in the kynurenine pathway, the O2-dependent oxidation of L-tryptophan to N-formylkynurenine, the others being indolamine-2,3-dioxygenase 2 (IDO2)[7] and tryptophan 2,3-dioxygenase (TDO).[8] IDO is an important part of the immune system and plays a part in natural defense against various pathogens.[9][10] It is produced by the cells in response to inflammation and has an immunosuppressive function because of its ability to limit T-cell function and engage mechanisms of immune tolerance.[11] Emerging evidence suggests that IDO becomes activated during tumor development, helping malignant cells escape eradication by the immune system. Expression of IDO has been described in a number of types of cancer, such as acute myeloid leukemia, ovarian cancer or colorectal cancer. IDO is part of the malignant transformation process and plays a key role in suppressing the anti-tumor immune response in the body, so inhibiting it could increase the effect of chemotherapy as well as other immunotherapeutic protocols.[12][13][14] Furthermore, there is data implicating a role for IDO1 in the modulation of vascular tone in conditions of inflammation via a novel pathway involving singlet oxygen.[15]
^Uyttenhove C, Pilotte L, Théate I, Stroobant V, Colau D, Parmentier N, et al. (October 2003). "Evidence for a tumoral immune resistance mechanism based on tryptophan degradation by indoleamine 2,3-dioxygenase". Nature Medicine. 9 (10): 1269–74. doi:10.1038/nm934. PMID14502282. S2CID10618102.
