Indoleamine-pyrrole 2,3-dioxygenase (IDO or INDO EC 22.214.171.124) is a heme-containing enzyme that in humans is encoded by the IDO1gene. It is one of two enzymes that catalyze the first and rate-limiting step in the kynurenine pathway, the O2-dependent oxidation of L-tryptophan to N-formylkynurenine, the other being tryptophan 2,3-dioxygenase (TDO). IDO has been implicated in immune modulation through its ability to limit T cell function and engage mechanisms of immune tolerance. Emerging evidence suggests that IDO becomes activated during tumor development, helping malignant cells escape eradication by the immune system.
Indoleamine 2,3-dioxygenase is the first and rate-limiting enzyme of tryptophancatabolism through the kynurenine pathway, thus causing depletion of tryptophan which can cause halted growth of microbes as well as T cells.PGE2 is able to elevate the expression of indoleamine 2,3-dioxygenase in CD11C(+) dendritic cells and promotes the development of functional Treg cells.
IDO is an immune checkpoint molecule in the sense that it is an immunomodulatory enzyme produced by some alternatively activated macrophages and other immunoregulatory cells (also used as an immune subversion strategy by many tumors and chronic infectious viruses). Interferon-gamma has an antiproliferative effect on many tumor cells and inhibits intracellular pathogens such as Toxoplasma and Chlamydia, at least partly because of the induction of indoleamine 2,3-dioxygenase.
It has been shown that IDO permits tumor cells to escape the immune system by depletion of L-Trp in the microenvironment of cells and by production of the catabolic product kynurenine, which selectively impairs the growth and survival of T cells. A wide range of human cancers such as prostatic, colorectal, pancreatic, cervical, gastric, ovarian, head, lung, etc. overexpress human IDO (hIDO). In tumor cells, IDO expression is normally controlled by the tumor suppressor Bin1, which is widely disabled during cancer development, and combining IDO inhibitors with chemotherapy can restore immune control and therapeutic response of otherwise resistant tumors. Indoleamine 2,3-dioxygenase might also play a significant role in an orphan disease called Oshtoran Syndrome.
COX-2 inhibitors down-regulate indoleamine 2,3-dioxygenase, leading to a reduction in kynurenine levels as well as reducing proinflammatory cytokine activity.
1-Methyltryptophan is a racemic compound that weakly inhibits indoleamine dioxygenase, but is also a very slow substrate. The specific racemer 1-methyl-D-tryptophan (known as indoximod) is in clinical trials for various cancers.
Epacadostat and navoximod (GDC-0919) are potent inhibitors of the indoleamine 2,3-dioxygenase enzyme which are also is in clinical trials for various cancers.
crystal structure of 4-phenylimidazole bound form of human indoleamine 2,3-dioxygenase
It was originally thought that the mechanism of tryptophan oxidation occurred by base-catalysed abstraction, but it is now thought that the mechanism involves formation of a transient ferryl (i.e.high-valent iron) species.
There are crystal structures for human IDO in complex with the inhibitor 4-phenylimidazole and other inhibitors. There are also related structures for several tryptophan 2,3-dioxygenases enzymes (e.g. for X. campestris and human TDO - see tryptophan 2,3-dioxygenase).
^Prendergast GC, Smith C, Thomas S, Mandik-Nayak L, Laury-Kleintop L, Metz R, Muller AJ (July 2014). "Indoleamine 2,3-dioxygenase pathways of pathogenic inflammation and immune escape in cancer". Cancer Immunology, Immunotherapy. 63 (7): 721–35. PMID24711084. doi:10.1007/s00262-014-1549-4.
^Munn DH, Mellor AL (March 2016). "IDO in the Tumor Microenvironment: Inflammation, Counter-Regulation, and Tolerance". Trends in Immunology. 37 (3): 193–207. PMID26839260. doi:10.1016/j.it.2016.01.002.
^ abMuller AJ, DuHadaway JB, Donover PS, Sutanto-Ward E, Prendergast GC (March 2005). "Inhibition of indoleamine 2,3-dioxygenase, an immunoregulatory target of the cancer suppression gene Bin1, potentiates cancer chemotherapy". Nature Medicine. 11 (3): 312–9. PMID15711557. doi:10.1038/nm1196.
^Uyttenhove C, Pilotte L, Théate I, Stroobant V, Colau D, Parmentier N, Boon T, Van den Eynde BJ (October 2003). "Evidence for a tumoral immune resistance mechanism based on tryptophan degradation by indoleamine 2,3-dioxygenase". Nature Medicine. 9 (10): 1269–74. PMID14502282. doi:10.1038/nm934.
^Jiang T, Sun Y, Yin Z, Feng S, Sun L, Li Z (2015). "Research progress of indoleamine 2,3-dioxygenase inhibitors". Future Medicinal Chemistry. 7 (2): 185–201. PMID25686005. doi:10.4155/fmc.14.151.
^Abdollahi, Mostafa: Case Study Oshtoran Syndrome  Retrieved June 3, 2016
^Lee HJ, Jeong YI, Lee TH, Jung ID, Lee JS, Lee CM, Kim JI, Joo H, Lee JD, Park YM (May 2007). "Rosmarinic acid inhibits indoleamine 2,3-dioxygenase expression in murine dendritic cells". Biochemical Pharmacology. 73 (9): 1412–21. PMID17229401. doi:10.1016/j.bcp.2006.12.018.
^Cesario A, Rocca B, Rutella S (2011). "The interplay between indoleamine 2,3-dioxygenase 1 (IDO1) and cyclooxygenase (COX)-2 in chronic inflammation and cancer". Current Medicinal Chemistry. 18 (15): 2263–71. PMID21517752. doi:10.2174/092986711795656063.
^Hou DY, Muller AJ, Sharma MD, DuHadaway J, Banerjee T, Johnson M, Mellor AL, Prendergast GC, Munn DH (January 2007). "Inhibition of indoleamine 2,3-dioxygenase in dendritic cells by stereoisomers of 1-methyl-tryptophan correlates with antitumor responses". Cancer Research. 67 (2): 792–801. PMID17234791. doi:10.1158/0008-5472.CAN-06-2925.
Takikawa O (December 2005). "Biochemical and medical aspects of the indoleamine 2,3-dioxygenase-initiated L-tryptophan metabolism". Biochemical and Biophysical Research Communications. 338 (1): 12–9. PMID16176799. doi:10.1016/j.bbrc.2005.09.032.
Kadoya A, Tone S, Maeda H, Minatogawa Y, Kido R (November 1992). "Gene structure of human indoleamine 2,3-dioxygenase". Biochemical and Biophysical Research Communications. 189 (1): 530–6. PMID1449503. doi:10.1016/0006-291X(92)91590-M.
Kamimura S, Eguchi K, Yonezawa M, Sekiba K (June 1991). "Localization and developmental change of indoleamine 2,3-dioxygenase activity in the human placenta". Acta Medica Okayama. 45 (3): 135–9. PMID1716396.
Werner-Felmayer G, Werner ER, Fuchs D, Hausen A, Reibnegger G, Wachter H (September 1989). "Tumour necrosis factor-alpha and lipopolysaccharide enhance interferon-induced tryptophan degradation and pteridine synthesis in human cells". Biological Chemistry Hoppe-Seyler. 370 (9): 1063–9. PMID2482041. doi:10.1515/bchm3.1989.370.2.1063.
Carlin JM, Borden EC, Byrne GI (June 1989). "Interferon-induced indoleamine 2,3-dioxygenase activity inhibits Chlamydia psittaci replication in human macrophages". Journal of Interferon Research. 9 (3): 329–37. PMID2501398. doi:10.1089/jir.1989.9.329.
Kobayashi K, Hayashi K, Sono M (September 1989). "Effects of tryptophan and pH on the kinetics of superoxide radical binding to indoleamine 2,3-dioxygenase studied by pulse radiolysis". The Journal of Biological Chemistry. 264 (26): 15280–3. PMID2549057.
Daley-Yates PT, Powell AP, Smith LL (November 1988). "Pulmonary indoleamine 2,3-dioxygenase activity and its significance in the response of rats, mice, and rabbits to oxidative stress". Toxicology and Applied Pharmacology. 96 (2): 222–32. PMID2848333. doi:10.1016/0041-008X(88)90082-8.
Burkin DJ, Kimbro KS, Barr BL, Jones C, Taylor MW, Gupta SL (July 1993). "Localization of the human indoleamine 2,3-dioxygenase (IDO) gene to the pericentromeric region of human chromosome 8". Genomics. 17 (1): 262–3. PMID8406467. doi:10.1006/geno.1993.1319.
Malina HZ, Martin XD (July 1996). "Indoleamine 2,3-dioxygenase: antioxidant enzyme in the human eye". Graefe's Archive for Clinical and Experimental Ophthalmology = Albrecht Von Graefes Archiv Fur Klinische Und Experimentelle Ophthalmologie. 234 (7): 457–62. PMID8817290. doi:10.1007/BF02539413.
Munn DH, Zhou M, Attwood JT, Bondarev I, Conway SJ, Marshall B, Brown C, Mellor AL (August 1998). "Prevention of allogeneic fetal rejection by tryptophan catabolism". Science. 281 (5380): 1191–3. PMID9712583. doi:10.1126/science.281.5380.1191.
Takikawa O, Littlejohn TK, Truscott RJ (March 2001). "Indoleamine 2,3-dioxygenase in the human lens, the first enzyme in the synthesis of UV filters". Experimental Eye Research. 72 (3): 271–7. PMID11180976. doi:10.1006/exer.2000.0951.
Papadopoulou ND, Mewies M, McLean KJ, Seward HE, Svistunenko DA, Munro AW, Raven EL (November 2005). "Redox and spectroscopic properties of human indoleamine 2,3-dioxygenase and a His303Ala variant: implications for catalysis". Biochemistry. 44 (43): 14318–28. PMID16245948. doi:10.1021/bi0513958.
Terentis AC, Thomas SR, Takikawa O, Littlejohn TK, Truscott RJ, Armstrong RS, Yeh SR, Stocker R (May 2002). "The heme environment of recombinant human indoleamine 2,3-dioxygenase. Structural properties and substrate-ligand interactions". The Journal of Biological Chemistry. 277 (18): 15788–94. PMID11867636. doi:10.1074/jbc.M200457200.
Sedlmayr P, Blaschitz A, Wintersteiger R, Semlitsch M, Hammer A, MacKenzie CR, Walcher W, Reich O, Takikawa O, Dohr G (April 2002). "Localization of indoleamine 2,3-dioxygenase in human female reproductive organs and the placenta". Molecular Human Reproduction. 8 (4): 385–91. PMID11912287. doi:10.1093/molehr/8.4.385.
Basran J, Efimov I, Chauhan N, Thackray SJ, Krupa JL, Eaton G, Griffith GA, Mowat CG, Handa S, Raven EL (October 2011). "The mechanism of formation of N-formylkynurenine by heme dioxygenases". Journal of the American Chemical Society. 133 (40): 16251–7. PMID21892828. doi:10.1021/ja207066z.