title: Human indoleamine 2,3-dioxygenase is a catalyst of physiological heme peroxidase reactions: implications for the inhibition of dioxygenase activity by hydrogen peroxide.
abstract: The heme enzyme indoleamine 2,3-dioxygenase (IDO) is a key regulator of immune responses through catalyzing l-tryptophan (l-Trp) oxidation. Here, we show that hydrogen peroxide (H(2)O(2)) activates the peroxidase function of IDO to induce protein oxidation and inhibit dioxygenase activity. Exposure of IDO-expressing cells or recombinant human IDO (rIDO) to H(2)O(2) inhibited dioxygenase activity in a manner abrogated by l-Trp. Dioxygenase inhibition correlated with IDO-catalyzed H(2)O(2) consumption, compound I-mediated formation of protein-centered radicals, altered protein secondary structure, and opening of the distal heme pocket to promote nonproductive substrate binding; these changes were inhibited by l-Trp, the heme ligand cyanide, or free radical scavengers. Protection by l-Trp coincided with its oxidation into oxindolylalanine and kynurenine and the formation of a compound II-type ferryl-oxo heme. Physiological peroxidase substrates, ascorbate or tyrosine, enhanced rIDO-mediated H(2)O(2) consumption and attenuated H(2)O(2)-induced protein oxidation and dioxygenase inhibition. In the presence of H(2)O(2), rIDO catalytically consumed nitric oxide (NO) and utilized nitrite to promote 3-nitrotyrosine formation on IDO. The promotion of H(2)O(2) consumption by peroxidase substrates, NO consumption, and IDO nitration was inhibited by l-Trp. This study identifies IDO as a heme peroxidase that, in the absence of substrates, self-inactivates dioxygenase activity via compound I-initiated protein oxidation. l-Trp protects against dioxygenase inactivation by reacting with compound I and retarding compound II reduction to suppress peroxidase turnover. Peroxidase-mediated dioxygenase inactivation, NO consumption, or protein nitration may modulate the biological actions of IDO expressed in inflammatory tissues where the levels of H(2)O(2) and NO are elevated and l-Trp is low.
authors.affiliation: Centre for Vascular Research and School of Medical Sciences, University of New South Wales, Sydney, New South Wales 2052, Australia.
authors.forename: Mohammed
authors.initials: M
authors.lastname: Freewan
authors.forename: Martin D
authors.initials: MD
authors.lastname: Rees
authors.forename: Tito S Sempértegui
authors.initials: TS
authors.lastname: Plaza
authors.forename: Elias
authors.initials: E
authors.lastname: Glaros
authors.forename: Yean J
authors.initials: YJ
authors.lastname: Lim
authors.forename: Xiao Suo
authors.initials: XS
authors.lastname: Wang
authors.forename: Amanda W S
authors.initials: AW
authors.lastname: Yeung
authors.forename: Paul K
authors.initials: PK
authors.lastname: Witting
authors.forename: Andrew C
authors.initials: AC
authors.lastname: Terentis
authors.forename: Shane R
authors.initials: SR
authors.lastname: Thomas
mesh_terms: D055162:Biocatalysis; D002942:Circular Dichroism; D004926:Escherichia coli / Q000235:genetics; D006418:Heme / Q000737:chemistry* / Q000378:metabolism; D006801:Humans; D006861:Hydrogen Peroxide / Q000737:chemistry*; D050503:Indoleamine-Pyrrole 2,3,-Dioxygenase / Q000737:chemistry* / Q000378:metabolism; D007700:Kinetics; D009569:Nitric Oxide / Q000737:chemistry; D010084:Oxidation-Reduction; D010544:Peroxidases / Q000737:chemistry* / Q000378:metabolism; D011487:Protein Conformation; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D012996:Solutions; D013059:Spectrum Analysis, Raman
chemical_list: D050503:Indoleamine-Pyrrole 2,3,-Dioxygenase; D011994:Recombinant Proteins; D012996:Solutions; D009569:Nitric Oxide; D006418:Heme; D006861:Hydrogen Peroxide; D010544:Peroxidases