title: Post-translational regulation of human indoleamine 2,3-dioxygenase activity by nitric oxide.
abstract: The heme protein indoleamine 2,3-dioxygenase (IDO) is induced by the proinflammatory cytokine interferon-gamma (IFNgamma) and plays an important role in the immune response by catalyzing the oxidative degradation of L-tryptophan (Trp) that contributes to immune suppression and tolerance. Here we examined the mechanism by which nitric oxide (NO) inhibits human IDO activity. Exposure of IFNgamma-stimulated human monocyte-derived macrophages (MDM) to NO donors had no material impact on IDO mRNA or protein expression, yet exposure of MDM or transfected COS-7 cells expressing active human IDO to NO donors resulted in reversible inhibition of IDO activity. NO also inhibited the activity of purified recombinant human IDO (rhIDO) in a reversible manner and this correlated with NO binding to the heme of rhIDO. Optical absorption and resonance Raman spectroscopy identified NO-inactivated rhIDO as a ferrous iron (Fe(II))-NO-Trp adduct. Stopped-flow kinetic studies revealed that NO reacted most rapidly with Fe(II) rhIDO in the presence of Trp. These findings demonstrate that NO inhibits rhIDO activity reversibly by binding to the active site heme to trap the enzyme as an inactive nitrosyl-Fe(II) enzyme adduct with Trp bound and O2 displaced. Reversible inhibition by NO may represent an important mechanism in controlling the immune regulatory actions of IDO.
authors.affiliation: Centre for Vascular Research, Faculty of Medicine, University of New South Wales, Sydney 2052, Australia. shane.thomas@unsw.edu.au
authors.forename: Shane R
authors.initials: SR
authors.lastname: Thomas
authors.forename: Andrew C
authors.initials: AC
authors.lastname: Terentis
authors.forename: Hong
authors.initials: H
authors.lastname: Cai
authors.forename: Osamu
authors.initials: O
authors.lastname: Takikawa
authors.forename: Aviva
authors.initials: A
authors.lastname: Levina
authors.forename: Peter A
authors.initials: PA
authors.lastname: Lay
authors.forename: Mohammed
authors.initials: M
authors.lastname: Freewan
authors.forename: Roland
authors.initials: R
authors.lastname: Stocker
mesh_terms: D001665:Binding Sites; D002478:Cells, Cultured; D004789:Enzyme Activation; D006801:Humans; D050503:Indoleamine-Pyrrole 2,3,-Dioxygenase / Q000037:antagonists & inhibitors* / Q000378:metabolism; D007700:Kinetics; D008264:Macrophages / Q000378:metabolism; D009569:Nitric Oxide / Q000494:pharmacology*; D020030:Nitric Oxide Donors / Q000378:metabolism; D010100:Oxygen / Q000378:metabolism; D013057:Spectrum Analysis; D014364:Tryptophan / Q000378:metabolism
chemical_list: D050503:Indoleamine-Pyrrole 2,3,-Dioxygenase; D020030:Nitric Oxide Donors; D009569:Nitric Oxide; D014364:Tryptophan; D010100:Oxygen