title: Mechanism of reaction of myeloperoxidase with nitrite.
abstract: Myeloperoxidase (MPO) is a major neutrophil protein and may be involved in the nitration of tyrosine residues observed in a wide range of inflammatory diseases that involve neutrophils and macrophage activation. In order to clarify if nitrite could be a physiological substrate of myeloperoxidase, we investigated the reactions of the ferric enzyme and its redox intermediates, compound I and compound II, with nitrite under pre-steady state conditions by using sequential mixing stopped-flow analysis in the pH range 4-8. At 15 degrees C the rate of formation of the low spin MPO-nitrite complex is (2.5 +/- 0.2) x 10(4) m(-1) s(-1) at pH 7 and (2.2 +/- 0.7) x 10(6) m(-1) s(-1) at pH 5. The dissociation constant of nitrite bound to the native enzyme is 2.3 +/- 0.1 mm at pH 7 and 31.3 +/- 0.5 micrometer at pH 5. Nitrite is oxidized by two one-electron steps in the MPO peroxidase cycle. The second-order rate constant of reduction of compound I to compound II at 15 degrees C is (2.0 +/- 0.2) x 10(6) m(-1) s(-1) at pH 7 and (1.1 +/- 0.2) x 10(7) m(-1) s(-1) at pH 5. The rate constant of reduction of compound II to the ferric native enzyme at 15 degrees C is (5.5 +/- 0.1) x 10(2) m(-1) s(-1) at pH 7 and (8.9 +/- 1.6) x 10(4) m(-1) s(-1) at pH 5. pH dependence studies suggest that both complex formation between the ferric enzyme and nitrite and nitrite oxidation by compounds I and II are controlled by a residue with a pK(a) of (4.3 +/- 0.3). Protonation of this group (which is most likely the distal histidine) is necessary for optimum nitrite binding and oxidation.
authors.affiliation: Institute of Chemistry, University of Agricultural Sciences, Muthgasse 18, A-1190 Vienna, Austria.
authors.forename: U
authors.initials: U
authors.lastname: Burner
authors.forename: P G
authors.initials: PG
authors.lastname: Furtmuller
authors.forename: A J
authors.initials: AJ
authors.lastname: Kettle
authors.forename: W H
authors.initials: WH
authors.lastname: Koppenol
authors.forename: C
authors.initials: C
authors.lastname: Obinger
mesh_terms: D006801:Humans; D006861:Hydrogen Peroxide / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D009504:Neutrophils / Q000201:enzymology*; D009569:Nitric Oxide / Q000378:metabolism; D009573:Nitrites / Q000737:chemistry*; D010084:Oxidation-Reduction; D009195:Peroxidase / Q000737:chemistry*; D013053:Spectrophotometry
chemical_list: D009573:Nitrites; D009569:Nitric Oxide; D006861:Hydrogen Peroxide; D009195:Peroxidase