title: The reaction of nitric oxide with ubiquinol: kinetic properties and biological significance.
abstract: The reaction of nitric oxide (*NO) with ubiquinol-0 and ubiquinol-2, short-chain analogs of coenzyme Q, was examined in anaerobic and aerobic conditions in terms of formation of intermediates and stable molecular products. The chemical reactivity of ubiquinol-0 and ubiquinol-2 towards *NO differed only quantitatively, the reactions of ubiquinol-2 being slightly faster than those of ubiquinol-0. The ubiquinol/*NO reaction entailed oxidation of ubiquinol to ubiquinone and reduction of *NO to NO-, the latter identified by its reaction with metmyoglobin to form nitroxylmyoglobin and indirectly by measurement of nitrous oxide (N2O) by gas chromatography. Both the rate of ubiquinone accumulation and *NO consumption were linearly dependent on ubiquinol and *NO concentrations. The stoichiometry of *NO consumed per either ubiquinone formed or ubiquinol oxidized was 1.86 A 0.34. The reaction of *NO with ubiquinols proceeded with intermediate formation of ubisemiquinones that were detected by direct EPR. The second order rate constants of the reactions of ubiquinol-0 and ubiquinol-2 with *NO were 0.49 and 1.6 x 10(4) M(-1)s(-1), respectively. Studies in aerobic conditions revealed that the reaction of *NO with ubiquinols was associated with O2 consumption. The formation of oxyradicals - identified by spin trapping EPR- during ubiquinol autoxidation was inhibited by *NO, thus indicating that the O2 consumption triggered by *NO could not be directly accounted for in terms of oxyradical formation or H2O2 accumulation. It is suggested that oxyradical formation is inhibited by the rapid removal of superoxide anion by *NO to yield peroxynitrite, which subsequently may be involved in the propagation of ubiquinol oxidation. The biological significance of the reaction of ubiquinols with *NO is discussed in terms of the cellular O2 gradients, the steady-state levels of ubiquinols and *NO, and the distribution of ubiquinone (largely in its reduced form) in biological membranes with emphasis on the inner mitochondrial membrane.
authors.affiliation: Laboratory of Oxygen Metabolism, University Hospital, University of Buenos Aires, Argentina.
authors.forename: J J
authors.initials: JJ
authors.lastname: Poderoso
authors.forename: M C
authors.initials: MC
authors.lastname: Carreras
authors.forename: F
authors.initials: F
authors.lastname: Schöpfer
authors.forename: C L
authors.initials: CL
authors.lastname: Lisdero
authors.forename: N A
authors.initials: NA
authors.lastname: Riobó
authors.forename: C
authors.initials: C
authors.lastname: Giulivi
authors.forename: A D
authors.initials: AD
authors.lastname: Boveris
authors.forename: A
authors.initials: A
authors.lastname: Boveris
authors.forename: E
authors.initials: E
authors.lastname: Cadenas
mesh_terms: D000332:Aerobiosis; D000693:Anaerobiosis; D000818:Animals; D016227:Benzoquinones / Q000737:chemistry; D004578:Electron Spin Resonance Spectroscopy; D005609:Free Radicals / Q000737:chemistry; D006736:Horses; D007700:Kinetics; D008786:Metmyoglobin / Q000737:chemistry / Q000378:metabolism; D008956:Models, Chemical; D009211:Myoglobin / Q000737:chemistry; D009569:Nitric Oxide / Q000737:chemistry*; D013056:Spectrophotometry, Ultraviolet; D014451:Ubiquinone / Q000031:analogs & derivatives* / Q000737:chemistry
chemical_list: D016227:Benzoquinones; D005609:Free Radicals; D009211:Myoglobin; D008786:Metmyoglobin; D014451:Ubiquinone; D009569:Nitric Oxide; C109863:ubiquinone-O; C003741:ubiquinol