The reaction of nitric oxide with ubiquinol: kinetic properties and biological significance

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 indi rectly 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 Å 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 × 104 M−1s−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.