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 indi rectly by measurement of nitrous oxide (N 2O) 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 × 10 4 M −1s −1, respectively. Studies in aerobic conditions revealed that the reaction of NO with ubiquinols was associated with O 2 consumption. The formation of oxyradicals — identified by spin trapping EPR— during ubiquinol autoxidation was inhibited by NO, thus indicating that the O 2 consumption triggered by NO could not be directly accounted for in terms of oxyradical formation or H 2O 2 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 O 2 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.