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

The reaction of nitric oxide ((NO)-N-.) 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)-N-. differed only quantitatively, the reactions of ubiquinol-2 being slightly faster than those of ubiquinol-0. The ubiquinol/(NO)-N-. reaction entailed oxidation of ubiquinol to ubiquinone and reduction of (NO)-N-. 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)-N-. consumption were linearly dependent on ubiquinol and (NO)-N-. concentrations. The stoichiometry of (NO)-N-. consumed per either ubiquinone formed or ubiquinol oxidized was 1.86 Angstrom 0.34. The reaction of (NO)-N-. 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)-N-. were 0.49 and 1.6 x 10(4) M(-1)s(-1) respectively. Studies in aerobic conditions revealed that the reaction of (NO)-N-. with ubiquinols was associated with O-2 consumption. The formation of oxyradicals - identified by spin trapping EPR- during ubiquinol autoxidation was inhibited by (NO)-N-., thus indicating that the O-2 consumption triggered by (NO)-N-. 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)-N-. to yield peroxynitrite, which subsequently may be involved in the propagation of ubiquinol oxidation. The biological significance of the reaction of ubiquinols with (NO)-N-. is discussed in terms of the cellular O-2 gradients, the steady-state levels of ubiquinols and (NO)-N-., and the distribution of ubiquinone (largely in its reduced form) in biological membranes with emphasis on the inner mitochondrial membrane. (C) 1999 Elsevier Science Inc.