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, s hort-chain analogs of coenzyme Q, was examined in anaerobic and aerobic con ditions in terms of formation of intermediates and stable molecular product s. The chemical reactivity of ubiquinol-0 and ubiquinol-2 towards (NO)-N-. differed only quantitatively, the reactions of ubiquinol-2 being slightly f aster than those of ubiquinol-0. The ubiquinol/(NO)-N-. reaction entailed o xidation of ubiquinol to ubiquinone and reduction of (NO)-N-. to NO-, the l atter identified by its reaction with metmyoglobin to form nitroxylmyoglobi n and indi rectly by measurement of nitrous oxide (N2O) by gas chromatograp hy. Both the rate of ubiquinone accumulation and (NO)-N-. consumption were linearly dependent on ubiquinol and (NO)-N-. concentrations. The stoichiome try 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 direc t 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 oxyradica ls - identified by spin trapping EPR- during ubiquinol autoxidation was inh ibited by (NO)-N-., thus indicating that the O-2 consumption triggered by ( NO)-N-. could not be directly accounted for in terms of oxyradical formatio n or H2O2 accumulation. It is suggested that oxyradical formation is inhibi ted by the rapid removal of superoxide anion by (NO)-N-. to yield peroxynit rite, which subsequently may be involved in the propagation of ubiquinol ox idation. 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 (lar gely in its reduced form) in biological membranes with emphasis on the inne r mitochondrial membrane. (C) 1999 Elsevier Science Inc.