Reduction of ubiquinone-1 by ascorbic acid is a catalytic and reversible process controlled by the concentration of molecular oxygen

SUMMARY

To address whether reduction by vitamin C may contribute to the in vivo maintenance of coenzyme Q in the reduced form, we studied the reduction of ubiquinone-1 by ascorbate at pH 7.4. Addition of ascorbate to ubiquinone-1 resulted in rapid O₂ consumption and an increase in the steady-state concentration of ascorbyl radical. The initial rate of O₂ consumption was proportional to the product of ubiquinone-1] and ascorbate] whereas ascorbyl radical] was proportional to the square root of this parameter; both dependencies were in quantitative agreement with each other. The extent of O₂ consumption greatly exceeded the amounts of ubiquinone-1 initially present. Formation of ubiquinol-1 from ubiquinone-1 by ascorbate was reversible, moderate under aerobic conditions, but substantial in the absence or near absence of oxygen. At high O₂ concentration, ascorbate promoted the oxidation of ubiquinol-1 to ubiquinone-1. Addition of sodium dodecyl sulphate dramatically decreased the rate of reaction between ubiquinone-1 and ascorbate, most likely as a result of phase separation of the reagents. A preliminary reaction scheme with putative rate constants for the relevant reactions is presented that quantitatively describes the kinetic behaviour of the process studied. The key reactions in the scheme are electron transfer from ascorbate to ubiquinone-1 with formation of the ascorbyl and ubisemiquinone radical. The reaction of the latter with O₂ is postulated to be responsible for O₂ consumption, with ubiquinone-1 acting as a catalyst. Together, the results demonstrate that the extent of reduction of ubiquinone-1 by ascorbate was controlled by the O₂ concentration and the physical availability of the reactants. As the O₂ concentration in human blood is relatively high and ubiquinone-10 is located exclusively within the lipid phase of lipoproteins where negatively charged ascorbate has little access, our results suggest that direct reduction by ascorbate is unlikely to be responsible for the high reduction percentage observed for plasma coenzyme Q.