Role of membrane charge and semiquinone structure on oxygen consumption rates

Rates of oxygen consumption of air-saturated aqueous solutions containing either 2,3-dimethoxy-5-methylbenzoquinone (UQ-0), 1,4-naphthoquinone (NQ) or phenanthroquinone (PHQ) and ascorbate were measured in the presence or absence of large unilamellar vesicles (LUVs) composed of either dimyristoylphosphatidylcholine (DMPC) or equimolar mixtures of DMPC with dimyristoylphospahtidic acid (DMPA) or hexadecyltrimethylammonium bromide (CTAB), at physiological pH. Semiquinone hydrophobicities follow the same order as that of the corresponding quinones. Rates increase with positively charged LUV concentration and decrease with either neutral or negatively charged LUV concentration in samples containing the hydrophobic quinones NQ and PHQ. Rates remain essentially unchanged in the presence of the most hydrophilic quinone, UQ-0. An increase in the ionic strength of the solution partially inhibits the observed changes in the rate of oxygen consumption, R-ox, caused by the presence of positively charged membranes, thus implying that such changes are of electrostatic nature. Similar trends with membrane charge are observed for the rates of oxygen consumption in the presence of PHQ and dithiothreitol. The observed increase in the ascorbate oxidation rate in the presence of positively charged lipids occurs in systems where a decrease in semiquinone disproportionation is also detected, thus, implying that an increase in the quinone one-electron redox potential, caused by semiquinone-positively-charged-membrane interaction, could contribute to the observed effects.