Antioxidative activity of ubiquinol-10 at physiologic concentrations in human low density lipoprotein

Ubiquinol-10 is a powerful lipid-soluble antioxidant found in cell membranes and lipoproteins in vivo. Its mechanism of action on lipid peroxidation has been determined in model and biological systems. Data concerning antioxidative activity of ubiquinol-10 in lipoproteins, however, are still controversial. The present work examines its role in the prevention of low density lipoprotein (LDL) oxidation, specifically its influence on a copper-mediated oxidative modification of human LDL in vitro. We found that ubiquinol-10 incorporated in LDL in subnormal concentrations (0.05-0.13 mol/mol LDL incorporated in comparison with 0.10-1.20 mol/mol LDL reported as normally in human LDL) slightly but not significantly decreased production of lipid peroxidation products (lipid peroxides, conjugated dienes, thiobarbituric acid-reactive substances) during the first hours of oxidation. The extent of apolipoprotein B modification (LDL fluorescence at 360/430 nm) was also decreased. Increasing the ubiquinol-10 concentration in LDL to 0.55-1.48 mol/mol LDL made it significantly more resistant to copper-mediated oxidation than native LDL. Adding the same amounts of either ubiquinone-10 or alpha-tocopherol to the LDL suspension had almost no effect on its oxidation. Ubiquinol-10 decreased alpha-tocopherol consumption during LDL oxidation and was consumed more rapidly than the latter. These results demonstrate that LDL ubiquinol-10 content is an important factor influencing LDL susceptibility to oxidation by copper and suggest that it represents the first line of defense against oxidative modification in human LDL.