Abstract
Ubiquinone-10 plays a central role in energy production and its reduced form, ubiquinol-10 is also capable of acting as a potent radical scavenging antioxidant against membrane lipid peroxidation. Efficiency of this protection depends mostly on its localization in lipid bilayer. The intrinsic fluorescence of ubiquinol-10 and of the exogenous probe, Laurdan, has been used to determine the location of ubiquinol-10 in unilamellar liposomes of egg phosphatidylcholine (EggPC) and dimyristoyl phosphatidylcholine. Laurdan fluorescence moiety is positioned at the hydrophilic-hydrophobic interface of the phospholipid bilayer and its parameters reflect the membrane polarity and microheterogeneity, which we have used to explore the coexistence of microdomains with distinct physical properties. In liquid-crystalline bilayers ubiquinol has a short fluorescence lifetime (0.4 ns) and a high steady-state anisotropy. In a concentration-dependent manner, ubiquinol-10 influences the Laurdan excitation, emission and generalized polarization measurements. In EggPC liposomes ubiquinol-10 induces a decrease in membrane water mobility near the probe, while in dimyristoyl liposomes a decrease in the membrane water content was found. Moreover the presence of ubiquinol results in the formation of coexisting phospholipid domains of gel and liquid-crystalline phases. The results indicate that ubiquinol-10 molecules are mainly located at the polar-lipid interface, inducing changes in the physico-chemical properties of the bilayer microenvironment.
Highlights
Ubiquinone-10 (UQ) is an integral redox and proton-translocating component of the mitochondrial electron transport chain [1] and it is widely distributed in other subcellular membranes [2].It has been well established that ubiquinol-10 (UQH2), a reduced form of UQ (Scheme 1), protects either membrane phospholipids and serum low-density lipoproteins from lipid peroxidation or mitochondrial membrane proteins and DNA from free-radical-induced oxidative damage [3]
UQH2-derived semiquinones can generate superoxide radicals, initiating a variety of prooxidative reactions and as this depends on the membrane localizations of UQH2 [6], its net antioxidative capacity is strongly dependent on its membrane position
At all concentrations used for the experiments we did not notice any decrease in UQH2 fluorescence intensity, indicating no aggregation of the molecules
Summary
It has been well established that ubiquinol-10 (UQH2), a reduced form of UQ (Scheme 1), protects either membrane phospholipids and serum low-density lipoproteins from lipid peroxidation or mitochondrial membrane proteins and DNA from free-radical-induced oxidative damage [3]. The membrane presence of enzymes capable of regenerating ubiquinol through the reduction of ubiquinone, or any ubisemiquinone radicals, is critical for effective UQH2 antioxidant activity [4]. UQH2 can act both directly, by preventing the formation of lipid peroxyl radicals and indirectly by regenerating a-tocopherol [5]. UQH2-derived semiquinones can generate superoxide radicals, initiating a variety of prooxidative reactions and as this depends on the membrane localizations of UQH2 [6], its net antioxidative capacity is strongly dependent on its membrane position. The intrinsic fluorescence of reduced plastoquinols and a-tocopherol have been used to observe their incorporation in liposomes [10]
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