リポソーム膜におけるα-トコフェロールの脂質過酸化抑制ならびにアスコルビン酸による再生のダイナミクス

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We studied the location of α-tocopherol(α-Toc) in liposome membranes, and dynamics of radical trapping and tocopherol recycling by ascorbic acid (AsA). No specific interaction was suggested to occur between the OH-group of α-Toc and the polar interfacial region of the bilayer, because of the absence of stretching bands of P = O and C = O of phosphatidylcholine by incorporation of α-Toc into liposomes, and no difference of the spin-lattice relaxation times at the 5a-position of α-Toc between positively and negatively charged liposomes. Low exposure of the chromanol at the surface of liposome membranes was supposed by little interaction with acrylamide, a water-soluble fluorescene quencher with a very low capacity to penetrate through phospholipid bilayers. Since the oxidation rate of α-Toc by positively charged Fe^<3+> was 150 times slower in negatively charged liposomes than in negatively charged SDS micelles, less than 0.65 mol% of the OH-groups of α-Toc was probably exposed at the membrane surface of liposomes. The lack of difference in the oxidation rates of α-Toc in liposomes in gel state and liquid crystalline state suggested that the OH-group was not located deeply in the hydrophobic region of membranes. Among the n-(N-oxy-4,4'-dimethyloxazolidine-2-yl) stearic acids (n-NS) labeled at different depths of membranes, 5-NS most effectively quenched the intrinsic fluorescence of α-Toc, indicating that the OH-group of α-Toc is located in a position corresponding to an inner 5-methylene carbon under the surface, where the OH-group may most efficiently trap the lipid peroxy-radicals floating up from the inner membrane core. The lipid peroxy-radicals were confirmed to float up to the membrane surface, because AsA was rapidly oxidized by 2,2'-azobis-(dimethylvaleronitrile)(AMVN), a hydrophobic radical precursor, when negatively charged AsA was ionically trapped at the positively charged surface of the liposome membranes. AsA completely suppressed the consumption of α-Toc by AMVN-induced lipid peroxidation in negatively charged liposomes. However, AsA did not penetrate into negatively charged membranes, because the ESR spectra of 5-NS and 16-NS labeled in these liposomes were not affected by the addition of AsA. These findings indicate that α-Toc is oxidized just under the membrane surface by lipid radicals, and then floats up to the surface, where it is regenerated to α-Toc by AsA.
日本ビタミン学会の論文
1994-06-25

リポソーム膜におけるα-トコフェロールの脂質過酸化抑制ならびにアスコルビン酸による再生のダイナミクス

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