A Kinetic Study of the Radical-Scavenging Action of Tocotrienols in the Membranes of Egg Yolk Phosphatidylcholine Vesicles

• Published in: Journal of Nutritional Science and Vitaminology Vol. 60; no. 6; pp. 443 - 446
• Main Authors: FUKUZAWA, Kenji, OUCHI, Aya, NAGAOKA, Shin-ichi, SHIBATA, Akira, MUKAI, Kazuo
• Format: Journal Article
• Language: English
• Published: Japan Center for Academic Publications Japan 2014
• Subjects: antioxidant; Egg Yolk - chemistry; Free Radical Scavengers - chemistry; Lipid Peroxidation; membrane; Molecular Structure; Phosphatidylcholines - chemistry; tocopherol; tocotrienol; Tocotrienols - chemistry; vitamin E
• ISSN: 0301-4800; 1881-7742
• DOI: 10.3177/jnsv.60.443

Vitamin E is localized in membranes and functions as an efficient inhibitor of lipid peroxidation in biological systems. In this study, we measured the second-order rate constants (ks) for the reaction of tocotrienol homologues (α-, β-, γ-, and δ-Toc-3Hs) with the aroxyl radical (ArO•) used as a model for lipid peroxyl radicals (LOO•) in the membranes of egg yolk phosphatidylcholine (EYPC) vesicles by stopped-flow spectrophotometry, and compared them to those of tocopherol homologues (α-, β-, γ-, and δ-TocHs). The relative rate constants of Toc-3H homologues to α-Toc-3H in membranes (α/β/γ/δ=100/83.7/63.2/20.2) were not much different to those of TocH homologues to α-TocH (α/β/γ/δ=100/88.4/83.8/17.3). Each ks value of Toc-3H homologues in membranes was 60-80% of that of the corresponding TocH homologues except for the almost identical ks values of δ-homologues, but there was no difference in EtOH solution between each ks value of the corresponding homologues of Toc-3H and TocH. These results indicate that the difference of the alkyl-side chain structure of vitamin E causes a change in the mobility of vitamin E molecules and/or the location of their antioxidant OH-groups in membranes, resulting in lowered radical-trapping rates of Toc-3Hs. By use of the ratio of the kinh value of α-TocH with LOO• (3.20×106 M−1s−1) to the ks value of α-TocH with ArO• (8.05×104 M−1s−1) in chlorobenzene (that is, 39.8), the kinh value for the reaction of α-TocH with LOO• in membrane was estimated to be 1.03×105 M−1s−1.