A kinetic study of the radical-scavenging action of tocotrienols in the membranes of egg yolk phosphatidylcholine vesicles.

ABSTRACT

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-, ß-, 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-, ß-, γ-, and δ-TocHs). The relative rate constants of Toc-3H homologues to α-Toc-3H in membranes (α/ß/γ/δ=100/83.7/63.2/20.2) were not much different to those of TocH homologues to α-TocH (α/ß/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• (3.20×10(6) M(-1)s(-1)) to the ks value of α-TocH with ArO• (8.05×10(4) M(-1)s(-1)) in chlorobenzene (that is, 39.8), the kinh value for the reaction of α-TocH with LOO• in membrane was estimated to be 1.03×10(5) M(-1)s(-1).