STRUCTURAL AND DYNAMIC MEMBRANE-PROPERTIES OF ALPHA-TOCOPHEROL AND ALPHA-TOCOTRIENOL - IMPLICATION TO THE MOLECULAR MECHANISM OF THEIR ANTIOXIDANT POTENCY

d-Alpha-tocopherol and d-alpha-tocotrienol are two vitamin E constitue nts having the same aromatic chromanol ''head'' but different hydrocar bon ''tails''. Alpha-tocotrienol has been shown to be more potent in p rotecting against free radical-induced oxidative stress than alpha-toc opherol. Simple models of phospholipid membrane systems were used to i nvestigate the mechanism of the antioxidant potency of alpha-tocotrien ol in terms of its effects on membrane order and reorientational dynam ics. Chemiluminescence and fluorescence measurements demonstrated that alpha-tocotrienol exhibits significantly greater peroxyl radical scav enging potency than alpha-tocopherol in phosphatidylcholine liposomes, whereas both antioxidants have identical activity in hexane. This sug gests that the antioxidant potency of alpha-tocotrienol requires the m embrane environment. When alpha-tocopherol and alpha-tocotrienol were examined for their effects on phospholipid molecular order using conve ntional ESR spin labeling with 5- and 16-position-labeled doxylstearic acid, although both vitamin E constituents disordered the gel phase a nd stabilized the liquid-crystalline phase, no differences were observ ed between the effects of the two compounds. A slightly greater increa se (19% vs 15%) in ordering of the liquid-crystalline state due to alp ha-tocopherol, however, was discerned in noninvasive H-2 NMR experimen ts. The difference is most noticeable near C10-C13 positions of the ph ospholipid chain, possibly suggesting alpha-tocotrienol is located clo ser to the membrane surface. Saturation-transfer ESR, furthermore, rev ealed that on the time scale tau(c) = 10(-7)-10(-3) s the rates of rot ation about the long molecular axis and of the wobbling motion of the axis are modified to differing extents by the two forms of the vitamin E. Calculation of the ratio of correlation times tau(c)(L''L) and tau (c)(C'/C) associated with the two motions suggests that alpha-tocotrie nol imposes more motional anisotropy on the membrane. Thus, the differ ent effects of alpha-tocotrienol on the molecular properties of the me mbrane may explain its greater antioxidant potency.