Action of vitamin E as antioxidant in phospholipid liposomal membranes as studied by spin label technique

The action of alpha-tocopherol in the phosphatidylcholine liposomal membranes was studied by spin label technique in order to elucidate the role of vitamin E in the membranes. The fluidity of the liposomal membranes decreased with increasing concentrations of saturated phosphatidylcholine, cholesterol, and alpha-tocopherol. However, the physical effect of alpha-tocopherol was quite small at its physiological concentrations. The effects of structure of various kinds of chromanols on the membrane fluidity were studied. The fluidity of the liposomal membranes decreased with increasing length of the side chain at 2-position of 6-chromanols. This effect was smaller when the side chain at the 2-position did not possess branched methyl group. The effect of tocopherols on the membrane fluidity was larger in the inner part of the membranes. Ascorbic acid residing in the aqueous phase interacted with the nitroxide spin probe incorporated into the membranes and the rate was smaller as the nitroxide group was buried deeper in the phospholipid bilayer. The spin probes incorporated into liposomal membranes were consumed as the oxidation proceeded. alpha-Tocopherol suppressed both the oxidation of membranes and the decrease of spin probe, but when alpha-tocopherol was depleted, the spin probe decreased and the membranes became more rigid as the oxidation proceeded.