Effect of alpha-tocopherol on lipid peroxidation and acyl chain mobility of liver microsomes from vitamin...

Effects of alpha-tocopherol on lipid peroxidation and membrane fluidity were studied in liver microsomes from vitamin E (alpha-tocopherol)-deficient rats using NADPH as a substrate. Microsomes containing various contents of alpha-tocopherol were prepared by incubation with various concentrations of alpha-tocopherol in ethanol solution. NADPH-dependent lipid peroxidation decreased the content of polyunsaturated fatty acids, arachidonic acid and 4, 7, 10, 13, 16, 19-docosahexaenoic acid. The treatment with alpha-tocopherol before peroxidation reduced the production of lipid peroxides and the change in fatty acid composition even at the lowest content of alpha-tocopherol dealt with in this experiment, 0.2 molar fraction, while addition of alpha-tocopherol after peroxidation resulted in a slight inhibition of peroxide production and small alteration in fatty acid composition. By an ESR measurement using stearate spin probe, the alpha-tocopherol incorporated into microsomes did not alter the acyl chain mobility up to 0.2 molar fraction but reduced the mobility above 0.2 molar fraction. The acyl chain mobility was markedly decreased by lipid peroxidation. The decrease of membrane fluidity was repressed in microsomes treated with alpha-tocopherol before peroxidation, but was not repressed in microsomes treated after peroxidation. The experiment using artificial membranes of egg yolk phosphatidylcholine and rat liver phosphatidylcholine revealed that the effect of alpha-tocopherol on membrane fluidity depends on the fatty acid composition of phospholipid, especially the content of arachidonic acid. On the other hand, the mobility of the fatty acyl chain was not affected by spermine at concentrations which could inhibit lipid peroxidation. These results suggest that the inhibitory effect of alpha-tocopherol on lipid peroxidation is due to antioxidant activity rather than the indirect effect of membrane stabilization.