Vitamin E, membrane order, and antioxidant behavior in lung microsomes and reconstituted lipid vesicles

Abstract

Vitamin E, a dietary antioxidant, is known to inhibit peroxidation of membrane lipids and to protect the lungs of vitamin E-deficient animals and to a lesser extent vitamin E-sufficient animals from oxidant injury. Since the protective interaction between vitamin E and biological membranes may be related to alterations in composition and physical state of membrane lipids, we evaluated the effect of vitamin E deficiency on lung microsomal lipids and membrane fluidity. Both intact microsomes and lipid vesicles prepared from the total lipid extracts of these microsomes were used. The percentage incorporation of vitamin E and cholesterol, membrane fluidity, and lipid peroxidation were measured in microsomes as well as their lipid vesicles. Fluidity was measured by monitoring changes in fluorescence anisotropy for 1,6-diphenyl-1,3,5-hexatriene (DPH). Lipid peroxidation was measured by thiobarbituric acid reaction. There were significant increases in the phospholipid ( p < 0.01), the total cholesterol ( p < 0.05), and the total saturated fatty acids ( p < 0.05) and decreases in total polyunsaturated fatty acid ( p < 0.01) content of vitamin E-deficient microsomes. There were no detectable peroxidative products in freshly isolated microsomes from either vitamin E-sufficient or -deficient lungs. However, lipids from vitamin E-deficient microsomal membranes were more susceptible to free radical initiated peroxidation than lipids from vitamin E-sufficient microsomes. Fluidity in vitamin E-deficient microsomes or in their lipid vesicles was significantly ( p < 0.05) decreased compared to the respective controls. In vitamin E-deficient microsomes or their lipid vesicles, the incorporation rate of vitamin E was two- to three-fold greater than in vesicles of vitamin E-sufficient microsomes or their lipid vesicles. However, the percentage incorporation of cholesterol was identical in both vitamin E-deficient and vitamin E-sufficient microsomes or in their respective lipid vesicles. As a result of vitamin E incorporation, fluidity was significantly decreased ( p < 0.05) in vitamin E-sufficient vesicles and was further decreased ( p < 0.001) in vitamin E-deficient vesicles. Incorporation of cholesterol also decreased fluidity in both vitamin E-deficient and vitamin E-sufficient vesicles but to the same extent ( p < 0.001). Lipid peroxide formation was two-fold greater in the vitamin E-deficient than in the vitamin E-sufficient vesicles. The increased incorporation of vitamin E (but not cholesterol) diminished ( p < 0.001) lipid peroxide formation in both vitamin E-deficient and vitamin E-sufficient vesicles. These results suggest that alterations in the physical state of membrane lipids in vitamin E deficiency favors incorporation of more vitamin E. Although both vitamin E and cholesterol decreased membrane fluidity, only vitamin E protects against membrane lipid peroxidation.