Antioxidant Effects of α-Tocopherol and Ascorbate in Liposomes Exposed to Nitrogen Dioxide

Abstract

This study investigated the rate constants of α-tocopherol and ascorbate oxidation by nitrogen dioxide in the presence of saturated and polyunsaturated phospholipids, and further evaluated whether the oxidation rate of these vitamins was sufficient to protect membranes from nitrogen dioxide-induced peroxidation. Liposomes of unsat-urated phospholipids containing vitamin E permit exposure to nitrogen dioxide and quantitation of vitamin E oxidation. Liposomes made of l-α-dipalmitoyl phosphatidyl-choline containing α-tocopherol in the bilayer at a ratio of 1.50 (α-tocopherol: palmitic acid) revealed a second-order rate constant for α-tocopherol oxidation of 8.9 × 10⁻-² M⁻¹ s⁻¹. Under the same conditions with liposomes made from l-α-dilinoleoyl phosphatidylcholine, the second-order rate constant was 4.9 × 10⁻-¹M⁻¹s⁻¹. α-Tocopherol quinone was the product of the nitrogen dioxide induced α-tocopherol oxidation. Ascorbate (50 μM) in the presence of l-α-dilinoleoyl phosphatidylcholine liposomes was oxidized at similar rates for all concentrations of nitrogen dioxide tested, and thus did not follow a second-order rate model. These liposome studies permit the conclusion that the presence of unsaturated bonds in membrane lipids has a very small effect on the oxidation rate of α-tocopherol. Take with earlier results from this laboratory (Shoaf et al., 1989), there is a 30-fold preferential oxidation of α-tocopherol over l-α-dilinoleoyl phosphatidylcholine.