Antioxidant effects of αtocopherol and ascorbate in liposomes exposed to nitrogen dioxide
This study investigated the rate constants of αtocopherol 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-αdipalmitoyl phosphatidyl-choline containing αtocopherol in the bilayer at a ratio of 1.50 (αtocopherol: palmitic acid) revealed a second-order rate constant for αtocopherol oxidation of 8.9 × 10--2 M-1 s-1. Under the same conditions with liposomes made from l-αdilinoleoyl phosphatidylcholine, the second-order rate constant was 4.9 × 10--1M-1s-1. αTocopherol quinone was the product of the nitrogen dioxide induced αtocopherol oxidation. Ascorbate (50 μM) in the presence of l-αdilinoleoyl 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 αtocopherol. Take with earlier results from this laboratory (Shoaf et al., 1989), there is a 30-fold preferential oxidation of αtocopherol over l-αdilinoleoyl phosphatidylcholine. © 1989 Informa UK Ltd All rights reserved: reproduction in whole or part not permitted.
Shoaf, CR; Wolpert, RL; Menzel, DB
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