Synergistic Interactions between Vitamin A and Vitamin E against Lipid Peroxidation in Phosphatidylcholine Liposomes

Abstract

Interactions between α-tocopherol and all-transretinol in suppressing lipid peroxidation were studied in a unilamellar liposomal system of phosphatidylcholine from either egg or soybean, in which peroxidation was initiated by the water-soluble azo initiator 2,2′-azobis(2-amidino-propane)hydrochloride and peroxidation was measured as production of conjugated diene hydroperoxides. While all-transretinol alone was poorly effective, the combination of all-transretinol with α-tocopherol caused an inhibition period far beyond the sum of the inhibition periods observed with individual antioxidants, providing evidence of synergistic interactions. Furthermore, the inhibition rate calculated in the presence of both all-transretinol and α-tocopherol,Rinh(E+A), was lower thanRinh(E)observed with α-tocopherol alone, suggesting that the extension of the inhibition time cannot be ascribed only to the antioxidant activity of α-tocopherol. The extent of synergism was linear with a molar ratio all-transretinol/α-tocopherol ranging from 0.1 to 1.0, whereas a drop was observed at a ratio of 2.0. Synergistic antioxidant interactions between all-transretinol and α-tocopherol were also evident when peroxidation was evaluated as production of malondialdehyde. A time course study, in which peroxidation of liposomes and depletion of antioxidants were concomitantly monitored, while showing that most of α-tocopherol was consumed to bring about the inhibition period, indicated that autooxidative reactions substantially contributed to the rapid depletion of all-transretinol, when the antioxidants were allowed to act separately. On the other hand, when α-tocopherol and all-transretinol were combined, the consumption of both antioxidants was significantly delayed, indicating reciprocal protection. Regeneration mechanisms cannot be accounted for by our results. The observed synergism between all-transretinol and α-tocopherol does not appear as the result of specific structural interactions in the lipid bilayer. Combination of all-transretinol with butylated hydroxytoluene, which reduced markedly all-transretinol oxidation, resulted in a synergistic antioxidant activity greater than that observed with comparable amounts of α-tocopherol. In light of the known antioxidant mechanism of retinoids, the data suggest that by limiting autooxidation of all-transretinol, α-tocopherol strongly promotes its antioxidant effectiveness. The concerted radical scavenging action in turn results in a synergistic protection of the lipid system against peroxidative stress and, ultimately, slows down the α-tocopherol consumption.