SYNERGISTIC INTERACTIONS BETWEEN VITAMIN-A AND VITAMIN-E AGAINST LIPID-PEROXIDATION IN PHOSPHATIDYLCHOLINE LIPOSOMES

Interactions between alpha-tocopherol and all-trans retinol in suppres sing lipid peroxidation were studied in a unilamellar liposomal system of phosphatidylcholine from either egg or soybean, in which peroxidat ion was initiated by the water-soluble azo initiator 2,2'-azobis(2 -am idino-propane)hydrochloride and peroxidation was measured as productio n of conjugated diene hydroperoxides. While all-trans retinol alone wa s poorly effective, the combination of all-trans retinol with alpha-to copherol caused an inhibition period far beyond the sum of the inhibit ion periods observed with individual antioxidants, providing evidence of synergistic interactions. Furthermore, the inhibition rate calculat ed in the presence of both all-trans retinol and alpha-tocopherol, R(i nh(E+A)), was lower than R(inh(E)) observed with alpha-tocopherol alon e, suggesting that the extension of the inhibition time cannot be ascr ibed only to the antioxidant activity of alpha-tocopherol. The extent of synergism was linear with a molar ratio all-trans retinol/alpha-toc opherol ranging from 0.1 to 1.0, whereas a drop was observed at a rati o of 2.0. Synergistic antioxidant interactions between all-trans retin ol and alpha-tocopherol were also evident when peroxidation was evalua ted as production of malondialdehyde. A time course study, in which pe roxidation of liposomes and depletion of antioxidants were concomitant ly monitored, while showing that most of alpha-tocopherol was consumed to bring about the inhibition period, indicated that autooxidative re actions substantially contributed to the rapid depletion of all-trans retinol, when the antioxidants were allowed to act separately. On the other hand, when alpha-tocopherol and all-trans retinol were combined, the consumption of both antioxidants was significantly delayed, indic ating reciprocal protection. Regeneration mechanisms cannot be account ed for by our results. The observed synergism between all-trans retino l and alpha-tocopherol does not appear as the result of specific struc tural interactions in the lipid bilayer. Combination of all-trans reti nol with butylated hydroxytoluene, which reduced markedly all-trans re tinol oxidation, resulted in a synergistic antioxidant activity greate r than that observed with comparable amounts of alpha-tocopherol. In l ight of the known antioxidant mechanism of retinoids, the data suggest that by limiting autooxidation of all-trans retinol, alpha-tocopherol strongly promotes its antioxidant effectiveness. The concerted radica l scavenging action in turn results in a synergistic protection of the lipid system against peroxidative stress and, ultimately, slows down the alpha-tocopherol consumption. (C) 1996 Academic Press, Inc.