The effects of the carotenoids beta-carotene and astaxanthin on the peroxidation of liposomes induced by ADP and Fe(2+) were examined. Both compounds inhibited production of lipid peroxides, astaxanthin being about 2-fold more effective than beta-carotene. The difference in the modes of destruction of the conjugated polyene chain between beta-carotene and astaxanthin suggested that the conjugated polyene moiety and terminal ring moieties of the more potent astaxanthin trapped radicals in the membrane and both at the membrane surface and in the membrane, respectively, whereas only the conjugated polyene chain of beta-carotene was responsible for radical trapping near the membrane surface and in the interior of the membrane. The efficient antioxidant activity of astaxanthin is suggested to be due to the unique structure of the terminal ring moiety.
The effects of various pentamethine trinuclear cyanine dyes, each of which has three alkyl chains, on ADP/Fe2+-induced lipid peroxidation in rat liver mitochondria were examined. Although the dye having the shortest -C2H5 chains (tri-S-C2(5)) did not show any appreciable effect, the dyes having -C4H9 (tri-S-C4(5)), -C7H15 (tri-S-C7(5)), and -C12H25 (tri-S-C12(5)) chains significantly inhibited lipid peroxidation, the most potent inhibitory effect being observed with tri-S-C7(5). The mode of antiperoxidation effect of the dyes was dependent on the length of the alkyl chains. The relatively hydrophilic dye tri-S-C4(5) was suggested to scavenge radicals more efficiently at or near the membrane surface rather than in the interior of the lipid membrane, whereas the more hydrophobic dye tri-S-C7(5) was suggested to scavenge radicals efficiently in the membrane rather than at or near the membrane surface. The hydrophilic/hydrophobic balance of the dye was found to regulate the site of action of the dyes.
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