Antioxidant synergism between carotenoids in membranes. Astaxanthin as a radical transfer bridge

In phosphatidyl choline liposomes, carotenoids increased the lag phase for oxidation initiated by the water soluble azo-initiator AAPH (zeaxanthin > lycopene > astaxanthin > β-carotene) and more significantly for the oxidation initiated by the lipid soluble AMVN (zeaxanthin > astaxanthin > lycopene > β-carotene). For combination of astaxanthin with β-carotene or astaxanthin with lycopene, significant antioxidant synergism was observed for initiation of oxidation in the lipid phase. Astaxanthin anchored in the water/lipid interface is suggested to scavenge radicals in the interface and transfer an electron from a non-polar and more reducing carotenoid in the membrane interior. Spectroscopic detection of steady state concentration of carotenoid radical cations during electrochemical oxidation of carotenoid mixtures confirmed conversion of astaxanthin radical cations to radical cations of other carotenoids. Decrease in membrane fluidity as measured by fluorescence anisotropy was most significant for astaxanthin (and zeaxanthin), which may further hamper diffusion and bimolecular reactions of radicals, with an antioxidative effect.