Rate constants for quenching singlet oxygen and activities for inhibiting lipid peroxidation of carotenoids and α-tocopherol in liposomes

The ¹O₂ quenching rate constants (k _Q ) of α-tocopherol (α-Toc) and carotenoids such as β-carotene, astaxanthin, canthaxanthin, and lycopene in liposomes were determined in light of the localization of their active sites in membranes and the micropolarity of the membrane regions, and compared with those in ethanol solution. The activities of α-Toc and carotenoids in inhibiting ¹O₂-dependent lipid peroxidation (reciprocal of the concentration required for 50% inhibition of lipid peroxidation: IC₅₀]⁻¹) were also measured in liposomes and ethanol solution and compared with their k _Q values. The k _Q and IC₅₀]⁻¹ values were also compared in two photosensitizing systems containing Rose bengal (RB) and pyrenedodecanoic acid (PDA), respectively, which generate ¹O₂ at different sites in membranes. The k _Q values of α-Toc were 2.9×10⁸M⁻¹s⁻¹ in ethanol solution and 1.4×10⁷ M⁻¹s⁻¹ (RB system) or 2.5×10⁶ M⁻¹s⁻¹ (PDA system) in liposomes. The relative IC₅₀]⁻¹ value of α-Toc in liposomes was also five times higher in the RB system than in the PDA-system. In consideration of the local concentration of the OH-group of α-Toc in membranes, the k _Q value of α-Toc in liposomes was recalculated as 3.3×10⁶ M⁻¹s⁻¹ in both the RB and PDA systems. The k _Q values of all the carotenoids tested in two photosensitizing systems were almost the same. The k _Q value of α-Toc in liposomes was 88 times less than in ethanol solution, but those of carotenoids in liposomes were 600–1200 times less than those in ethanol solution. The IC₅₀]⁻¹ value of α-Toc in liposomes was 19 times less than that in ethanol solution, whereas those of carotenoids in liposomes were 60–170 times less those in ethanol solution. There were no great differences (less than twice) in the k _q and IC₅₀]⁻¹ values of any carotenoids. The k _Q values of all carotenoids were 40–80 times higher than that of α-Toc in ethanol solution but only six times higher that of α-Toc in liposomes. The IC₅₀]⁻¹ values of carotenoid were also higher than that of α-Toc in ethanol solution than in liposomes, and these correlated well with the k _Q values.