Singlet oxygen scavenging by α‐tocopherol and β‐carotene: Kinetic studies in phospholipid membranes and ethanol solution

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Abstract
The rate constants (ks of 1O2 scavenging for α-tocopherol (α-Toc) and β-carotene (β-Car) were measured in liposome membranes, and compared with those in EtOH solution. 1O2 was site-specifically generated by photoirradiation using two photosensitizers, water-soluble Rose bengal (RB) and lipid-soluble 12-(1-pyrene)-dodecanoic acid (PDA). The ks value for β-Car in EtOH solution was 1.3 × 1010 M−1 s−1, which was 36 times that for α-Toc (3.6 × 108 M−1 s−1), but there was no difference between their ks values in liposomes (1.8 × 107 M−1 s−1 for β-Car and 1.2 × 107 M−1 s−1 for α-Toc). In the liposomes, the ks value for α-Toc was affected by the membrane site where 1O2 was generated, which depended on the localization of the photosensitizer, being high at the membrane surface in the RB-system and low in the inner region of the membrane in the PDA-system. In contrast, the ks value for β-Car was not affected by the 1O2-generating site. These differences were supposed to be caused by differences in the relative concentrations of 1O2 and active sites of α-Toc and β-Car in the membranes. α-Toc and β-Car inhibited 1O2-dependent peroxidation of egg yolk phosphatidylcholine (egg PC). The concentrations of α-Toc required for 50% inhibition of lipid peroxidation (IC50) were higher than those of β-Car, being more than 6 times higher in EtOH solution and less than 2 times higher in liposomes. The ratio of the antioxidant activity of β-Car to that of α-Toc was more in EtOH solution than in liposomes, and was well correlated with the ratio of their 1O2 scavenging rate constants.

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