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

The rate constants (k(s)) of O-1(2) scavenging for alpha-tocopherol (alpha-Toc) and beta-carotene (beta-Car) were measured in liposome membranes, and compared with those in EtOH solution. O-1(2) was site-specifically generated by photoirradiation using two photosensitizers, water-soluble Rose bengal (RB) and lipid-soluble 12-(1-pyrene)-dodecanoic acid (PDA). The k(s) value for beta-Car in EtOH solution was 1.3 x 10(10) M-1 s(-1), which was 36 times that for alpha-Toc (3.6 x 10(8) M-1 s(-1)), but there was no difference between their k(s) values in liposomes (1.8 x 10(7) M-1 s(-1) for beta-Car and 1.2 x 10(7) M-1 s(-1) for alpha-Toc). In the liposomes, the k(s) value for alpha-Toc was affected by the membrane site where O-1(2) 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 k(s) value for beta-Car was not affected by the O-1(2)-generating site. These differences were supposed to be caused by differences in the relative concentrations of O-1(2) and active sites of alpha-Toc and beta-Car in the membranes. alpha-Toc and beta-Car inhibited O-1(2)-dependent peroxidation of egg yolk phosphatidylcholine (egg PC). The concentrations of alpha-Toc required for 50% inhibition of lipid peroxidation (IC50) were higher than those of beta-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 beta-Car to that of alpha-Toc was more in EtOH solution than in liposomes, and was well correlated with the ratio of their O-1(2) scavenging rate constants.