Affiliations: Faculty of Pharmaceutical Sciences, Tokushima University, Shomachi, Tokushima, Japan | Department of Nutrition, School of Medicine, Tokushima University, Kuramoto‐cho, Tokushima, Japan | Department of Chemical Science and Technology, Tokushima University, Minami‐josanjima‐cho, Tokushima, Japan
Note: [] Corresponding author.
Abstract: The rate constants (k_{\rm s}) of ^{1}O_{2} scavenging for \alpha‐tocopherol (\alpha‐Toc) and \beta‐carotene (\beta‐Car) were measured in liposome membranes, and compared with those in EtOH solution. ^{1}O_{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_{\rm s} value for \beta‐Car in EtOH solution was 1.3 \times 10^{10} M^{-1 } s^{-1}, which was 36 times that for \alpha‐Toc (3.6 \times 10^{8} M^{-1} s^{-1}), but there was no difference between their k_{\rm s} values in liposomes (1.8 \times 10^{7} M^{-1} s^{-1} for \beta‐Car and 1.2 \times 10^{7} M^{-1} s^{-1 } for \alpha‐Toc). In the liposomes, the k_{\rm s} value for \alpha‐Toc was affected by the membrane site where ^{1}O_{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_{\rm s} value for \beta‐Car was not affected by the ^{1}O_{2}‐generating site. These differences were supposed to be caused by differences in the relative concentrations of ^{1}O_{2} and active sites of \alpha‐Toc and \beta‐Car in the membranes. \alpha‐Toc and \beta‐Car inhibited ^{1}O_{2}‐dependent peroxidation of egg yolk phosphatidylcholine (egg PC). The concentrations of \alpha‐Toc required for 50% inhibition of lipid peroxidation (IC_{50}) 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 ^{1}O_{2} scavenging rate constants.
Keywords: [TeX:] \beta‐carotene, vitamin E, [TeX:] \alpha‐tocopherol, singlet oxygen, liposomes, lipid peroxidation
Journal: Biofactors, vol. 7, no. 1-2, pp. 31-40, 1998
