title: Singlet oxygen scavenging by alpha-tocopherol and beta-carotene: kinetic studies in phospholipid membranes and ethanol solution.
abstract: The rate constants (ks) of 1O2 scavenging for alpha-tocopherol (alpha-Toc) and beta-carotene (beta-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 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 ks 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 ks value for alpha-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 beta-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 alpha-Toc and beta-Car in the membranes. alpha-Toc and beta-Car inhibited 1O2-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 1O2 scavenging rate constants.
authors.affiliation: Faculty of Pharmaceutical Sciences, Tokushima University, Japan.
authors.forename: K
authors.initials: K
authors.lastname: Fukuzawa
authors.forename: Y
authors.initials: Y
authors.lastname: Inokami
authors.forename: A
authors.initials: A
authors.lastname: Tokumura
authors.forename: J
authors.initials: J
authors.lastname: Terao
authors.forename: A
authors.initials: A
authors.lastname: Suzuki
mesh_terms: D004563:Electrochemistry; D000431:Ethanol / Q000378:metabolism*; D016166:Free Radical Scavengers; D007700:Kinetics; D007850:Lauric Acids; D008027:Light; D008081:Liposomes / Q000378:metabolism*; D010084:Oxidation-Reduction; D010100:Oxygen; D010713:Phosphatidylcholines / Q000378:metabolism; D010743:Phospholipids / Q000378:metabolism; D017319:Photosensitizing Agents; D012395:Rose Bengal; D026082:Singlet Oxygen; D012996:Solutions; D014810:Vitamin E / Q000378:metabolism*; D019207:beta Carotene / Q000378:metabolism*
chemical_list: D016166:Free Radical Scavengers; D007850:Lauric Acids; D008081:Liposomes; D010713:Phosphatidylcholines; D010743:Phospholipids; D017319:Photosensitizing Agents; D012996:Solutions; D019207:beta Carotene; D014810:Vitamin E; D026082:Singlet Oxygen; D012395:Rose Bengal; D000431:Ethanol; C040596:12-(1-pyrene)dodecanoic acid; D010100:Oxygen