Scavenging of Singlet Oxygen by α-Tocopherol in Liposomes

Abstract

Many studies have shown that vitamin E (α-tocopherol, α-Toc) can inactivate singlet oxygen (1O2). In most of these studies, however, organic solvents rather than membranes have been used although α-Toc is mainly located in membranes in biological systems. We studied the kinetics of 1O2 scavenging by α-Toc in liposomes, as model membranes, in comparison with the kinetics in ethyl alcohol (EtOH) solution. Liposomes were prepared from dimyristoylphosphatidyl-choline (DMPC) and dicetylphosphate (DCP). 1O2 was generated site specifically by photoirradiation using two photosensitizers, water-soluble methylene blue (MB) and lipid-soluble 12-(l-pyrene)dodecanoic acid (PDA). The generation site in the PDA system was the hydrophobic inner region of membranes where the pyrene residue of PDA is located. In the MB system, 1O2 was generated at the membrane surface where MB is located, because positively charged MB increases the zeta (ζ) potential of DCP-DMPC liposomes by interaction with negatively charged DCP at the surface. The rate constants of 1O2 scavenging by α-Toc were determined by the competitive reaction of α-Toc and 1,3-diphenyl-isobenzofuran (DPBF), a specific 1O2 trap. The rate constant values were lower in liposomes than in EtOH solution and were higher in the MB system than in the PDA system.