Fluorescence depolarization studies of phase transition and fluidity in lecithin liposomes containing .ALPHA.-tocopherol.

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Abstract
The effects of .alpha.-tocopherol and related compounds on the membrane fluidity of liposomes composed of dipalmitoyl-lecithin or egg lecithin were studied by measuring the fluorescence polarization of DPH(1,6-diphenyl-1,3,5-hexatriene), a hydrophobic fluorescent probe distributed in the hydrocarbon region of lipid bilayers. The transition temperature of dipalmitoyl-lecithin liposomes was lowered on incorporation of .alpha.-tocopherol into the membranes. A similar effect was observed on incorporation of phytol or phytanic acid into liposomes, but not on incorporation of 2,2,5,7,8-pentamethyl-1-hydroxychroman as an .alpha.-tocopherol model compound (TMC) which lacks an isoprenoid side chain. Above the phase-transition temperatures, incorporation of .alpha.-tocopherol or TMC dose-dependently increased the polarization values of DPH in dipalmitoyl-lecithin and egg lecithin liposomes, while incorporation of phytol or phytanic acid had little effect or slightly lowered the polarization. The effect of .alpha.-tocopherol in lowering the phase-transition temperature apparently depends on its hydrophobic side chain, and its effect in decreasing the membrane fluidity above the phase-transition temperature depends on its hydrophylic chroman ring portion. Cholesterol had an effect similar to .alpha.-tocopherol on membrane fluidity. .alpha.-Tocopherol and cholesterol had independent effects in increasing the polarization of DPH above the phase-transition temperature when the 2 compounds were incorporated together into liposomes.