Competitive carotenoid and cholesterol incorporation into liposomes: effects on membrane phase transition, fluidity, polarity and anisotropy

Pure 1,2-dipalmitoyl-sn-glycero-3-phosphorylcholine (DPPC) or mixed DPPC:1, 2-dipalmitoyl phosphatidyletanolamine (DPPE):1,2-dipalmitoyl diphosphatidyl serine (DPPS) (17:5:3) liposomes were incorporated with 5 mol% dietary caro tenoids (beta-carotene, lutein and zeaxanthin) or with cholesterol (16 and 48 mol%) in the absence or presence of 15 mol% carotenoids, respectively Th e carotenoid incorporation yields ranged from 0.42 in pure to 0.72 in mixed phospholipid liposomes. They decreased significantly, from 3 to 14%, in th e corresponding cholesterol-doped liposomes, respectively. Highest incorpor ation yields were achieved by zeaxanthin and lutein in phospholipid liposom es while in cholesterol-containing liposomes, lutein was highest incorporat ed. The effects on membrane structure and dynamics were determined by diffe rential scanning calorimetry, steady-state fluorescence and anisotropy meas urements. Polar carotenoids and cholesterol cause similar, dose-dependent e ffects: ordering and rigidification revealed by broadening of the transitio n peak, and increase of anisotropy. Membrane hydrophobicity is determined b y cholesterol content and carotenoid polarity. In cholesterol-doped liposom es, beta-carolene is less incorporated than in cholesterol-free liposomes. Our observations suggest effects of carotenoids, even at much lower effecti ve concentrations than cholesterol (8 to 80-fold), on membrane structure an d dynamics. Although they are minor constituents of animal membranes, carot enoids may act as modulators of membrane phase transition, fluidity, polari ty and permeability, and therefore, can influence the membrane physiology a nd pathology. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.