Permeability of large unilamellar digalactosyldiacylglycerol vesicles for protons and glucose – influence of α-tocopherol, β-carotene, zeaxanthin and cholesterol

Abstract

The permeability of large unilamellar vesicles formed from digalactosyldiacylglycerol for glucose and protons was measured. The vesicle composition was modified by addition of different terpenoids: α-tocopherol, cholesterol, zeaxanthin and β-carotene. The digalactosyldiacylglycerol species composition was dominated by the species 18:2/18:2 and 18:1/18:2. Using the self-quenching properties of the fluorescent probe 6-carboxyfluorescein trapped in the aqueous space of the vesicles, the permeability for glucose was determined with a glucose gradient of 800 mM. The calculated permeability coefficient for glucose was in the range of 4.85.10 –10 to 1.12.10 –9 cm.s –1. For proton permeability measurements, the pH-sensitive fluorescent dye pyranine was used. The proton permeability was measured with a pH gradient of 0.6 pH units from 7.0 to 7.6 with valinomycin present to dissipate any diffusion potential across the membrane. The permeability coefficients for protons were in the range of 3.5.10 –8 to 1.0.10 –7 cm.s –1. Digalactosyldiacylglycerol vesicles with 5 % α-tocopherol or 10 % cholesterol or 2 % zeaxanthin reduced the permeability for protons, the two latter significantly as compared to digalactosyldiacylglycerol vesicles. α-Tocopherol (5 %) decreased the permeability for glucose remarkably and so did cholesterol (10 %). β-Carotene (< than 1 %) and zeaxanthin (2 %) in galactolipid vesicles, however, increased the permeability. The significance of these results is discussed in relation to the physiological functions of galactolipids and terpenoids in chloroplast membranes.