LOW PERMEABILITY OF LIPOSOMAL MEMBRANES COMPOSED OF BIPOLAR TETRAETHER LIPIDS FROM THERMOACIDOPHILIC ARCHAEBACTERIUM SULFOLOBUS-ACIDOCALDARIUS

The physical origin of the extremely high thermal stability of tetraet her liposomes composed of the polar lipid fraction E (PLFE) from the t hermoacidophilic archaebacterium Sulfolobus acidocaldarius has been in vestigated. The leakage rate of trapped 5,6-carboxyfluorescein (5(6)CF ) and the proton permeability in PLFE liposomes have been measured usi ng fluorescence probe techniques in the temperature range of 25-85 deg rees C, The results are compared with those obtained from nonarchaebac terial liposomes. Egg yolk phosphatidylglycerol (eggPG) and PLFE lipos omes exhibit similar large negative zeta-potentials (-31 to -34 mV) an d low permeability coefficients for 5(6)CF, indicating that membrane s urface charge is responsible for the low leakage rate of 5(6)CF in PLF E liposomes. This assertion is confirmed by the observation of an incr eased leakage rate of 5(6)CF with deceasing membrane surface negative charge via varying the content of egg yolk phosphatidylcholine (eggPC) in eggPC/eggPG binary mixtures, Gel-state dipalmitoylphosphatidylchol ine bilayers and PLFE liposomes exhibit similar permeability coefficie nts for 5(6)CF, suggesting that lipid packing also plays an important role in the low leakage rate of 5(6)CF. PLFE liposomes, especially tho se similar to 60 nm in diameter, are remarkably thermally stable in re gard to proton permeability, which increases by less than 2 x 10(-10) cm/s from 25 to 82 degrees C,, The proton permeability comparison of v arious liposomes reveals that the tight and rigid lipid packing is the major contributor of the extremely low proton permeation in PLFE lipo somes; the inositol moiety and the branched methyl groups may also con tribute, but to a much lesser extent.