| Abstract | The total polar lipids (TPL) extracted from the archaeobacterium Metha-nosarcina mazei were predominantly in the form of sodium and potassium salts. Upon passage of this natural salt form of TPL (ns-TPL) through a silica gel G column (s-TPL), the molar ratio of the monovalent cations, sodium plus potassium, to that of the divalent cations, magnesium plus calcium, decreased from about 11:1 to 2:1. Liposomes (archaeosomes) made from ns-TPL were unable to efficiently retain low molecular weight aqueous markers such as 5(6)-carboxyfluorescein (CF). In phosphate buffered saline (PBS, pH 7.14), between 60-90% and 90-100% of entrapped CF leaked out during 21 h storage at room temperature (20-22°) and 3 h at 50°, respectively. However, archaeosomes made with the s-TPL, as well as from TPL converted to the salt forms of predominantly sodium, potassium, calcium or magnesium, were significantly less leaky. The archaeosomes made with s-TPL were the most stable, showing less than 7% leakage after 21 h at room temperature and only 14% leakage of CF after 3 h at 50°. The stability of s-TPL archaeosomes (21 h, 20-22°) was not affected when Tris-HCI buffer (pH 7.4) was used instead of PBS. However, the inclusion of 1 or 10 mM EGTA in the Tris-HCI buffer increased the amount of CF leaked, to about 25% and 100%, respectively. Except for some differences in phosphatidylserine and phosphati-dylglycerol, the lipid compositions of ns-TPL, s-TPL, and the magnesium form of TPL were similar, as determined by thin layer chromatography of labeled lipids. Archaeosomes prepared from s-TPL and ns-TPL had -“P NMR spectra that were similar to each other, but distinct from those of vesicles prepared from the ester lipid dimyristoylphosphatidylcholine. The types and relative proportions of cations associated with the lipids of M. mazei prior to their hydration and vesicle formation have a major influence, although other factors such as lipid composition may have an effect, on the permeability of the bilayer to low molecular weight compounds. |
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