Membrane damage by a toxin from the sea anemone Stoichactis helianthus. II. Effect of membrane lipid composition in a liposome system.

In the first paper of this series, it was shown that a toxin from the sea anemone Stoichactis helianthus increased the permeability of black lipid membranes due to transmembrane channel formation. In the present study, we have used liposomes to examine the reactivity of the toxin with different phospholipids. Membrane damage was assessed by measuring the release of 86Rb+ and 14C-labeled membrane lipid. For the different lipids, the rank order of marker release was: sphingomyelin greater than C18 : 2 phosphatidylcholine greater than C18 : 1 phosphatidylcholine greater than C18 : 0 phosphatidylcholine greater than C16 : 0 phosphatidylcholine = C14 : 0 phosphatidylcholine. In C14 : 0 and C16 : 0 phosphatidylcholine liposomes there was no 14C-labeled lipid release and only 13 to 16% 86 Rb+ release which corresponds to the 86Rb+ content in the outermost aqueous shell of multilamellar liposomes. This indicates that membrane damage was limited to the outermost bilayer. In liposomes prepared with the other lipids, the extent of release of both markers increased proportionately with the length and the degree of unsaturation of the lipids' acyl side chains. Spingomyelin liposomes were the most susceptible with 47% of the 14C-labeled lipid marker and 90% of the 86Rb+ marker being released. The large extent of 14C-labeled lipid release is attributed to a detergent-like activity of the toxin which presumably is due to the amphipathic nature of the protein. Thus, the toxin can inflict membranrtance of one mechanism or the other apparently varies depending on membrane structure and lipid composition.