As shown previously, the action of activated complement (C) on a lipid bilayer produces two distinct effects, namely, formation of transmembrane channels and transfer of phospholipid from the bilayer to the aqueous phase. In the present work, we show that both phenomena are affected by the structure of the membrane lipids. Increasing acyl chain length of lecithin liposomes containing cholesterol was found to decrease release of 86Rb+ by antibody (Ab) and C or by purified C5b,6 + C7 + C8 + C9. These changes in 86Rb+ release suggest that channel formation is affected by membrane thickness. Similarly, release of 14C-PC by Ab + C diminished with increasing acyl chain length.

Release of 86Rb+ from egg lecithin liposomes by Ab + C was found to diminish with increasing cholesterol concentration. On the other hand, higher release of 14C-PC was observed with increasing cholesterol concentration. Cholesterol itself was not released by Ab + C.

The effect of membrane cholesterol was also studied with sheep erythrocytes by incubation of the cells with a cholesterol-enriched medium. In accord with the results obtained with liposomes, the susceptibility of erythrocytes to hemolysis by C5b-9 was found to diminish with increasing cholesterol enrichment of the membrane, which indicates that cholesterol affects channel formation.

The effect of unsaturation was tested by treatment of C18:0 or C18:1 lecithin liposomes containing 43% molar cholesterol with Ab + C. 86Rb+ release was decreased, whereas 14C-PC release was slightly increased when unsaturated chains were present. Membrane fluidity is not believed to be the sole controlling factor since the changes in this parameter by varying cholesterol or by unsaturation did not correlate with the effects on marker release.

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