The interactions between cholesterol and phospholipids located in the inner leaflet of human erythrocytes membrane (DPPE and DPPS) in binary and ternary films--the effect of sodium and calcium ions.

The studies on the influence of cholesterol on phospholipids accumulated in inner leaflet of membrane are performed rather rarely, especially in the presence of electrolytes, which are present in membrane environment. Therefore, in this work the interactions between cholesterol and saturated phosphatidylethanolamine (PE) and phosphatidylserine (PS) were studied in binary (phospholipid/cholesterol) and ternary (PS/PE/cholesterol) monolayers in the presence and absence of sodium and calcium ions. The composition of ternary films was estimated to reflect the proportion of PSs to PEs in inner layer of human erythrocyte membrane. The influence of electrolytes on pure PS and PE films was also analyzed. It was found that both sodium and calcium ions affect the condensation of DPPS films, and influence the interactions in DPPS/cholesterol monolayers. On the other hand, no effect of these ions on DPPE films as well as on DPPE/cholesterol interactions in the mixed systems was observed. The results obtained for ternary mixtures prove that in the presence of Na(+) the interactions between the lipids are more favorable than in the absence of these ions. This is in contrast to the effect of Ca(2+). All the results were thoroughly analyzed in the context of the structure of polar heads of the investigated phospholipids.