Langmuir monolayers studies on the relationship between the content of cholesterol in model erythrocyte membranes and the influence of β-sitosterol.

Plant sterols, which are well known dietary taken cholesterol lowering agents, can incorporate into erythrocyte membranes. However, the influence of these compounds on membrane properties seems to be unclear. Since the composition of erythrocytes undergoes changes, e.g., in pathological cases, in this work the relationship between the proportion of lipids in model membranes (Langmuir monolayers) and the effect of β-sitosterol was analyzed. The investigations were carried out for cholesterol/phosphatidylcholine (POPC)/sphingomyelin films differing in the proportion of cholesterol and containing the plant sterol in various concentrations. Additionally, the experiments based on systematic replacement of cholesterol in model membranes were performed. Based on the analysis of mean area and the excess area per molecule, compressional modulus and the excess free energy of mixing values as well as BAM images it was evidenced that the effect of the plant sterol depends on the composition of model membrane. However, it was also found that, in the investigated range of monolayer composition, the increase of total sterol content in the mixed film caused by the presence of β-sitosterol practically does not influence the molecular packing and interactions in the investigated monolayers. On the other hand, the replacement of cholesterol by β-sitosterol causes a decrease of condensation and weakens the interactions between molecules in the mixed film. These effects are the strongest for the systems of the highest content of cholesterol. The most important finding is that the variations in the composition of membranes, widely occurring in natural systems, may influence the effect of phytosterols.