The study on the interaction between phytosterols and phospholipids in model membranes

Sterols are one of the major components of cellular membranes. Although in mammalian membranes cholesterol is a predominant sterol, in the human organism plant sterols (phytosterols) can also be found. Phytosterols, especially if present in concentrations higher than normal (phytosterolemia), may strongly affect membrane properties. In this work, we studied phytosterol–phospholipid interactions in mixed Langmuir monolayers serving as model membranes. Investigated were two phytosterols, β-sitosterol and stigmasterol and a variety of phospholipids, both phosphatidylethanolamines and phosphatidylcholines. The phospholipids had different polar heads, different length and saturation of their hydrocarbon chains. The interactions between molecules in mixed sterol/phospholipid films were characterized with the mean area per molecule (A12) and the excess free energy of mixing (ΔGExc). The effect of the sterols on the molecular organization of the phospholipid monolayers was analyzed based on the compression modulus values. It was found that the incorporation of the phytosterols into the phospholipid monolayers increased their condensation. The plant sterols revealed higher affinity towards phosphatidylcholines as compared to phosphatidylethanolamines. The phytosterols interacted more strongly with phospholipids possessing longer and saturated chains. Moreover, both the length and the saturation of the phosphatidylcholines influenced the stoichiometry of the most stable complexes. Our results, compared with those presented previously for cholesterol/phospholipid monolayers, allowed us to draw a conclusion that the structure of sterol (cholesterol, β-sitosterol, stigmasterol) does not affect the stoichiometry of the most stable complexes formed with particular phospholipids, but influences their stability. Namely, the strongest interactions were found for cholesterol/phospholipids mixtures, while the weakest for mixed systems containing stigmasterol.