Influence of Plant Sterols on the Phase Properties of Phospholipid Bilayers

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Abstract
The effects of stigmasterol, sitosterol, campesterol, and cholesterol on the phase properties of dipalmitoylphosphatidylcholine bilayers have been compared by differential scanning calorimetry and x-ray diffraction. The sterols were equally effective at progressively reducing the cooperativity and the enthalpy of the dipalmitoylphosphatidylcholine phase transition as their concentrations in the bilayer were increased. Moreover, both differential scanning calorimetry and x-ray diffraction indicated that the dipalmitoylphosphatidylcholine transition was eliminated by each of the sterols when they were present at a concentration of 33 mole%. This indicates that the interaction between phospholipid and both plant and animal sterols is stoichiometric, each sterol associating with two phospholipid molecules. At concentrations above 33 mole% the sterols were no longer completely solvated by the phospholipid, and sterol-sterol interaction resulted. Cholesterol, even at concentrations as high as 50 mole%, did not disrupt the lamellar structure of the bilayer. When these high concentrations of plant sterols were intercalated into the phospholipid, crystallinity, which presumably derives from sterol-sterol interaction, was detectable in the bilayer by x-ray diffraction. This observation is consistent with previous reports to the effect that the C17 chains of the plant sterols render them less soluble in phospholipid than is cholesterol. It is clear that this solvation difference is of insufficient magnitude to affect the stoichiometry of dipalmitoylphosphatidylcholine-sterol interaction, but it could well account for the less effective modulation of lipid bilayer permeability exhibited by plant sterols in comparison with cholesterol.