Effects of natural and enantiomeric cholesterol on the thermotropic phase behavior and structure of egg sphingomyelin bilayer membranes.

Phospholipids, sphingolipids, and sterols are the major lipid components of the plasma membranes of eukaryotic cells. Because these three lipid classes occur naturally as enantiomerically pure compounds, enantiospecific lipid-lipid and lipid-sterol interactions could in principle occur in the lipid bilayers of eukaryotic plasma membranes. Although previous biophysical studies of phospholipid and phospholipid-sterol model membrane systems have consistently failed to observe such enantiomerically selective interactions, a recent monolayer study of the interactions of natural and enantiomeric cholesterol with egg sphingomyelin has apparently revealed the existence of enantiospecific sterol-sphingolipid interactions. To determine whether enantiospecific sterol-sphingolipid interactions also occur in more biologically relevant lipid-bilayer systems, differential scanning calorimetric, x-ray diffraction, and neutral buoyant-density measurements were utilized to study the effects of natural and enantiomeric cholesterol on the thermotropic phase behavior and structure of egg sphingomyelin bilayers. The calorimetry experiments show that the natural and enantiomeric cholesterol have essentially identical effects on the temperature, enthalpy, and cooperativity of the gel/liquid-crystalline phase transition of egg sphingomyelin bilayers within the limits of experimental error. As well, the x-ray diffraction and neutral buoyancy experiments indicate that bilayers formed from mixtures of natural or enantiomeric cholesterol and egg sphingomyelin have, within experimental uncertainty, the same structure and mass density. We thus conclude that significant enantioselective cholesterol-sphingolipid interactions do not occur in this lipid-bilayer model membrane system.