Displacement of sterols from sterol/sphingomyelin domains in fluid bilayer membranes by competing molecules

The formation of sterol and palmitoyl sphingomyelin enriched ordered domains in a fluid bilayer was examined using domain selective fluorescent reporter molecules (cholestatrienol and trans-parinaric acid containing lipids) together with a quencher molecule in the fluid phase. The aim of the study was to explore how stable the ordered domains were and how different, biologically interesting, membrane intercalators could affect domain stability and sterol distribution between domains. We show that sterols easily can be displaced from ordered domains by a variety of saturated, single- and double-chain membrane intercalators with a small polar group as a common denominator. Of the two-chain intercalators examined, both palmitoyl ceramide and palmitoyl dihydroceramide were effective in displacing sterols from ordered domains. Of the single-chain intercalators, hexadecanol and hexadecyl amide displaced the sterol from sterol/sphingomyelin domains, whereas palmitic acid, sphingosine and sphinganine failed to do so. All molecules examined stabilized the sphingomyelin-rich domains, as reported by trans-parinaric-sphingomyelin and by scanning calorimetry. Parallels between the displacement of sterol from ordered domains in our model membrane system and the ability of the above mentioned molecules to alter the chemical activity and distribution of sterols in biological membranes are discussed.