On the importance of the phosphocholine methyl groups for sphingomyelin/cholesterol interactions in membranes - a study with ceramide phosphoethanolamine

¹ Åbo Akademi University

^* To whom correspondence should be addressed. E-mail: jpslotte{at}abo.fi.

Submitted on December 16, 2004
Revised on January 3, 2005
Accepted on 6 January 2005

	Abstract

In this study we have examined how the headgroup size and properties affect the membrane properties of sphingomyelin and interactions with cholesterol. We prepared N-palmitoyl ceramide phosphoethanolamine (PCPE) and compared its membrane behavior with D-erythro-N-palmitoyl-sphingomyelin(PSM), both in monolayers and bilayers. The pure PCPE monolayer did not show a phase transition at 22^°C (in contrast to PSM), but displayed a much higher inverse isothermal compressibility coefficient as compared to the PSM monolayer, indicating stronger intermolecular interactions between PCPEs than between PSMs. At 37^°C the PCPE monolayer was more expanded (than at 22^°C) and displayed a rather poorly defined phase transition. When cholesterol was co-mixed into the monolayer, a condensing effect of cholesterol on the lateral packing of the lipids in the monolayer could be observed. The phase transition from an ordered to a disordered state in bilayer membranes was determined by diphenylhexatriene steady state anisotropy. Whereas the PSM bilayer became disordered at 41^°C, the PCPE bilayer main transition occurred around 64^°C. The diphenylhexatriene steady state anisotropy values were similar in both PCPE and PSM bilayers before and after the phase transition, suggesting that the order in the hydrophobic core in both bilayer types was rather similar. The emission from Laurdan was blue shifted in PCPE bilayers in the gel phase when compared to the emission spectra from PSM bilayers, and the blue shifted component in PCPE bilayers was retained also after the phase transition, suggesting that Laurdan molecules sensed a more hydrophobic environment at the PCPE interface compared to the PSM interface both below and above the bilayer melting temperature. Whereas PSM was able to form sterol-enriched domains in dominantly fluid bilayers (as determined from cholestatrienol dequenching experiments), PCPE failed to form such domains, suggesting that the size and/or properties of the headgroup was important for stabilizing sphingolipid/sterol interaction. In conclusion, our study has highlighted how the headgroup in sphingomyelin affect its membrane properties and interactions with cholesterol.

Key Words: bilayer membrane, cholestatrienol, cholesterol desorption, head group size, membrane domain, monolayer membrane

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