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Domain Formation and Stability in Complex Lipid Bilayers as Reported by Cholestatrienol

Department of Biochemistry and Pharmacy, Åbo Akademi University, Tykistökatu 6 A, FIN 20520 Turku, Finland

Correspondence: Address reprint requests to Bodil Ramstedt, Tel.: 358-2-2154816; Fax: 358-2-2154010; E-mail: boramste{at}abo.fi.

In this study, we used cholestatrienol (CTL) as a fluorescent reporter molecule to study sterol-rich L_o domains in complex lipid bilayers. CTL is a fluorescent cholesterol analog that mimics the behavior of cholesterol well. The ability of 12SLPC to quench the fluorescence of cholestatrienol gives a measure of the amount of sterol included in L_o domains in mixed lipid membranes. The stability of sterol-rich domains formed in complex lipid mixtures containing saturated sphingomyelins, phosphatidylcholines, or galactosylceramide as potential domain-forming lipids were studied. The amount of sterol associated with sterol-rich domains seemed to always increase with increasing temperature. The quenching efficiency was highly dependent on the domain-forming lipid present in complex lipid mixtures. Sphingomyelins formed stable sterol-enriched domains and were able to shield CTL from quenching better than the other lipids included in this study. The saturated phosphatidylcholines also formed sterol-rich domains, but the quenching efficiency in membranes with these was higher than with sphingomyelins and the domains melted at lower temperatures. PGalCer was not able to form sterol-enriched domains. However, we found that PGalCer stabilized sterol-rich domains formed in PSM-containing bilayers. Using a fluorescent ceramide analog, we also demonstrated that N-palmitoyl-ceramide displaced the sterol from sphingolipid-rich domains in mixed bilayer membranes.

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