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Interactions of Cholesterol with Lipid Membranes and Cyclodextrin Characterized by Calorimetry

Alekos Tsamaloukas ^*, Halina Szadkowska ^*, Peter J. Slotte  and Heiko Heerklotz ^*

^* Biozentrum of the University of Basel, Division of Biophysical Chemistry, Basel, Switzerland; and Åbo Akademi University, Department of Biochemistry and Pharmacy, Turku, Finland

Correspondence: Address reprint requests to Heiko Heerklotz, E-mail: heiko.heerklotz{at}unibas.ch.

Interactions of cholesterol (cho) with different lipids are commonly believed to play a key role in the formation of functional domains in membranes. We introduce a novel approach to characterize cho-lipid interactions by isothermal titration calorimetry. Cho is solubilized in the aqueous phase by reversible complexation with methyl-ß-cyclodextrin (cyd). Uptake of cho into the membrane is measured upon a series of injections of lipid vesicles into a cyd/cho solution. As an independent assay, cho release from membranes is measured upon titrating lipid/cho mixed vesicles into a cyd solution. The most consistent fit to the data is obtained with a mole fraction (rather than mole ratio) partition coefficient and considering a cho/cyd stoichiometry of 1:2. The results are discussed in terms of contributions from 1), the transfer of cho from cyd into a hypothetical, ideally mixed membrane and 2), from nonideal interactions with POPC. The latter are exothermic but opposed by a strong loss in entropy, in agreement with cho-induced acyl chain ordering and membrane condensation. They are accompanied by a positive heat capacity change which cannot be interpreted in terms of the hydrophobic effect, suggesting that additive-induced chain ordering itself increases the heat capacity. The new assays have a great potential for a better understanding of sterol-lipid interactions and yield suggestions how to optimize cho extraction from membranes.

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