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Biophys. J. BioFAST: First Published May 27, 2005. doi:10.1529/biophysj.105.061846
© 2005 by the Biophysical Society.

A more recent version of this article appeared on August 1, 2005.
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MEMBRANES

Interactions of cholesterol with lipid membranes and cyclodextrin characterized by calorimetry

Alekos D. Tsamaloukas 1, Halina Szadkowska 2, Peter J. Slotte 3 and Heiko H. Heerklotz 1*

1 Biocenter of the Univ. of Basel
2 Biocenter
3 Abo Akademi

* To whom correspondence should be addressed. E-mail: heiko.heerklotz{at}unibas.ch.

Submitted on March 3, 2005
Revised on April 7, 2005
Accepted on 19 May 2005


  Abstract
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-beta-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 (i) the transfer of cho from cyd into a hypothetical, ideally mixed membrane and (ii) from non-ideal 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.

Key Words: ITC, cholesterol, cyclodextrin, enthalpy, lipid membranes, lipid rafts




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Copyright © 2005 by the Biophysical Society.