title: Interactions of cholesterol with lipid membranes and cyclodextrin characterized by calorimetry.
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 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.
authors.affiliation: Biozentrum of the University of Basel, Division of Biophysical Chemistry, Switzerland.
authors.forename: Alekos
authors.initials: A
authors.lastname: Tsamaloukas
authors.forename: Halina
authors.initials: H
authors.lastname: Szadkowska
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Slotte
authors.forename: Heiko
authors.initials: H
authors.lastname: Heerklotz
mesh_terms: D002151:Calorimetry / Q000379:methods*; D002784:Cholesterol / Q000737:chemistry*; D003198:Computer Simulation; D003505:Cyclodextrins / Q000737:chemistry*; D008051:Lipid Bilayers / Q000737:chemistry*; D008560:Membrane Fluidity; D021962:Membrane Microdomains / Q000737:chemistry*; D008956:Models, Chemical; D044367:Phase Transition; D010713:Phosphatidylcholines / Q000737:chemistry*; D013816:Thermodynamics
chemical_list: D003505:Cyclodextrins; D008051:Lipid Bilayers; D010713:Phosphatidylcholines; D002784:Cholesterol; C028694:1-palmitoyl-2-oleoylphosphatidylcholine