Ethanol-Induced Reorganization of the Liquid-Ordered Phase:  Enhancement of Cholesterol−Phospholipid Association

Abstract

This paper records, what is believed to be, the first evidence for the reorganization of the liquidordered phase by ethanol. Specifically, ethanol has been found to significantly enhance sterolphospholipid association in liquid-ordered bilayers derived from 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) plus cholesterol, and also 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) plus cholesterol. The evidence for such reorganization comes from a series of nearestneighbor recognition (NNR) experiments that have been carried out, where low concentrations of equilibrating lipid dimers (i.e., "reporter molecules") have been used to detect changes in the phase composition of host membranes made from varying mixtures of DPPC/cholesterol, and also DSPC/ cholesterol, in the presence and in the absence of ethanol. These findings have important biological implications, which are briefly discussed.

Drunken Membranes: Short-Chain Alcohols Alter Fusion of Liposomes to Planar Lipid Bilayers

“…It is known that ethanol increases membrane fluidity and that cholesterol decreases fluidity by increasing the order and packing of the lipids (65). Furthermore, ethanol's disordering effect is more potent in cholesterol-containing membranes (66,67). If the suppression of fusion by trans addition is due to increased fluidity of the planar membrane then this same suppression would be expected with cis addition also, because in both cases the planar bilayer is exposed to alcohol.…”

Cholesterol−Phospholipid Complexation in Fluid Bilayers as Evidenced by Nearest-Neighbor Recognition Measurements

“…If one then considers ternary mixtures of DMPC/DSPC/cholesterol, containing this same cholesterol concentration and variable ratios of DMPC/DSPC, one would expect that these membranes would be dominated by the l o phase at 60 °C. As discussed previously, exchangeable phospholipids such as 1 and 2 have proven to be excellent mimics of phosphocholines, based on their monolayer properties, melting behavior, and their NNR features in membranes containing variable quantities of cholesterol . Thus, the experimental system chosen for the present study is a good model for the liquid-ordered phase of phospholipid bilayers.…”

“…As discussed previously, exchangeable phospholipids such as 1 and 2 have proven to be excellent mimics of phosphocholines, based on their monolayer properties, melting behavior, and their NNR features in membranes containing variable quantities of cholesterol. 20 Thus, the experimental system chosen for the present study is a good model for the liquid-ordered phase of phospholipid bilayers. Given the excellent agreement between our experimental results and our model for complexation, the picture that emerges for this lipid system is as follows: (1) The liquid-ordered phase is homogeneous with all lipids and lipid complexes mixing ideally; (2) a specific complex of cholesterol with four long-chain phospholipids is in rapid equilibrium with free cholesterol and free phospholipid; (3) the complex is fully soluble in the liquidordered phase; (4) cholesterol has a high selectivity for the longchain phospholipid; (5) supernumerary cholesterol is freely soluble in this phase up to a high limit; and (6) the formation of the liquid-ordered phase creates conditions that are favorable for complex formation, not vice versa.…”

Cholesterol-Phospholipid Association in Fluid Bilayers: A Thermodynamic Analysis from Nearest-Neighbor Recognition Measurements

“…1) (12). Since the mixing of 1 with 2 has been shown to be similar to the mixing of DPPC with cholesterol, van 't Hoff plots of K in the singlephase regions of the diagram [that is, where ln K ¼ ÿDH°/R (1/T) 1 DS°/R are expected to yield DH o and DS o values that are relevant not only to the mixing between 1 and 2, but also to the mixing between DPPC and cholesterol (15).…”