An Upside Down View of Cholesterol’s Condensing Effect: Does Surface Occupancy Play a Role?

Abstract

The condensing action of cholesterol has been compared with that of a structural isomer having its hydroxyl group located at the C-25 position (i.e., 25-OH'); that is, an isomer favoring an "upside down" orientation in lipid membranes. Surface pressure-area isotherms of mixed monolayers made from 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC)/cholesterol and DMPC/25-OH' have established that 25-OH' has a weaker condensing effect than cholesterol. Nearest-neighbor recognition measurements in liposomes made from 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) have also shown that 25-OH' has a weaker condensing effect in the physiologicallyrelevant fluid bilayer state. These findings provide support for surface occupancy playing a role in the condensing action of cholesterol.The role that cholesterol plays in controlling the conformation and function of integral membrane proteins as well as lipids in cell membranes continues to be a subject of considerable interest. 1,2 Studies with model membranes have already shown that certain high-melting phospholipids can combine with cholesterol to form "condensed complexes" as well as a "liquid-ordered" phase. 3,4 Although the ability of cholesterol to condense fluid phospholipids was demonstrated more than 80 years ago, the mechanism of its condensing action remains to be established. 5 One model that has been proposed (i.e., the "umbrella model") considers that both acyl chains of a phospholipid and a neighboring cholesterol molecule are required to share limited space beneath the phospholipid's head group, and that the hydrophobic nature of cholesterol forces it together with these acyl chains. 6 Alternatively, with the "template model", the flexible acyl chains of the phospholipid are able to complement, perfectly, the planar nucleus of a neighboring cholesterol molecule, resulting in a high number of hydrophobic contacts and tight packing. 7 In both of these models, hydrophobic interactions have been proposed to play a major role.In the present study, we have tested whether surface occupancy plays a role in cholesterol's condensing action. We define "surface occupancy" as the taking up of space in the interphase region of phospholipid membranes; that is, the region where water makes direct contact with exposed CH 2 groups. Since phospholipid head groups occupy only about half of the surface area of liquid phospholipid bilayers, the remaining surface is occupied by partially hydrated CH 2 groups. In principle, by replacing the space occupied by these wet CH 2 groups, the sterol nucleus of cholesterol should release them into the hydrophobic interior of the membrane, resulting in a partial straightening of the acyl chains, stronger chain-chain interactions and tighter packing.AUTHOR EMAIL ADDRESS (slr0@lehigh.edu). SUPPORTING INFORMATION. Experimental procedures and tables of data. This material is available free of charge via the Internet at http://pubs.acs.org. NIH Public Access Author ManuscriptLangmuir. Author manuscript; available in PMC 2011 April 20....

Comparison of Cholesterol and 25-Hydroxycholesterol in Phase-Separated Langmuir Monolayers at the Air–Water Interface

“…While 25OH is likely to have a preferred vertical orientation at the air−water interface both orientations have been demonstrated possible in previous experiments. 20 Hence, the reorientation process is likely to be statistical in nature. Unfortunately, our experimental tools are not sensitive to the statistical distribution of 25OH vertical orientations at the present time.…”

“…Work by Janout and colleagues on a related system strongly supports our hypothesis. 20 In that study, the hydroxyl group has been removed from the 3-position of the steroid ring and been placed at the 25-position of the steroid tail, mono-25OH. The sterol remains monofunctional.…”

Surface Occupancy Plays a Major Role in Cholesterol’s Condensing Effect

“…The fact that 25-OH′ was found to exhibit a relatively weak condensing effect was interpreted as being due to weaker condensing action within the hydrocarbon interior of the membrane. A caveat for this interpretation, however, is that the double bond of the 25-OH′molecule has the potential to act as a second headgroup that could result in an unnatural "looped" conformation 20,21 The greater compressibility of 25-OH′ at the air/water interface relative to that of cholesterol lends credibility to such a possibility. 21 The primary aim of the work reported herein was to establish firmly that the surface occupancy of the steroid nucleus does indeed play a major role in cholesterol's condensing effect.…”

“…A caveat for this interpretation, however, is that the double bond of the 25-OH′molecule has the potential to act as a second headgroup that could result in an unnatural "looped" conformation 20,21 The greater compressibility of 25-OH′ at the air/water interface relative to that of cholesterol lends credibility to such a possibility. 21 The primary aim of the work reported herein was to establish firmly that the surface occupancy of the steroid nucleus does indeed play a major role in cholesterol's condensing effect. With this purpose in mind, we have compared the condensing power of cholesterol to that of a steroid analogue that is devoid of any functionality that can serve as a headgroup (i.e., 5α-cholestane (Chart 2)).…”

Elucidating the Effects of Cholesterol on the Molecular Packing of Double-chained Cationic Lipid Langmuir Monolayers by Infrared Reflection-absorption Spectroscopy

“…7 suggest that the incorporation of cholesterol in the monolayers at a LE phase apparently reduced gauche defects in the hydrocarbon chains of the molecules. The rigid nucleus of cholesterol has been suggested to play an important role in the condensing effect of cholesterol on a fluidic monolayer 48 . The rigid nucleus of cholesterol would maximize the hydrocarbon chain contact with neighboring molecules 49 .…”