Cholesterol-Induced Interfacial Area Condensations of Galactosylceramides and Sphingomyelins with Identical Acyl Chains

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Abstract
The interfacial interactions occurring between cholesterol and either galactosylceramides (GalCers) or sphingomyelins (SMs) with identical acyl chains have been investigated using Langmuir film balance techniques. Included among the synthesized GalCers and SMs were species containing palmitoyl (16:0), stearoyl (18:0), oleoyl [18:1Δ9(c)], nervonoyl [24:1Δ15(c)], or linoleoyl [18:2Δ9,12(c)] acyl residues. The cholesterol-induced condensations in the average molecular areas of the monolayers were determined by classic mean molecular area vs composition plots as well as by expressing the changes in terms of sphingolipid cross-sectional area reduction over the surface pressure range from 1 to 40 mN/m (at 1 mN/m intervals). The results show that, at surface pressures approximating bilayer conditions (30 mN/m), acyl heterogeneity in naturally occurring SMs (bovine or egg SM) enhanced the area condensation induced by cholesterol compared to their predominant molecular species (e.g. 18:0 SM in bovine SM; 16:0 SM in egg SM). Nonetheless, cholesterol always had a greater condensing effect on SM compared to GalCer when these sphingolipids were acyl chain matched and in similar phase states (prior to mixing with cholesterol). Also, the cholesterol-induced area changes for a given sphingolipid type (e.g. SM or GalCer) were similar whether the acyl chains were saturated, cis-Δ9-monounsaturated, or cis-Δ9,12-diunsaturated if the sphingolipids were in similar phase states (prior to mixing with cholesterol) and compared at equivalent surface pressures. These results indicate that, under conditions where hydrocarbon structure is matched, the sphingolipid head group plays a dominant role in determining the extent to which cholesterol reduces sphingolipid cross-sectional area. Despite the larger cholesterol-induced area condensations observed in SMs compared to those in GalCers, the molecular-packing densities showed that equimolar GalCer−cholesterol films were generally packed as tight as or slightly tighter than those of the SM−cholesterol films. The results are discussed in terms of a molecular model for sphingolipid−cholesterol interactions. Our findings also not only raise questions as to whether cholesterol-induced condensation data provide a reliable measure of the affinity, i.e. interaction strength, between cholesterol and different lipids but also provide insight regarding the stability of sterol/sphingolipid-rich microdomains thought to exist in caveolae and other cell membrane regions.