Cholesterol Decreases the Interfacial Elasticity and Detergent Solubility of Sphingomyelins

• Published in: Biochemistry (Easton) Vol. 40; no. 20; pp. 5954 - 5963
• Main Authors: Li, Xin-Min, Momsen, Maureen M, Smaby, Janice M, Brockman, Howard L, Brown, Rhoderick E
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 22-05-2001
• Subjects: 1,2-Dipalmitoylphosphatidylcholine - chemistry; Animals; Cattle; Chickens; Cholesterol - chemistry; Detergents - chemistry; Dimyristoylphosphatidylcholine - chemistry; Elasticity; Fatty Acids, Unsaturated - chemistry; Octoxynol - chemistry; Phosphatidylcholines - chemistry; Solubility; Sphingomyelins - chemistry
• ISSN: 0006-2960; 1520-4995
• DOI: 10.1021/bi002791n

The interfacial interactions of cholesterol with sphingomyelins (SMs) containing various homogeneous acyl chains have been investigated by Langmuir film balance approaches. Low in-plane elasticity among the packed lipids was identified as an important physical feature of the cholesterol−sphingomyelin liquid-ordered phase that correlates with detergent resistance, a characteristic property of sphingolipid−sterol rafts. Changes in the in-plane elastic packing, produced by cholesterol, were quantitatively assessed by the surface compressional moduli (C s -1) of the monolayer isotherms. Of special interest were C s -1 values determined at high surface pressures (>30 mN/m) that mimic the biomembrane situation. To identify structural features that uniquely affect the in-plane elasticity of the sphingomyelin−cholesterol lateral interaction, comparisons were made with phosphatidylcholine (PC)−cholesterol mixtures. Cholesterol markedly decreased the in-plane elasticity of either SM or PC regardless of whether they were fluid or gel phase without cholesterol. The magnitude of the reduction in in-plane elasticity induced by cholesterol was strongly influenced by acyl chain structure and by interfacial functional groups. Liquid-ordered phase formed at lower cholesterol mole fractions when SM's acyl chain was saturated rather than monounsaturated. At similar high cholesterol mole fractions, the in-plane elasticity within SM−cholesterol liquid-ordered phase was significantly lower than that of PC−cholesterol liquid-ordered phase, even when PCs were chain-matched to the SMs. Sphingoid-base functional groups (e.g., amide linkages), which facilitate or strengthen intermolecular hydrogen bonds, appear to be important for forming sphingomyelin−cholesterol, liquid-ordered phases with especially low in-plane elasticity. The combination of structural features that predominates in naturally occurring SMs permits very effective resistance to solubilization by Triton X-100.