Monounsaturated PE does not phase-separate from the lipid raft molecules sphingomyelin and cholesterol: role for polyunsaturation?
We investigated interactions of the lipid raft molecules sphingomyelin (SM) and cholesterol (CHOL) in monolayers and bilayers composed of 1-palmitoyl-2-oleoyl-sn-glycerophosphatidylethanolamine (POPE) or 1-palmitoyl-2-docosahexaenoyl-sn-glycerophosphatidylethanolamine (PDPE) at 35 degrees C. Techniques employed were pressure-area (pi-A) isotherms generated from Langmuir-Blodgett films, solid-state (2)H and (31)P NMR spectroscopies, and differential scanning calorimetry (DSC). Condensation calculated from pi-A isotherms and reduction in the enthalpy of the gel-liquid-crystalline transition in DSC scans showed CHOL has a strong affinity for POPE, comparable to that observed between SM-CHOL. Order parameters derived from (2)H NMR spectra of the perdeuterated sn-1 chain of POPE-d(31) increased by >50% upon addition of equimolar CHOL to POPE-d(31)/SM (1:1 mol) bilayers. Close proximity of CHOL to POPE even in the presence of SM is indicated. Chemical shift anisotropy (Deltasigma(csa)) measured from (1)H-decoupled (31)P NMR spectra also implied intimate lipid mixing in POPE/SM/CHOL (1:1:1 mol). In contrast, pi-A isotherms and corroborating DSC studies of PDPE/SM (1:1 mol) indicate phase separation between SM and PDPE, which was maintained in the presence of CHOL. The cholesterol-associated increase in order of the perdeuterated sn-1 chain of PDPE determined by (2)H NMR was 2-fold less for PDPE-d(31)/SM/CHOL (1:1:1 mol) than POPE-d(31)/SM/CHOL (1:1:1 mol). Our findings support the notion that acyl chain dependent lateral phase separation occurs in the presence of a docosahexaenoic acid (DHA)-containing phospholipid (PDPE), but not an oleic acid-containing phospholipid (POPE). We propose that monounsaturated lipids do not promote formation of stable lipid rafts and that polyunsaturation may be important for raft stability.
Department of Biology, Indiana University/Purdue University Indianapolis, 723 West Michigan Street, Indianapolis, IN 46202-5132, USA.
Saame Raza
SR
Shaikh
Michael R
MR
Brzustowicz
Noah
N
Gustafson
William
W
Stillwell
Stephen R
SR
Wassall
D000818:Animals; D002152:Calorimetry, Differential Scanning; D002784:Cholesterol / Q000737:chemistry* / Q000378:metabolism; D004531:Eggs; D005227:Fatty Acids / Q000737:chemistry; D005231:Fatty Acids, Unsaturated / Q000737:chemistry* / Q000378:metabolism; D009682:Magnetic Resonance Spectroscopy; D021962:Membrane Microdomains / Q000737:chemistry* / Q000378:metabolism; D010714:Phosphatidylethanolamines / Q000737:chemistry* / Q000378:metabolism; D013109:Sphingomyelins / Q000737:chemistry* / Q000378:metabolism
D005227:Fatty Acids; D005231:Fatty Acids, Unsaturated; D010714:Phosphatidylethanolamines; D013109:Sphingomyelins; C057561:1-palmitoyl-2-oleoylphosphatidylethanolamine; D002784:Cholesterol