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-glycerophosphatidylethanplamine (PDPE) at 35 degreesC. Techniques employed were pressure-area (pi-A) isotherms generated from Langmuir-Blodgett films, solid-state H-2 and P-31 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 H-2 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 H-1-decoupled P-31 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 H-2 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 monoun-saturated lipids do not promote formation of stable lipid rafts and that polyunsaturation may be important for raft stability.