Docosahexaenoic acid enhances segregation of lipids between : 2H-NMR study

Abstract

Solid-state (2)H-NMR of [(2)H(31)]-N-palmitoylsphingomyelin ([(2)H(31)]16:0SM, PSM*), supplemented by differential scanning calorimetry, was used for the first time, to our knowledge, to investigate the molecular organization of the sphingolipid in 1:1:1 mol mixtures with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (16:0-18:1PE, POPE) or 1-palmitoyl-2-docosahexaenoyl-sn-glycero-3-phosphoethanolamine (16:0-22:6PE, PDPE) and cholesterol. When compared with (2)H-NMR data for analogous mixtures of [(2)H(31)]16:0-18:1PE (POPE*) or [(2)H(31)]16:0-22:6PE (PDPE*) with egg SM and cholesterol, molecular interactions of oleic acid (OA) versus docosahexaenoic acid (DHA) are distinguished, and details of membrane architecture emerge. SM-rich, characterized by higher-order, and PE-rich, characterized by lower-order, domains <20 nm in size are formed in the absence and presence of cholesterol in both OA- and DHA-containing membranes. Although acyl chain order within both domains increases on the addition of sterol to the two systems, the resultant differential in order between SM- and PE-rich domains is almost a factor of 3 greater with DHA than with OA. Our interpretation is that the aversion that cholesterol has for DHA--but not for OA--excludes the sterol from DHA-containing, PE-rich (nonraft) domains and excludes DHA from SM-rich/cholesterol-rich (raft) domains. We attribute, in part, the diverse health benefits associated with dietary consumption of DHA to an alteration in membrane domains.

FIGURE 1

²H-NMR spectra for a 50 wt % aqueous dispersion in 50 mM Tris buffer (pH 7.5) of POPE/PSM*(1:1 mol) (left panel) and POPE/PSM*/cholesterol (1:1:1 mol) (right panel). Spectra were recorded at −23°C (a and e), 12°C (b and f), 27°C (c and g), and 52°C (d and h).

FIGURE 2

²H-NMR spectra for a 50 wt % aqueous dispersion in 50 mM Tris buffer (pH 7.5) of PDPE/PSM* (1:1 mol) (left panel) and PDPE/PSM*/cholesterol (1:1:1 mol) (right panel). Spectra were recorded at -23°C (a and e), 12°C (b and f), 27°C (c and g), and 52°C (d and h).

FIGURE 3

Variation of the first moment as a function of temperature for (a) POPE/PSM* (1:1 mol) in the absence (solid squares) and presence (solid circles) of cholesterol (1:1:1 mol), and (b) POPE/PSM* (1:1 mol) in the absence (solid squares) and presence (solid circles) of cholesterol (1:1:1 mol). M₁ is plotted logarithmically for clarity and the “X” designates the midpoint of the sharp drop in moment observed when the sterol is absent. DSC cooling scans for (c) POPE/PSM (1:1 mol) and (d) PDPE/PSM (1:1 mol). The scans are inverted so that transitions appear as positive peaks.

FIGURE 4

Fast Fourier transform depaked spectra for POPE/PSM* (1:1 mol) in the absence (a) and presence (b) of cholesterol (1:1:1 mol), and for PDPE/PSM* (1:1 mol) in the absence (c) and presence (d) of cholesterol (1:1:1 mol) at 43°C. The arrows specify assignment of the C2 position.

FIGURE 5

Order parameter profiles generated from depaked spectra for (a) POPE/PSM* (1:1 mol) in absence (solid circles) and presence (open circles) of cholesterol (1:1:1 mol), and for (b) PDPE/PSM* (1:1 mol) in absence (solid circles) and presence (open circles) of cholesterol (1:1:1 mol) at 43°C.

FIGURE 6

A graphic depiction of DHA versus OA-induced lateral segregation of lipid molecules in PE/SM/cholesterol (1:1:1 mol) membranes. PE-rich and SM-rich domains coexist in PDPE/SM (top left) and POPE/SM (bottom left) membranes in the absence of sterol. On addition of cholesterol to PDPE/SM, the sterol is preferentially taken up into SM-rich domains for which it has high affinity and further displaces DHA for which it has low affinity. The formation of DHA-containing PE-rich/cholesterol-poor nonraft and SM-rich/cholesterol-rich raft domains is the result (top right). On addition of cholesterol to POPE/SM, in contrast, the sterol incorporates into OA-containing PE-rich, albeit to a less extent, as well as SM-rich domains (bottom right). The tremendous aversion of cholesterol for DHA is not possessed by OA.