Molecular Organization of Cholesterol in Unsaturated Phosphatidylethanolamines:  X-ray Diffraction and Solid State²H NMR Reveal Differences with Phosphatidylcholines

Abstract

The major mammalian plasma membrane lipids are phosphatidylcholines (PCs), phosphatidylethanolamines (PEs), and cholesterol. Whereas PC-cholesterol interactions are well studied, far less is known about those between PE and cholesterol. Here, we investigated the molecular organization of cholesterol in PEs that vary in their degree of acyl chain unsaturation. For heteroacid sn-1 saturated (palmitoyl), sn-2 unsaturated (various acyl chain) PEs, cholesterol solubility determined by X-ray diffraction was essentially identical with 1 (oleoyl, 51 +/- 3 mol %) and 2 (linoleoyl, 49 +/- 2 mol %) double bonds before decreasing progressively with 4 (arachidonyl, 41 +/- 3 mol %) and 6 (docosahexaenoyl, 31 +/- 3 mol %) double bonds. With 6 double bonds in each chain, cholesterol solubility was further reduced to 8.5 +/- 1 mol %. However, (2)H NMR experiments established that the orientation of cholesterol in the same heteroacid PE membranes was unaffected by the degree of acyl chain unsaturation. A tilt angle of 15 +/- 1 degrees was measured when equimolar [3alpha-(2)H(1)]cholesterol was added, regardless of the number of double bonds in the sn-2 chain. The finding that solubility of cholesterol in sn-1 saturated PEs depends on the amount of polyunsaturation in the sn-2 chain of PE differs from the equivalent PCs that universally incorporate approximately 50 mol % sterol. Unlike PCs, a differential in affinity for cholesterol and tendency to drive lateral segregation is inferred between polyunsaturated PEs. This distinction may have biological implications reflected by the health benefits of dietary polyunsaturated fatty acids that are often taken up into PE > PC.

Fish oil increases raft size and membrane order of B cells accompanied by differential effects on function

“…They showed a DHAcontaining PC underwent a greater increase in molecular order upon adding cholesterol compared with a DHAcontaining PE ( Table 2 ). These results were in agreement with previous studies to show cholesterol was more soluble in n-3 PUFAs containing PCs than PEs ( 27 ). It is also important to note that the DHA-containing PC underwent a substantial increase in molecular order when exposed to cholesterol in the mixture with SM, which was consistent with the increase in membrane molecular order observed ex vivo when comparing cross-linked rafts with the absence of cross-linking with FO ( Fig.…”

“…Overall, the data were highly consistent with predictions from detergent-free model membrane studies in which PCs containing DHA were found to have higher solubility for cholesterol, and by implication to have more favorable interactions with rafts, than PEs containing DHA ( 27 ). We confi rmed this by comparing the order of cholesterol on the perdeuterated sn-1 chain on a DHA-containing PC (16:0-22:6PC-d 31 ) vs. a DHA-containing PE (16:0-22:2PE-d 31 ) in mixtures with SM (1:1) and SM/cholesterol (1:1:1), mimicking rafts, using 2 H NMR spectroscopy ( Table 2 ).…”

The location and behavior of α‐tocopherol in membranes

“…Partition coefficients for cholesterol are also smaller in unilamellar vesicles composed of PC with DHA than less unsaturated chains [53]. We have provided unequivocal substantiation of poor affinity for the sterol by the greatly reduced solubility that was measured in PUFAcontaining membranes by x-ray diffraction and solid-state 2 H NMR [54]. In fact, we have shown that cholesterol sequesters to the bilayer center when forced to mix with PUFA chains in dipolyunsaturated lipids [55,56].…”

Structure and dynamics of polyunsaturated hydrocarbon chains in lipid bilayers—significance for GPCR function

“…Fatty acid unsaturation may play an important role in the formation of domains since it is known that cholesterol prefers interaction with saturated lipids (Nakagawa et al, 1979;Rujanavech and Silbert, 1986;Smaby et al, 1997). More recently Stephen Wassall's laboratory studied interaction of cholesterol with polyunsaturated lipid species (Shaikh et al, 2006;Shaikh et al, 2004). Here our results on lateral organization in mixed bilayers composed of 18:0-22:6n3-PC/PE/PS/cholesterol are summarized.…”