Controlling membrane cholesterol content. A role for polyunsaturated (Docosahexaenoate) phospholipids

The molecular organization of cholesterol in 1,2-didocosahexaenoylphosphatidylcholine (22: 6-22:6PC) and 1-stearoyl-2-docosahexaenoylphosphatidylcholine (18:0-22:6PC) bilayers was investigated. Using low- and wide-angle X-ray diffraction (XRD), we determined that the solubility of the sterol at. 20 degreesC was 11 +/- 3 mol % in 22:6-22:6PC vs 55 +/- 3 mol % in 18:0-22:6PC bilayers. Solubility in the dipolyunsaturated membrane rose to 17 +/- 3 mol % at 40 degreesC, while in the saturated-polyunsaturated membrane there was no change within experimental uncertainty. We compared the molecular orientation of [3alpha-H-2(1)]cholesterol incorporated into 22:6-22:6PC bilayers to its solubility limit and into 18:0-22:6PC bilayers to a comparable concentration (10 mol %) in solid-state H-2 NMR experiments. The sterol possessed a tilt angle alpha(0) = 24degrees V in 22:6-22:6PC that was independent of temperature over a range from 20 to 40 degreesC. In contrast, the value was alpha(0) = 21degrees +/- 1degrees in 18:0-22:6 bilayers at 20 degreesC and increased to alpha(0) = 24degrees +/- 1degrees at 40 degreesC. We attribute the low solubility of cholesterol in 22:6-22:6PC membranes to steric incompatibility between the rigid steroid moiety and the highly disordered docosahexaenoic acid (DHA) chain, which has the potential to promote lateral heterogeneity within DHA-rich membranes. Considering 22:6-22:6PC to be the most unsaturated phospholipid found in vivo, this model membrane study provides a point of reference for elucidating the role of sterol-lipid interactions in controlling local compositional organization. Our results form the basis for a model that is consistent with cholesterol's ability to modulate the activity of certain neural transmembrane proteins.