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Lipid composition of plasma membranes isolated from light-grown barley (Hordeum vulgare) leaves: identification of cerebroside as a major component.

The total lipid composition of highly purified plasma membranes from light-grown barley (Hordeum vulgare) leaves was investigated. The plasma membranes were separated from intracellular membranes by subfractionation of the microsomal fraction using aqueous polymer two-phase partitioning. A novel finding was that glucocerebroside was a major lipid of the plasma membrane (23 mol%). The most abundant lipid class in the plasma membrane was phospholipid (42 mol%), consisting mainly of phosphatidylcholine and phosphatidylethanolamine, together with free sterols at a level of 28 mol%. The only free sterols of the plasma membrane were campesterol (15%), stigmasterol (23%), and sitosterol (62%). The plasma membrane contained a relatively high proportion of saturated fatty acids compared to the bulk of intracellular membranes, the major components of the plasma membrane being palmitic (16:0), linoleic (18:2), and linolenic (18:3) acids in approximately equal amounts.

Cell Membrane↗

The interactions of 1-palmitoyl-2-oleylphosphatidylcholine and bovine brain cerebroside.

Model membranes composed of 1-palmitoyl-2-oleylphosphatidylcholine (POPC) and bovine brain galactocerebroside (BOV-CER) have been studied by differential scanning calorimetry (DSC). POPC is a naturally occurring phospholipid, and BOV-CER is a major component of the myelin membrane. POPC and BOV-CER are immiscible in the gel state over the composition range 0-70 mol% BOV-CER. At most POPC/BOV-CER ratios, broad dual-peaked acyl chain transitions are observed, characteristic of the co-existence of a fluid POPC-rich liquid-crystalline phase and a solid BOV-CER-rich gel phase over a wide temperature range.

Animals↗

Structure and metastability of N-lignocerylgalactosylsphingosine (cerebroside) bilayers.

Differential scanning calorimetry (DSC) and X-ray diffraction have been used to study hydrated N-lignocerylgalactosylsphingosine (NLGS) bilayers. DSC of fully hydrated NLGS shows an endothermic transition at 69-70 degrees C, immediately followed by an exothermic transition at 72-73 degrees C; further heating shows a high-temperature (Tc = 82 degrees C), high-enthalpy (delta H = 15.3 kcal/mol NLGS) transition. Heating to 75 degrees C, cooling to 20 degrees C and subsequent reheating shows no transitions at 69-73 degrees C; only the high-temperature (82 degrees C), high-enthalpy (15.3 kcal/mol) transition. Two exothermic transitions are observed on cooling; for the upper transition its temperature (about 65 degrees C) and enthalpy (about 6 kcal/mol NLGS) are essentially independent of cooling rate, whereas the lower transition exhibits marked changes in both temperature (30----60 degrees C) and enthalpy (2.2----9.5 kcal/mol NLGS) as the cooling rate decreases from 40 to 0.625 Cdeg/min. On reheating, the enthalpy of the 69-70 degrees C transition is dependent on the previous cooling rate. The DSC data provide clear evidence of conversions between metastable and stable forms. X-ray diffraction data recorded at 26, 75 and 93 degrees C show clearly that NLGS bilayer phases are present at all temperatures. The X-ray diffraction pattern at 75 degrees C shows a bilayer periodicity d = 65.4 A, and a number of sharp reflections in the wide-angle region indicative of a crystalline chain packing mode. This stable bilayer form converts to a liquid-crystal bilayer phase; at 93 degrees C, the bilayer periodicity d = 59.1 A, and a diffuse reflection at 1/4.6 A-1 is observed. The diffraction pattern at 22 degrees C represents a combination of the stable and metastable low-temperature bilayer forms. NLGS exhibits a complex pattern of thermotropic changes related to conversions between metastable (gel), stable (crystalline) and liquid-crystalline bilayer phases. The structure and thermotropic properties of NLGS are compared with those of hydrated N-palmitoylgalactosylsphingosine reported previously (Ruocco, M.J., Atkinson, D., Small, D.M., Skarjune, R.P., Oldfield, E. and Shipley, G.G. (1981) Biochemistry 20, 5957-5966).

Calorimetry, Differential Scanning↗