Phosphatidylcholine and Sphingomyelin

PC is the main component of mammalian membranes and lipoproteins. PC is also keenly involved in cell signaling [41]. SM is specifically enriched to the plasma membranes of cells and is also abundant in hpoproteins. Notably, SM and cholesterol are thought to form segregated, ordered domains within the cellular membranes [42]. Such domains, also referred as rafts, are presently under intensive investigation due to their putative roles in intracellular lipid and protein sorting, cellular signaling molecules, and various diseases [43]. 

Lipid class-specific fragments produced by coUision-activated degradation 

Steryl esters Cholesteryl esters [M+NH41+ RCOO]+ [C27H45l acid PI 369.35 

Abbreviations DAG, diacylglycerol MAG, monoacylglycerol TAG, triacylglycerol NGSL, neutral glycosphingolipids PA, phosphatidic acid PC, phosphatidylcholine PE, phosphatidylethanolamine PG, phosphatidylglycerol PI, phosphatidylinositol PS, phosphatidylserine SM, sphingomyelin. See the text for other details. 

Both PC and SM contain a phosphocholine head group, which makes the molecules zwitterionic and largely dictates their ionization and fragmentation behavior. PC and SM readily form [M+H]+ ions which upon CAD yield an abundant phosphocholine fragment with m/z 184 (cf. Table 1) and can thus be selectively detected by scanning for parents of this ion. In the presence of different salts, PC and SM form both cation and anion adducts, which may be utilized for the elucidation of the fatty acids present in the molecules [19,29,34,44,45]. In case of SM, also a fragment indicative of the long-chain base is found [45]. 

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There is also inside-outside (transverse) asymmetry of the phospholipids. The choline-containing phospholipids (phosphatidylcholine and sphingomyelin) are located mainly in the outer molecular layer the aminophospholipids (phosphatidylserine and phos-phatidylethanolamine) are preferentially located in the inner leaflet. Obviously, if this asymmetry is to exist at all, there must be limited transverse mobility (flip-flop) of the membrane phospholipids. In fact, phospholipids in synthetic bilayers exhibit an extraordinarily slow rate of flip-flop the half-life of the asymmetry can be measured in several weeks. However, when certain membrane proteins such as the erythrocyte protein gly-cophorin are inserted artificially into synthetic bilayers, the frequency of phospholipid flip-flop may increase as much as 100-fold. 

It can be seen from Figure 1 that the choline-containing phospholipids, phosphatidylcholine and sphingomyelin are localized predominantly in the outer monolayer of the plasma membrane. The aminophospholipids, conprising phosphatidylethanolamine and phosphatidylserine, by contrast, are enriched in the cytoplasmic leaflet of the membrane (Bretcher, 1972b Rothman and Lenard, 1977 Op den Kamp, 1979). The transmembrane distribution of the minor membrane lipid components has been determined by reaction with lipid-specific antibodies (Gascard et al, 1991) and lipid hydrolases (Biitikofer et al, 1990). Such studies have shown that phosphatidic acid, phosphatidylinositol and phosphatidylinositol-4,5-fc -phosphate all resemble phosphatidylethanolamine in that about 80% of the phospholipids are localized in the cytoplasmic leaflet of the membrane. 

Yen, C.L., Mar, M.H., and Zeisel, S.H., 1999, Chohne dehdency-induced apoptosis in PC12 ceUs is associated with diminished membrane phosphatidylcholine and sphingomyelin, accumulation of ceramide and diacylglycetol, and achvation of a caspase. FASEB. J. 13 135-142... 

Apo A-I is the main structural apolipoprotein on HDL particles it is synthesized in hepatic and enteric cells. Bound phosphatidylcholine and sphingomyelin participate in the creation of protein-phospholipid complexes. 

Plasma membrane lipids are asymmetrically distributed between the two monolayers of the bilayer, although the asymmetry, unlike that of membrane proteins, is not absolute. In the plasma membrane of the erythrocyte, for example, choline-containing lipids (phosphatidylcholine and sphingomyelin) are typically found in the outer (extracellular or exoplasmic) leaflet (Fig. 11-5), whereas phosphatidylserine, phosphatidyl-ethanolamine, and the phosphatidylinositols are much more common in the inner (cytoplasmic) leaflet. Changes in the distribution of lipids between plasma membrane leaflets have biological consequences. For example, only when the phosphatidylserine in the plasma membrane moves into the outer leaflet is a platelet able to play its role in formation of a blood clot. For many other cells types, phosphatidylserine exposure on the outer surface marks a cell for destruction by programmed cell death. 

Lipids also show asymmetrical distributions between the inner and outer leaflets of the bilayer. In the erythrocyte plasma membrane, most of the phosphatidylethanolamine and phosphatidylserine are in the inner leaflet, whereas the phosphatidylcholine and sphingomyelin are located mainly in the outer leaflet. A similar asymmetry is seen even in artificial liposomes prepared from mixtures of phospholipids. In liposomes containing a mixture of phosphatidylethanolamine and phosphatidylcholine, phosphatidylethanolamine localizes preferentially in the inner leaflet, and phosphatidylcholine in the outer. For the most part, the asymmetrical distributions of lipids probably reflect packing forces determined by the different curvatures of the inner and outer surfaces of the bilayer. By contrast, the disposition of membrane proteins reflects the mechanism of protein synthesis and insertion into the membrane. We return to this topic in chapter 29. 

Flip-Flop Diffusion The inner leaflet (monolayer) of the human erythrocyte membrane consists predominantly of phosphatidylethanolamine and phosphatidylserine. The outer leaflet consists predominantly of phosphatidylcholine and sphingomyelin. Although the phospholipid components of the membrane can diffuse in the fluid bilayer, this sidedness is preserved at all times. How ... 

Phospholipid is a generic term that refers to lipids containing a phosphoric acid residue. Thus, both phosphatidylcholine and sphingomyelin would fit in this category. 

This is a respectable type of separation, and yet it is obvious that not all compounds or classes of compounds are separated precisely and neatly from each. There simply is not such a magic system, and one must realize that further fractionation may be necessary. It is interesting to note that phosphatidylcholine and sphingomyelin tend to blend together. Usually the more unsaturated phosphatidylcholine species elute early on, followed by the more saturated one. By judicious handling of solvent eluate volume (by a fraction collector), a quite reasonable separation can be obtained. 

The ionic characteristics of phosphatidylethanolamine and phosphatidylserine provided a basis for their separation from the neutral phospholipids, such as phosphatidylcholine and sphingomyelin. This involved application in chloroform-methanol (1 1, v/v) of a cellular lipid extract to a... 

The proportions of corresponding phospholipid classes in the milk of various animal species are remarkably similar (Table 1.19). In each case, phosphatidylethanolamine, phosphatidylcholine and sphingomyelin are the... 

As shown in Table IVA, the cells bind approximately 5% of the total radioactivity added 32% of this eictivity is associated with the cellular lipid fraction and 64% with the nonlipid (presumably protein) fraction. Of the lipid-associated activity, the distribution of I into cellular phospholipids, lysophosphatides, triacylglycerides, and free fatty acids is shown in Table IVB. The iodination reaction labels a variety of lipids of all classes without preference to charge, acid-base properties, or number of fatty acyl constituents, appears not to label intracellular lipids (e.g., cardiolipin, a major mitochondrial component), and labels lipids on both the exterior and interior hydrophilic surfaces of the plasma membrane (i.e., phosphatidylcholine and sphingomyelin as well as phosphatidyleth-anolamine and phosphatidylserine) (Table IVB). ... 

Skim milk and milk serum have the highest portion of polar lipids as percent of the total lipids, and whole milk and cream have the least. Of the polar lipids, phos-phatidylethanolamine constitutes the largest component, with phosphatidylcholine and sphingomyelin (being present in about equal proportions) at a signihcantly lower level (Table 4) (16, 23, 24, 26). 

Zeisel, S. H., Char, D., and Shepard, M. F. (1986)- Choline, phosphatidylcholine and sphingomyelin in human and bovine milk and infant formulas, /. Nutr. 116,50-56,... 

Jungalwala, F.B. High-Performance liquid chromatography of phosphatidylcholine and sphingomyelin with detection in the region of 200 nm. Biochem. J. 1976, 155, 55-60. 

Since the total content of phosphatidylcholine and sphingomyelin often exceeds that of phosphatidylethanolamine and phosphatidylserine, the bilayer would be incomplete on the inside of the membrane were it not for the presence of proteins, which contribute more to the inside than to the outside surface. 

Liebisch G, Lieser B, Rathenberg J, et al. (2004) High-throughput quantification of phosphatidylcholine and sphingomyelin by electrospray ionization tandem mass spectrometry coupled with isotope correction algorithm. Biochim Biophys Acta 1686 108-117... 

The membrane constituents are lipids (phospholipids, glycosphingolipids, and cholesterol Figure 10-5), carbohydrates, and proteins. The ratio of protein lipid carbohydrate on a weight basis varies considerably from membrane to membrane. For example, the human erythrocyte membrane has a ratio of about 49 43 8, whereas myelin has a ratio of 18 79 3. The composition of the normal human erythrocyte membrane is shown in Table 10-2. All membrane lipids are amphipathic (i.e., polar lipids). The polar heads of the phospholipids may be neutral, anionic, or dipolar. The surface of the membrane bears a net negative charge. The distribution of lipid constituents in the bilayer is asymmetrical. For example, in the erythrocyte membrane, phosphatidylethanolamine and phosphatidylserine are located primarily in the internal monolayer, whereas phosphatidylcholine and sphingomyelin are located in the external monolayer. 

The product obtained when the alchohol hydroxyl group of sphingosine is esterified with phosphotylcholine is sphingomyelin. The conformations of phosphatidylcholine and sphingomyelin are similiar. 

Asymmetry. Biological membranes are asymmetric that is, the lipid composition of each half of a bilayer is different. For example, the human red blood cell membrane possesses substantially more phosphatidylcholine and sphingomyelin on its outside surface. Most of the membrane s phosphatidylserine and phos-phatidylethanolamine are on the inner side. Membrane asymmetry is not unexpected, because each side of a membrane is exposed to a different environment. Asymmetry originates during membrane synthesis, because phospholipid biosynthesis occurs on only one side of a membrane (Special Interest Box 12.3). The protein components of membranes (discussed below) also exhibit considerable asymmetry with distinctly different functional domains within membrane and on the cytoplasmic and extracellular faces of membrane. 

The two layers of the phospholipid bilayer membrane are not identical in composition. For instance, in human red blood cells, approximately 80% of the phospholipids in the outer layer of the membrane are phosphatidylcholine and sphingomyelin whereas phosphatidylethanolamine and phosphatidylserine make up approximately 80% of the inner layer. In addition, carbohydrate groups are found attached only to those phospholipids found on the outer layer of a membrane. Here they participate in receptor and recognition functions. 

Fig. 33.35. Comparison of phosphatidylcholine and sphingomyelin in amniotic fluid. Phosphatidylcholine is the major hpid in lung surfactant. The concentration of phosphatidylcholine relative to sphingomyelin rises at 35 weeks of gestation, indicating pulmonary maturity.

The major phospholipids of milk from mammals examined so far are phosphatidylethanolamine, phosphatidylcholine and sphingomyelin. These each comprise about 30% of the total phospholipids (Morrison, 1970). 

The fatty acid compositions of phosphatidylcholines and sphingomyelins from several mammals are shown in Table 3.221. In contrast to tri-acylglycerols, the fatty acids of these phospholipids are mainly long and very long chain. Palmitic, stearic, oleic and, sometimes, linoleic acids are major components of the phosphatidylcholine fraction. In contrast, sphingomyelin has much smaller amounts of unsaturated components. Palmitic, stearic, arachidic, behenic and lignoceric acids are major components. In sheep milk sphingomyelin, tricosanoic acid is a major constituent while in the other species shown nervonic acid is the major unsaturated fatty acid (Table 3.221). 

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