Sphingomyelin monolayers

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. 

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. 

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 ... 

Fig. 5. (A) Near-field fluorescence image of a 1 1 1 sphingomyelin/cholesterol/ DOPC monolayer containing 1% ganglioside (GM1), 0.005% GM1-BODIPY, and 1% Texas Ted-DPPE. The excitation wavelength is resonant with both GM1-BODIPY and Texas Red-DPPE. (B) Near-field fluorescent image of the same film shown in (A), except that the excitation wavelength is 568 nm and is resonant with only Texas Red-DPPE. The rectangles denote film region common to both images. Reprinted in part with permission from Ref. [20]. Copyright 2003 The American Chemical Society.

Bittman R, Kasireddy CR, Mattjus P, Slotte IP. Interaction 53. of cholesterol with sphingomyelin in monolayers and vesicles. Biochemistry 1994 33 11776-11781. 

In the plasma membrane of animals (1), the amount of cholesterol is usually around 20-30 mol%. The rest of the lipids are mainly PC, PE, and sphingomyelin (SM) lipids, with smaller amounts of PS, PI, and glycolipids. These lipids are distributed asymmetrically across the membrane, because most cholesterol, PC, and glycolipids are located in the extracellular (outer) leaflet, whereas PS and PE lipids are located mainly in the intracellular (inner) monolayer. The lipid composition can be highly different in other organelles, however, as is the case in mitochondria (1), in which the mitochondrial membrane is composed of two (inner and outer) membranes. There, the amounts of cholesterol, SM, and PS are negligible most lipids are PC and PE. The major difference compared with plasma membrane is the concentration of cardiolipins. They are actually found only in bacterial and in mitochondrial membranes, where their numbers are significant even in mitochondria they are located mainly on the iimer membrane. 

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. 

Many monolayer studies have been made with phospholipids and sterols which have helped to clarify the details of the molecular properties of both types of molecule required to bring about the so-called condensation effect [21-23,27]. After various speculations and suggestions, it was shown that a cis double bond at the 9,10 position of the acyl chain was not necessary for condensation, and that trans isomers and even fully saturated phospholipid could bring about this condensation effect [24]. The condensation effects are not restricted to particular phospholipid classes but have been demonstrated to occur with phosphatidylcholines and phosphatidylethanolamines as well as with phosphatidic acid [25], sphingomyelin, phosphatidylglycerol and phospholipid derivatives [26]. 

Another important observation is that phospholipids are asymmetrically distributed in the plasma membrane of eucariotic cells. It has been found that phosphatidylserin (PS), phosphatidylethanolamine (PE) and phosphoinositides (PI) are essentially located in the inner monolayer, whereas phosphatidylcholine (PC), sphingomyelin (SH) and glycolipids (GL) are mainly located in the outer monolayer [8]. An example of the asymmetrical distribution of phospholipids in human red cells and in the membrane of the influenza virus is shown in Table 4 [9], [10]. 

Mattjus P, Slotte JP. Does cholesterol discriminate between sphingomyelin and phosphatidylcholine in mixed monolayers containing both phospholipids Chem Phys Lipids. 1996 81(l) 69-80. 

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