Sphingolipids, defined

Fig. 14.—Schematic Representation of the Fragmentation Observed in the Positive F.a.b.-Mass Spectrum of a Permethylated Ganglioside Isolated from Granulocytes. [Other glyco-sphingolipids fragment in a similar way. Major cleavages are shown with solid lines, and minor cleavages with dotted lines. The masses of ions resulting from cleavages (a), (b), and (c) define the type of sphingosine and the type of fatty acid. In this example, (a) is 548, (b) is [M + H] minus 238, and (c) is [M + H] minus 533.]...

The main disadvantage of incorporating sphingolipids in liposomes is their high cost. Purified phosphodiglyceride derivatives may be obtained in bulk quantities and in highly defined synthetic preparations, whereas sphingolipid derivatives are not so readily available in similar purity. 

In aqueous systems, membrane lipids may exist in a gel-like solid state or as a two-dimensional liquid. In the case of pure phospholipids, these states interconvert at a well-defined transition temperature, Tc, that increases with alkyl chain length and decreases with introduction of alkyl chain unsaturation. In cell membranes, which have marked heterogeneity in both the polar and nonpolar domains of the bilayer, this state is described as liquid disordered . The presence of sufficient sphingolipids, with... 

Recent evidence also suggests that the distribution of phospholipids and proteins within the cell membrane is not random, but that certain areas of the cell membrane are organized into special regions or domains. 35,52 63 In particular, certain domains appear to consist primarily of lipids such as cholesterol and sphingolipids.27,50 These lipid domains are often described as lipid rafts that move freely about the cell membrane and these lipid rafts appear to be important in controlling various cell functions including cell signaling, endocytosis, and ion channel function.27,50 Future research will help further define the role of the lipid rafts and other specific domains within the cell membrane. 

Lipids are far more diverse chemically than other typical biomolecules such as amino acids, carbohydrates, and nucleotides. The definition of lipids includes simple fatty acids and their glycerol esters, sterols such as cholesterol, and phospholipids, sphingolipids, and cerebrosides. Lipids are generally defined by their common hydrophobic character, which makes them soluble in organic solvents such as chloroform. Virtually all lipids also have a hydrophilic group, which makes them surface active. 

Sphingolipids are asymmetrically distributed in the cell membrane and are predominantly present in the outer leaflet of the cell membrane. Thus, sphingolipid-and cholesterol-enriched membrane domains exist in the outer leaflet, while at present it can be only speculated whether similar structures are also present in the inner leaflet of the cell membrane. In particnlar, long acyl-chains of sphingolipids are certainly able to interact with famesylated, geranylated or pahnitoylated proteins integrated in the inner leaflet of the cell membrane, and, furthermore, to influence the structure and lateral distribution of the inner membrane. However, the mechanisms that mediate an organization of the irmer membrane leaflet similar to the structure of the outer membrane leaflet still need to be defined. 

Fig. 1. Basic structures of sphingolipid backbones and head groups. (A) An example of a ceramide backbone, including commonly used names and abbreviations for the components (B) common head groups attached directly to ceramide in mammalian sphingolipids and (C) an example of a complex glycosphingollpid, ganglio-side GMla, and the names for the component parts. Abbreviations are as defined in text.

Lipids are defined as water-insoluble compounds extracted from living organisms by weakly polar or nonpolar solvents. The definition is based on a physical property in contrast to the definitions of the other basic building blocks in this chapter (proteins, carbohydrates, and nucleic acids), which are based on chemical stracture. Consequently the term lipid covers a stmcturally diverse group of compounds including/afiy acids, glycerolipids, sphingolipids, terpenes, steroids, and carotenoids. There is no universally accepted scheme for their classification. 

Lipidome The lipidome is defined as the total lipid composition of a cell or organ. The lipidome includes but is not limited to fatty acids, acylglycerols, glycerophospholipids, sphingolipids, terpenes, steroids, and lipids that complex with proteins and polysaccharides. 

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