Glycosphingolipids carbohydrate moiety

The procedure of isolating the glycosphingolipids is based on their physicochemical properties. The presence of the lipophilic, lipid moiety results in their solubility in such typical solvents for lipids as chloroform-methanol. On the other hand, the presence of the carbohydrate moiety imparts to them hydrophilic properties that increase with increase in the length of the carbohydrate chain and with the presence of such polar monosaccharides as sialic acid. This very property makes it possible to subdivide the glycolipids further, according to their polarity. In principle, the isolation of glycosphingolipids from marine animals does not differ from their isolation from other sources, a procedure comprehensively elucidated in a number of reviews.80 88... 

Glycosphingolipids are sphingolipids, such as the cerebrosides, globosides, sulfatides, and gangliosides, that contain carbohydrate moieties (Figure 10.8). Glycosphingolipids are significant components of the membranes of nerves. 

Gangliosides are glycosphingolipids that contain the structure of a ceramide plus carbohydrate moieties. Sialic acid is a part of the carbohydrate component. Gangliosides differ from cerebrosides in containing sialic acid, whereas the latter contain no sialic acid. 

It is now possible to use h n.m.r. spectroscopy to assign the anomeric configuration of the carbohydrate moieties of glycosphingolipids. 

Mass spectrometry of intact glycosphingolipids by direct probe insertion and soft ionisation procedures has become one of the major means of structure determination for these compounds [460,896,974]. information is thereby obtained on the nature of both the iipid and carbohydrate moieties, aithough the presence of different moiecuiar species hampers the interpretation of the spectra greatiy. Detailed discussion of this aspect is beyond the scope of this book. 

There are over 50 known glycosphingolipids based on the composition and sequence of the carbohydrate oligosaccharide. D-Galactocerebrosides are prevalent in the neuronal cell membranes of the brain (see Fig. 9.14B). Glucocerebrosides occur in the membranes of other nerve tissues. In many instances the carbohydrate moieties are oligosaccharides. A representative structure, known as lacto-cerebroside is shown in Fig. 9.14C. 

Structural studies of glycosphingolipids involves determination of the structure of the oligosaccharide chain and of the lipid moiety. For the oligosaccharide chain, it is necessary to determine the composition, molar ratio, and sequence of the monosaccharides, their pyranose or furanose nature, and the position of glycosidic bonds and their configuration for the lipid moiety, the composition of the fatty acids and sphingosine bases must be determined. Used for these purposes are the classical, chemical methods, conventionally accepted in the chemistry of carbohydrates and lipids and based on the degradation of compounds, enzymic, and physicochemical methods, primarily mass spectrometry and n.m.r. spectroscopy. 

To determine the position of glycosidic bonds in glycosphingolipids, periodate oxidation and methylation are widely used. A characteristic feature of the periodate oxidation of glycosphingolipids is the stability, to oxidation under mild conditions,179 of the D-glucose residue that is located at the beginning of the carbohydrate chain and substituted at 0-4 this seems to be associated with steric hindrance caused by the close location of the lipid moiety. 

In contrast to the striking divergences in the carbohydrate parts, respective ceramide moieties resemble those of other cestode glycosphingolipids in so far as sphinganine (d18 0) and 4-hydroxy-sphingoid bases (tl 8 0, t20 0) as well as, in part hydroxylated, fatty acids with 16 to 28 carbon atoms, in particular C26 0 and C28 0, represent predominant ceramide constituents, which might be an indication of similar biosynthetic routes. 

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