Glycolipids Gangliosides

Understanding of ganglioside sialic acid biosynthesis has been aided by the use of radioactive ManNAc. This approach has been used to prepare whole rat-brain gangliosides labelled only in the sialic acid moiety (Quarles and Brady 1971, ROsner et al. 1973, Caputto et al. 1976, Yohe and Rosenberg 1977). Studies with radioactive Glc and GlcN suggested that the site of biosynthesis was in the 

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Glycolipids, gangliosides, phosphatidylinositol Phosphatidylethanolamine Phosphatidylserine Phosphatidylcholine, sphingomyelin Lysolecithin... 

Sphingolipids comprise glycolipids (gangliosides and cerebrosides) and the phospholipid, sphingomyelin. These compounds, too, are important membrane constituents. The biosynthesis of sphingolipids involves the common intermediate ceramide (Fig. 13-15). 

Figure 5 Pathways of biosynthesis of the GSL. Glycosyltransferases in the ER sequentially add sugars to produce the various glycolipids. Gangliosides (i.e., glycosphingolipids containing sialic acid) are produced by the addition of NANA to lactosyi-ceramide to form Gmi, which can then be further glycosylated to Gm2 and then Gms- Globosides also come from lactosyi-ceramide by addition of another Gal and then a GalNAc.

Christie (2) noted that a variety of diverse eompounds generally soluble in organic solvents are usually classified as lipids, i.e., fatty acids and their derivatives, steroids, terpenes, carotenoids, and bile acids. He suggested that many of these diverse compounds have little in the way of structure or function to make them related, and that many substances regarded as lipids may be more soluble in water, e.g., glycolipids, gangliosides, than in organic solvents. Table 1 provides a list of diverse lipophilic substances that have been examined by TLC. It includes typical sorbents, solvent systems, and references for these lipophilic substances. 

The fatty acids are compounds synthesised in nature via condensation of maionyi-coenzyme A units by a fatty acid synthetase compiex. By such definitions, cholesterol (but not steroid hormones) can be considered a lipid, as are phospholipids and glycolipids. Gangliosides, which are acidic glycolipids, are soluble in water and would not be treated as a lipid if some of the looser definitions were accepted. In the subsequent text of this book, the strict definition of lipid given above was followed in selecting the subject matter. 

Mass spectrometry has also been useful in revealing sialic acid as a bound component of glycolipids (gangliosides). Permethylated ganglio-sides have proved more useful for this purpose than the trimethylsilyl derivatives (Karlsson, 1973 Ledeen et aLy 1974 Karlsson, 1974 Karls-son et aLy 1974 Price et aLy 1975). Abundant primary ions are produced... 

There is a considerable literature on the distribution of sialic-acid-containing complex glycolipids (gangliosides) in brain and its components (Burton et aL, 1965 Wolfe, 1961 Lapetina et aL, 1967 Wiegandt, 1967 Wherrett and Mcllwain, 1962 Eichberg et aL, 1964 others). The gangliosides, whose function is essentially unknown, are concentrated in the grey matter. 

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