OH OH NH2
sylcerebroside yields the neutral glycosyl-sphingolipids (or neutral ceramides) (Table 6.7). These are usually written with a shorthand nomenclature, e.g. Glc-Gal-Glc-Cer would be glu-cosyl-(l-4)galactosyl-(l-4)glucosyl-ceramide. The deacylated product of galactosylcerebroside, O-sphingosyl-galactoside is called psychosine.
Some glycosphingolipids contain one or more molecules of sialic acid linked to one or more of the sugar residues of a ceramide oligosaccharide. These lipids are called gangliosides (Table 6.8). Sialic acid is N-acetyl neuraminic acid (NANA or NeuAc; Fig. 6.7). Glycolipids with one molecule of sialic acid are called monosialogangliosides; those with two sialic residues are disialogangliosides and so on. Disi-alogangliosides may have each sialic moiety linked to a separate sugar residue or both sialic acids may be linked to each other and one of them linked to a central sugar residue. Because of their complex structure and, hence, cumbersome chemical names, many shorthand notations have been employed. One of the most commonly used is that introduced by the Swedish scientist, Svennerholm. The parent molecule is denoted as GMI and other derived structures are as shown in Table 6.8. The subscripts M, D, T and Q refer to monosialo-, disialo-, trisialo-and quatra (tetra)sialo-gangliosides, respectively. A more recent nomenclature system, due to the German, Wiegandt, is also included in Table 6.8. The latter method has the advantage that, once the symbols have been learnt, the structure can be worked out from the shorthand notation and it can be applied to non-sialic acid-containing glycolipids as well. However, it has yet to supersede the
Svennerholm notation, which will be used in this book.
Instead of glycosylation of the alcohol moiety of the sphingosine base, esterification with phos-phocholine can take place. The phospholipid produced thus is called sphingomyelin.
Three structures constitute the main bases in animals with sphinganine and sphingosine being present in the sphingomyelin of most tissues, while a trihydroxylated derivative of sphinganine, called phytosphingosine (tl8:0 in Fig. 6.6) is found in kidney. This base is also prominent in phytoglyco-lipid, which is a sphingolipid exclusive to plants that contains inositol as well as several other monosaccharide residues.
Physical techniques have revolutionized the analysis and identification of sphingolipids. In many cases a combination of gas-liquid chroma-tography, mass spectrometry and nuclear magnetic resonance spectroscopy provides sufficient information to elucidate a complete structure. The position of attachment of the sugar residues can be worked out from the use of glycosidase enzymes or by permethylation analysis. On the other hand the molecular weight and sequence of some quite complex glycosphingolipids can often be established in one step by the use of the gentler techniques of mass spectrometry such as fast-atom bombardment.
As a generalization, sphingolipids have a rather specific distribution in cells where they are concentrated in the outer (external) leaflet of the plasma membrane.
Fig. 6.7 N-acetylneuraminic acid (NeuAc) or sialic acid.
Table 6.7 The structure of some glycosyl ceramides [Cer = ceramide; glc = glucose; gai = galactose; GalNAc = N-acetyl galactosamine]
Old trivial name
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