Glycolipids Glycosphingolipids Are Important In Nerve Tissues In The Cell Membrane

Glycolipids are widely distributed in every tissue of the body, particularly in nervous tissue such as brain. They occur particularly in the outer leaflet of the plasma membrane, where they contribute to cell surface carbohydrates.

The major glycolipids found in animal tissues are glycosphingolipids. They contain ceramide and one or more sugars. Galactosylceramide is a major glyco-

1 II

Choline

Figure 14-9. Lysophosphatidylcholine (lysolecithin). Figure 14-11. A sphingomyelin.

Choline

Ethanolamine

Figure 14-10. Plasmalogen.

sphingolipid of brain and other nervous tissue, found in relatively low amounts elsewhere. It contains a number of characteristic C24 fatty acids, eg, cerebronic acid. Galactosylceramide (Figure 14-12) can be converted to sulfogalactosylceramide (sulfatide), present in high amounts in myelin. Glucosylceramide is the predominant simple glycosphingolipid of extraneural tissues, also occurring in the brain in small amounts. Ganglio-sides are complex glycosphingolipids derived from glu-cosylceramide that contain in addition one or more molecules of a sialic acid. Neuraminic acid (NeuAc; see Chapter 13) is the principal sialic acid found in human tissues. Gangliosides are also present in nervous tissues in high concentration. They appear to have receptor and other functions. The simplest ganglioside found in tissues is GM3, which contains ceramide, one molecule of glucose, one molecule of galactose, and one molecule of NeuAc. In the shorthand nomenclature used, G represents ganglioside; M is a monosialo-containing species; and the subscript 3 is a number assigned on the basis of chromatographic migration. GM1 (Figure 14-13), a more complex ganglioside derived from GM3, is of considerable biologic interest, as it is known to be the receptor in human intestine for cholera toxin. Other gangliosides can contain anywhere from one to five molecules of sialic acid, giving rise to di-, trisialogangliosides, etc.

Ceramide

Sphingosine

Ceramide

1

H

O II

CH — CH —

CH—N —

C — R

Fatty acid

Phosphoric acid-^ |

Fatty acid

Choline

Figure 14-9. Lysophosphatidylcholine (lysolecithin). Figure 14-11. A sphingomyelin.

Ceramide

Sphingosine a f \

OH O

CH3—(CH2)12— CH = CH— CH— CH— N — C — CH(OH) — (CH2)21 — CH3

CH2OH

Galactose <

Figure 14-12. Structure of galactosylcer-amide (galactocerebroside, R = H), and sul-fogalactosylceramide (a sulfatide, R = SO42-).

CH2OH

Galactose <

Fatty acid (eg, cerebronic acid)

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