Ceramide + Phosphatidyl choline
+ UDP-Galactose |
Figure 28-2. The formation of sphingomyelin and cerebrosides from ceramide.
represented by the letter G followed by a subscript letter and a number. The
subscript letter indicates the number of sialic acid residues present in the mol-
ecule (M = 1, D = 2, T = 3), and the number is equal to 5 minus the number of
neutral sugars. Over 60 gangliosides have been characterized, and the structure
of one of these, ganglioside GM1, is schematically represented in Figure 28-3.
Glycosphingolipids function in cell-cell recognition and tissue immunity.
Gangliosides are found at nerve endings and may be important in nerve
Sphingolipids are constantly being turned over in the lysosomes of cells by
specific hydrolytic enzymes that remove the sugars in a stepwise fashion.
Defects in these enzymes can occur and result in disease states resulting from
the accumulation of undegraded sphingolipids. Lysosomal storage diseases
are a group of approximately 40 different diseases that occur in approximately
1 in 5000 live births. Many of these diseases are characterized by a deficiency
in a specific lysosomal enzyme, but some diseases can be caused by the inabil-
ity of enzymes to be translocated to the lysosome (I-cell disease), defective
transport of small molecules out of the lysosome (cystinosis), or a deficiency
in sphingolipid activator proteins, which are small molecular weight proteins
that participate in the degradation of sphingolipids. The net result in all of
Figure 28-3. The schematic structure of ganglioside GM1. The nomenclature is
as follows: G stands for ganglioside; the subscript letter indicates the number
of sialic acid residues; the numeral is equal to 5 - (number of neutral sugars).
Abbreviations: GalNAc = A-acetylgalactosamine, Gal = galactose; Glc = glucose;
NANA = A-acetylneuraminic acid (sialic acid).