Over 300 types of sphingolipids are synthesized in various mammalian cell types. Structurally, sphingolipids are composed of a long-chain aliphatic 2-amino-1,3-diol (sphingoid base), an attached amide-linked fatty acyl chain varying in length from 16 to 24 carbon atoms, and a polar head group at the 1-position (Figure 27.1). The diversity of sphingolipids originates from a variety of head groups: ceramide has a hydroxyl at the 1-position, sphingomyelin has phosphorylcholine head groups, and glycosphingolipids contain carbohydrate head groups. Glycosphingolipids are further classified according to the sequence of sugars and the chemical bonds which link them together: cerebrosides have a single glucose or galactose, other neutral lipids such as latotosylceramide and trihexosides have higher order glycose units, and acidic glycosphingolipids contain one or more sialic acid residues (gangliosides) or sulfate monoester groups (sulfatides). For every sphingolipid there is a corresponding lysosphingolipid, which has the identical polar head group at the 1-position, but lacks the amide-linked fatty acyl group at the 2-position. For example, deacylation of ceramide produces sphingosine, a representative of free long-chain bases (FLCBs). Sphingosine can be further converted to highly biologically active metabolites such as sphingosine-1-phosphate via sphingosine kinase-catalyzed phosphorylation at the 1-position,1 and N , N-dimethylsphingosine via amino-dimethylation.2