Figure 1.

Sphingolipid metabolic pathway. A set of enzymes catalyzes the condensation of serine and acyl-CoA (usually palmitate) and de novo synthesis of dihydroceramides (yellow box). Dihydroceramides are then oxidized to ceramides, which are characterized by a long sphingosine carbon backbone (in black) and a fatty acid of variable length (orange box) that defines the species, according to the number of carbon atoms and double bonds. Subsequently, a hydrogen in ceramides can be replaced by a polar group, given rise to more complex sphingolipids. Depending on this polar group (green box) different classes exist, such as sphingomyelins, simple hexosylceramides (i.e., glucosylceramides and galactosylceramides), lactosylceramides, and other complex glycosphingolipids. These complex sphingolipids can be catabolized and recycled to regenerate ceramide through the salvage pathway (red box), while sphingomyelin can be hydrolyzed directly to ceramide (blue box). SPT, serine palmitoyltransferase; CerS, ceramide synthase; DES, dihydroceramide desaturase; SMase, sphingomyelinase; SMS, sphingomyelin synthase; CGT, ceramide galactosyltransferase; GCS, glucosylceramide synthase; LCS, lactosylceramide synthase, CDase, ceramidase.