Sphingolipids are derivatives of sphingosine (trans-1,3-dihydroxy 2-amino-4-octadecene), an 18-carbon unsaturated amino alcohol. Amide linkage of a fatty acid to sphingosine yields ceramides. Esterification of choline to ceramides yields sphingomyelin, and ceramide glycosylation yields gangliosides. These molecules appear to be essential components of cell membranes, and intermediates in the pathways of sphingolipid synthesis and breakdown modulate processes including apoptosis.
De novo synthesis proceeds in four steps: the reaction of palmitoyl-CoA and serine to form 3-dehydrosphinganine, the reduction of 3-dehydrosphinganine to sphinganine, the reaction of sphinganine and a long-chain fatty acyl CoA to form dihydroceramide, and the desaturation of dihydroceramide to form ceramide.
Other sphingolipids involved in signaling are derived from ceramide and its biosynthetic intermediates. These include sphinganine (dihydrosphingosine) 1-phosphate, phytoceramide, sphingosine, and sphingosine 1-phosphate.
Sphingomyelin is synthesized in a single step in the membrane of the Golgi apparatus from ceramides generated in the endoplasmic reticulum (ER) membrane and transferred to the Golgi by CERT (ceramide transfer protein), an isoform of COL4A3BP that is associated with the ER membrane as a complex with PPM1L (protein phosphatase 1-like) and VAPA or VAPB (VAMP-associated proteins A or B). Sphingomyelin synthesis appears to be regulated primarily at the level of this transport process through the reversible phosphorylation of CERT.
These processes have not been experimentally studied in chickens, but are inferred here from their human counterparts.
