Mammals have different sphingoid bases that differ in their length and/or pattern of desaturation and hydroxyl groups. The predominant sphingoid base in mammalian ceramides is sphing-4-enine (sphingosine or SPH) which has a trans desaturation at carbon 4. FADS3 is a ceramide desaturase that introduces a cis double bond between carbon 14 and carbon 15 of the SPH-containing ceramides, producing sphinga-4,14-dienine-containing ceramides (SPD ceramides). SPD ceramides occur widely in mammalian tissues and cells. Due to their unusual structure containing a cis double bond, SPD ceramides may have an opposite, negative role in lipid microdomain formation relative to conventional ceramides. FADS3 also acts as a methyl-end fatty acyl coenzyme A (CoA) desaturase that introduces a cis double bond between the preexisting double bond and the terminal methyl group of the fatty acyl chain. Desaturates (11E)-octadecenoate (trans-vaccenoate, the predominant trans fatty acid in human milk) at carbon 13 to generate (11E,13Z)-octadecadienoate (also known as conjugated linoleic acid 11E,13Z-CLA).
