Lipid species annotation

Lipid classes and species were annotated according to the classification systems for lipids [47,48]. For glycerolipids, the “Lipid class(FA1_FA2)” format specifies the structures of the fatty acyl (FA) side chains (e.g., PC(10:0_18:1), whereas the “Lipid class(sn1/sn2)” format specifies the side chain regiochemistry, too (e.g., PC(16:0/18:1)). In sum formulas, e.g., PE(36:2), the total numbers of carbons followed by double bonds for all chains are indicated. For sphingolipids, first the number of hydroxyl groups in the long chain base (e.g., “t” for the three hydroxyls of phytosphingosine), the number of carbon atoms and then the number of double bonds are indicated followed by the N-acyl chain composition, such as Cer(t20:0/24:0). The sum formula, e.g., Cer(44:0:3), specifies first the total number of carbons in the long chain base and FA moiety then the sum of double bonds in the long chain base and the FA moiety followed by the sum of hydroxyl groups in the long chain base and the FA moiety.