Abstrakt
The core building block of sphingolipids is a sphingoid base, which is derived from the condensation of an amino acid, normally serine, with a coenzyme A-activated acyl chain, in most cases palmitoyl-CoA. The first step of de novo sphingolipid biosynthesis is catalyzed by the pyridoxal 5'-phosphate (PLP)-dependent enzyme serine palmitoyltransferase (SPT), a water-soluble enzyme in bacteria, but membrane-bound in eukaryotes.
In humans, SPT is composed of two large (SPTLC1/SPTLC2 or SPTLC1/SPTLC3) and a small subunit (SPTSSA or SPTSSB), and this composition is tissue and age dependent. Depending on the organism, SPT composition and substrate availability sphingoid bases of various chain lengths (straight, iso-, or anteiso-branched) will be produced which are further modified by the cellular metabolism.
Modifications include addition of cis and trans double bonds, hydroxyl groups and methyl groups, attachment of various acyl chains at the 2-amino group forming "ceramides," and addition of phosphate, choline, ethanolamine, various glycans, or phosphoinositol(glycan)s, and a second acyl chain at the 1-hydroxyl group. Combinatorial biochemistry thus will allow synthesis of several tens of thousands of amphiphilic lipophilic compounds, some of them more lipophilic like 1-O-acylceramides and some quite polar like sphingosine 1-phosphate or complex gangliosides.
One example of high complexity is human skin with an estimated composition of thousands of ceramide structures. All these compounds serve structure-specific but also cell- and tissue-specific functions, which depend on a well-balanced cell-dependent sphingolipid metabolism and turnover.
Here we focus on mammalian sphingolipids. While following their metabolic pathways, we will introduce the individual sphingolipid classes and discuss, how to analyze and differentiate all these sometimes structurally similar or isobaric species.
Furthermore, we focus on the most common ionization techniques and the analysis of non-modified compounds.