Heterogeneities

Capturing the lipid heterogeneity between organisms using ESI-MS
Natural lipids exist in many different chemical and structural forms. Shown here are representative electrospray ionization (ESI) mass spectra of total lipid extracts from mammalian cells (A) and the yeast S. cerevisiae (B). Both spectra are recorded in the negative polarity, which efficiently ionizes many phospholipids and sphingolipids. The majority of ions from membrane lipids are in a mass to charge (m/z) range between 700-900; fatty acyls are in the lower molecular range (m/z 250-350). A lipid molecular species can tentatively be assigned to a given m/z value. This is shown in panel A for m/z 834 and glycerophosphatidylserine (GPSer) with a fatty acyl composition of 18:0 and 22:6, GPSer(18:0/22:6). [Note: 18:0 and 22:6 denote fatty acyls with 18 carbon atoms and 0 double bonds and 22 carbon atoms with 6 double bonds,respectively]. Such tentative assignment based on single stage mass analysis is to be confirmed experimentally by tandem mass spectrometry, in which the fragments allow more rigorous identification of an ion of interest. This also allows assessing of the contributions from isobaric lipids (e.g. m/z 766 is annotated with GPEtn(18:0/20:4); de-methylated GPCho(18:2/18:2) would produce an ion with the same m/z). Note that in a yeast lipid extract, an ion with a very similar m/z of 835 corresponds to a glycerophosphatidylinositol (GPIns(16:0/18:1)) rather than a glycerophosphatidylserine. A distinguishing feature between various organisms is details in fatty acyl and headgroup chemistry of lipids. S. cerevisiae for example does not produce polyunsaturated fatty acyls (e.g. arachidonic acid, 20:4, with m/z 303 or docosahexaenoic acid, 22:6, m/z 327).

Fatty acyl heterogeneities are often a distinguishing feature between different organisms. S. cerevisiae for example does not have polyunsaturated fatty acyls and the chain lengths tend to be shorter as well. This leads to a general shift (towards smaller values) of the masses of major phospholipids in yeast when compared to a typical mammalian cell. Such apparently subtle yet distinct differences in the fine structure of fatty acyl moieties are also found in bacterial lipids which carry more complicated (side chain modified) fatty acyls (e.g. tuberculostearic acid). Within a given cell or tissue, fatty acyls are actively remodeled in acyl-CoA dependent and independent reactions. This can lead to selective (trans)acylation of membrane lipids, e.g. in ether-linked phosphatidylethanolamines, with polyunsaturated fatty acyls. Such fatty acid remodeling is dependent on diet and age and has been implicated in apoptotic cell signaling which makes this an attractive pathway for therapeutic intervention. Aberrant acylation of cardiolipin has been linked to diseases such as Barth syndrome which is caused by mutation in tafazzin, a phospholipid acyltransferase involved in remodeling of cardiolipin.

Sometimes striking differences are observed when headgroups of biological lipids are compared between organisms. In mammals, the gangliosides contain ceramide anchors. S. cerevisiae adds a phosphoryl mannosides to its ceramide backbone which leads to a class of compounds which are closely linked metabolically and which are not present in mammalian cells. Yeast mutants with a defect in the csg2 gene, which encodes for a mannosyltransferase lack mannosylated inositol phosphorylceramides (MIPC) as well as M(IP)2C. Instead the substrates of the reaction, inositol phosphorylceramide (IPC) would be expected to accumulate which was indeed supported experimentally.