Based on sphingosine

A signaling rheostat has been proposed where the relative level of SIP and ceramide determines cellular fate. This was based on the observation that ceramide, Fas and TNFa-mediated programmed cell death is suppressed by the addition of SIP (Cuvillier et al, 1996). The model assumes that ceramide is a physiological mediator of apoptosis although this is subject to controversy (Hofmann and Dixit, 1998 Kolesnick and Hannun, 1999). Indeed, sphingosine is also implicated as an apoptotic agent (see above). 

Measurements of the quantities of glycolipids inserted into the membrane have also been reported by a technique based on the use of C-labeled lipid anchors. In this method, the carbohydrate (a-o-Man) was covalently coupled to the anchor at the surface of a pre-formed vesicle. Indeed, the liposome structure was shown to remain intact in the treatment. Nevertheless, the measurement of the incorporated mannose was performed after separation of bound and unbound material by centrifugation. The yields of coupling were shown to increase with the increase of the initial mannose/ C-anchor ratio, but non covalent insertions were displayed at high initial mannose concentrations. Therefore, the aforementioned method was not as accurate as could have been expected for the use of radioactive materials [142]. Radiolabeled phospholipids were also used for such determinations thus the amounts of glycosphingolipids incorporated into liposomes were quantified by the use of H-phospholipids whereas the amounts of glycolipids were determined by a sphingosine assay [143]. 

Structural studies of glycosphingolipids involves determination of the structure of the oligosaccharide chain and of the lipid moiety. For the oligosaccharide chain, it is necessary to determine the composition, molar ratio, and sequence of the monosaccharides, their pyranose or furanose nature, and the position of glycosidic bonds and their configuration for the lipid moiety, the composition of the fatty acids and sphingosine bases must be determined. Used for these purposes are the classical, chemical methods, conventionally accepted in the chemistry of carbohydrates and lipids and based on the degradation of compounds, enzymic, and physicochemical methods, primarily mass spectrometry and n.m.r. spectroscopy. 

Sphingosine bases are usually isolated from glycosphingolipids by treatment with methanolic HC1 containing some water,148 and characterized, in the form of the free bases or their 2,4-dinitrophenyl (DNP) derivatives,149 151 by t.l.c. For quantitative determination of bases, a colorimetric procedure is used, based on color formation of the base with Methyl Orange.152 The composition of the sphingosine bases, as their Me3Si derivatives, is determined by g.l.c. and g.l.c.-m.s.,148,153,154 as well as by using for this purpose... 

Based on the MS data only sphingosine is present in the ceramide moiety CMH-C , ceramide monohexosides from T. cruzi wilh n fatty acids CMH-Con, CMH with oc-hydroxy fatty acids from T. cruzi tr, trace... 

The most common lipids in the human body are phospholipids and glycol-ipids, which are based on glycerol diesters (glycerides) or sphingosine amides (sphingosides Table 2.2.3). 

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