Hydrophobic ceramide

Neuraminidase isolated from the culture filtrate of Arthrobacter ureafaciens has been characterized in detail with respect to its action on glycolipids. Strong electrolytes had a reversible inhibitory effect on the action of the enzyme on brain gangliosides in accordance with Debye-Hiickel effect of ionic environment on ionic activity, and resulted in an acidic shift and a broadening of the pH optimum. Both ionic and non-ionic detergents markedly enhanced the activity of the thiol-sensitive enzyme on the gangliosides, and caused an acidic shift of the pH optimum. It was suggested that the hydrophobic ceramide moiety increases affinity of the lipid substrate to the enzyme, but inhibits hydrolysis of the substrate, possibly due to its hydrophobic interaction with hydrophobic portions of the enzyme molecule. 

Structurally, glycosphingolipids and gangliosides are distinct from other classes of lipids they are made of a hydrophobic ceramide lipid tail to which an oligosaccharide head group is attached. Characterization of glycosphingolipids and gangliosides is achieved by CID-MS/MS of the FAB-, ESI-, and MALDI-produced [M - - H]+ and [M — H] ions. Structural information on ceramide and carbohydrate portions can be also derived. 

Ferrying of molecules into cells via entry through caveolae may represent a way to traffic specifically cytotoxic molecules to specific action sites. For example, elevating the intracellular level of the sphingolipid ceramide is known to exert antimitogenic and proapoptotic effects. While ceramide is cell-permeable and displays antiapoptotic properties in vitro, systemic in vivo use of ceramide is hampered by its hydrophobicity. Using a C6-ceramide formulation in pegylated liposomes was shown to elicit a sixfold reduction in solid phase tumors, when compared to unloaded liposomes in a mouse model of breast adenocarcinoma [68],... 

Using a similar approach, Notman et al. [81], determined the free energy for pore formation in bilayers composed of ceramide, as a model for the stratum corneum of the skin, both in the presence and in the absence of DMSO. Without DMSO, the bilayer was in the gel phase, and interestingly, a hydrophobic pore was observed with a high free-energy barrier ( 60 kj/mol). In the presence of DMSO, the bilayer was more fluid, and the more typical hydrophilic pore was observed, with a much smaller activation energy of 20kJ/mol. This work provided a thermodynamic and structural explanation for the enhanced permeability of skin by DMSO. 

Notman, R., Anwar, J., Briels, W.J., Noro, M.G., den Otter, W.K. Simulations of skin barrier function free energies of hydrophobic and hydrophilic transmembrane pores in ceramide bilayers. Biophys. J. 2008, 95, 4763-71. 

More complex structures, often related to natural products are prepared by organic synthesis. Among them can be mentioned (f )-3-hydroxytetradecanoic acid (the double-tail hydrophobic moiety of lipid A), sphingosine derivatives related to the ceramides or 1,2- and l,3-dialkyl(acyl)glycerols related to glyco-glycerolipids, glycerophospholipids, and GPI anchors of membrane proteins. The preparations of the above derivatives were reported several years ago but some improvements have been published more recently. 

The apolar chains of the ceramide portion of gangliosides are responsible for the hydrophobic properties of gangliosides and for their availability to hydrophobic interactions. The formation of an hydrogen bond between the 3-hydroxyl group of the sphingo-sine and the carbonyl oxygen of the fatty acid, would tend to spread the two hydrocarbon chains reducing their tendency for mutual association, and thus to promote association of each chain with other molecules ( 3 ). 

The lipid composition changes dramatically during terminal differentiation. After extrusion from the lamellar bodies, the polar lipid precursors are enzymatically converted into more hydrophobic lipids. As a result, phospholipids are almost absent in the stratum corneum. The lipid lamellae surrounding the corneocytes are predominantly composed of ceramides, cholesterol, and free fatty acids. It is generally assumed that these lipids are present in nearly equimolar ratios. However, inspection of literature data shows that there is a high interindividual variability in the lipid composition [37],... 

Fig. 1.4.8. Concept for cell surface engineering exogenously added ceramide analogs can lead to modification of the hydrophobic and the hydrophilic part of membrane sphingolipids and alter the properties of membrane proteins.

Sphingolipids are derivatives of ceramide (N-acylsphingosine, compare Chapter 1.4). Most eukaryotic glycolipids and the phospholipid sphingomyelin contain ceramide as the hydrophobic backbone. 

Looking at the barrier in more detail, we find that it can be described as composed of two main components. Interspersed between the corneocytes we find the hydrophobic (water-repellent) substance, the barrier lipids. The keratinized corneocytes containing fibrous and amorphous proteins represent a hydrophilic (water-attracting) component. Neutral lipids (fatty acids, cholesterol) and ceramides dominate the lipid phase, and it is mainly these lipids that are responsible for the control and limitation of water transport through the skin.14 Visualization of the penetration pathway through the skin by tracer methods has demonstrated that the extracellular pathway is likely to be the only route through the barrier for substances other than water.15 Water diffusion through the keratinocytes... 

Several classes of ceramides have been described in human skin.14 Today it is considered that the ceramides are essential for the barrier properties. It has been suggested that the lower amount of ceramides found in stratum corneum in atopic dermatitis26,27 explains the increased TEWL seen in dry atopic skin. In this context it is of special interest to note that part of the long-chain ceramides of the horny layer are covalently bound to the proteins forming the corneocyte envelope.25 This suggests that such lipids constitute anchors of the hydrophobic phase to the corneocytes and thereby add to the cohesion of the cells of the horny layer. 

Another group of naturally occurring lipids with applications in liposome technology is comprised of the sphingophospholipids (mainly sphingomyelin) which are derivatives of ceramides [17]. Sphingomyelin (SM) is found in the outer leaflet of plasma membranes [17] and has many similarities with PC since they both have the same zwitterionic polar group and two hydrophobic acyl chains. 

MDRl is a member of a family of ATP-dependent drug efflux pumps (32). Several of these pumps, including MDRl, have been shown to mediate lipid translocation and phospholipid bilayers (33). This translocation activity has been implicated in part as the mechanism for dmg efflux by which the hydrophobic cytotoxic drug is translocated from the cytosolic to the external leaflet of the plasma membrane bilayer. MDRl first was shown to translocate glucosyl ceramide analogs from the cytosolic to the external leaflet of the plasma membrane (33). However, the major glucosyl ceramide translocation activity of MDRl later... 

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