Importance of interfacial water in permeabilization of ceramide bilayers

The role of structured water at the membrane-aqueous interface in regulating the water permeability functions of the epidermal horny layer (Stratum corneum) is not clear at present. The subject of this review is the effect of perturbed interfacial water structure on the relative water permeability properties in shrinkage experiments of reconstituted vesicle suspensions of ceramides and cholesterol in the gel phase. Water structure was perturbed by a series of reagents known to affect the phase equilibrium of lipid assemblies. Multilamellar bilayers containing synthetic ceramides N-palmitoyl-D-L-dihydrosphingosine, and N-oleoyl-D-sphingosine were used as model membranes. The reagent binding to the ceramide membrane was determined by fluorescence measurements with 1-anilino, 8-naphtalene sulphonate (ANS). A dependence of number of binding sites for ANS on the type of reagent was found. The change of relative water permeability with concentration of reagents was studied. Analysis of these data reveals that the anomalous behaviour of relative water permeability with the concentration of reagents used is due to concentration-dependent structural transitions of the bound water at the membrane-aqueous interface. Different possibilities of interaction of the reagents with the interface are discussed. At the end, it is suggested that the interfacial water in multilamellar ceramide structures may also contribute to the water holding and permeability barrier functions of cell membrane complex within the stratum corneum.