A model membrane approach to the epidermal permeability barrier: an X-ray diffraction study

The permeability of mammalian skin is determined in large part by lamellar lipid domains packed between cells of the upper layer of the epidermis, the stratum comeum. Although these lamellae have features in common with typical biological membranes, they differ in having a lipid population composed mainly of ceramides, cholesterol, and free fatty acids. In our initial studies of the relationship between lipid composition and phase behavior in this unusual system, we used deuterium NMR [Kitson et al. (1994) Biochemistry 33, 6707-6715] to examine aqueous dispersions of nonhydroxylated bovine brain ceramide, cholesterol, and perdeuterated palmitic acid, and found complex phase behavior as a function of temperature and pH, whereas analogous dispersions in which sphingomyelin replaced ceramide resulted in spectra consistent with a fluid lamellar phase under the same conditions. To extend these observations, we examined the same dispersions at pH 5.2 by means of X-ray diffraction. The significant findings are as follows: (1) the ceramide dispersions form complex crystalline phases between room temperature and about 40 degrees C; (2) the majority of the crystalline cholesterol is not in a separate phase; and (3) the analogous sphingomyelin dispersions form a fluid lamellar phase under the same conditions. We conclude that ceramides, even in the presence of considerable mole fractions of cholesterol, can form crystalline lamellar structures. We suggest that the existence of such structures in stratum corneum may be important in the function of the epidermal permeability barrier, and that the interaction between ceramide and cholesterol in other biological membranes may result in regions having unique physical properties.