Ceramides are the major component of the stratum corneum, accounting for 30%-40% of stratum corneum lipids by weight, and are composed of at least seven molecular groups (designated ceramides 1-7). Stratum corneum ceramides, together with cholesterol and fatty acids, form extracellular lamellae that are responsible for the epidermal permeability barrier. Previous studies indicated that beta-glucocerebrosidase- and sphingomyelinase-dependent ceramide production from glucosylceramides and sphingomyelins, respectively, is important for epidermal permeability barrier homeostasis. A recent study indicated that sphingomyelins are precursors of two stratum corneum ceramide molecular groups (ceramides 2 and 5). In this study, we have examined the role of glucosylceramides in the generation of each of the seven stratum corneum ceramide molecular groups. First, the structures of various glucosylceramide species in human epidermis were determined by gas chromatography-mass spectrometry, fast atom bombardment-mass spectrometry, and nuclear magnetic resonance. The results indicate that total epidermal glucosylceramides are composed of six distinct molecular groups, glucosylceramides 1-6. Glucosylceramide 1 contains sphingenine and nonhydroxy fatty acids, glucosylceramide 2, phytosphingosine and nonhydroxy fatty acids, glucosylceramide 3, phytosphingosine with one double bond and nonhydroxy fatty acids, glucosylceramide 4, sphingenine and alpha-hydroxy fatty acids, glucosylceramide 5, phytosphingosine and alpha-hydroxy fatty acids, and glucosylceramide 6, phytosphingosine with one double bond and alpha-hydroxy fatty acids. The nonhydroxy fatty acids typically have 16-24-carbon-length chains, whereas alpha-hydroxy fatty acids are limited to 24-, 25-, and 26-carbon chains. The sphingosine bases are C18 or C20 chains. Next, acylglucosylceramides and glucosylceramides were treated with beta-glucocerebrosidase and the ceramides released were compared with stratum corneum ceramides. Ceramide moieties of acylglucosylceramides and glucosylceramides 1, 2, 4-6 correspond to stratum corneum ceramides 1-7. These results, together with those of our previous reports characterizing epidermal sphingomyelins, indicate that all ceramide species, including omega-hydroxy fatty-acid-containing ceramides, are derived from glucosylceramides, and fractions of ceramides 2 and 5 are from sphingomyelins. Furthermore, structural analysis of glucosylceramides revealed that human epidermal glycosphingolipids display a unique lipid profile that is rich in very long chain hydroxylated (alpha- and omega-hydroxy) fatty acids and phytosphingosine.

Epidermal ceramides (Cer) comprise a heterogeneous family of seven species, including two unique -hydroxylated Cer, that are key components of the stratum corneum (SC) intercellular lamellar membranes responsible for the epidermal permeability barrier. Although both glucosylceramide (GlcCer) and the phospho-sphingolipid sphingomyelin (SM) are potential precursors of SC Cer, based on reported chemical structures of epidermal GlcCer and SC Cer, it is assumed that all major subfractions of SC Cer are generated from lamellar body-derived GlcCer. Yet, we and others have shown that SM-derived Cer are required for normal barrier homeostasis. Moreover, two pools of SM, one from plasma membrane, the other from lamellar body-derived contents, are potentially available for Cer production. To clarify the role of SM as a potential precursor of bulk or specific SC Cer, we compared Cer moieties in epidermal SM, Cer generated from epidermal SM by sphingomyelinase treatment, Cer within SC, and Cer that persist in Gaucher SC, where GlcCer cannot generate Cer due to an absence of ␤ -glucocerebrosidase. Using gas chromatographymass spectrometry, fast atom bombardment-mass spectrometry, and nuclear magnetic resonance for Cer characterization, epidermal SM comprise three major subfractions with distinctive amide-linked ( N -acyl) fatty acid (FA) compositions: that is, either long-chain FA (SM-1; C 22-26 ), short-chain FA (SM-2; primarily C 16 ), and short-chain ␣ -hydroxy FA (SM-3; C 16-18 ). In contrast, only trace quantities of -hydroxy FA were present. For each SM subfraction, the sphingoid base was either sphingosine or sphinganine, but phytosphingosine was not detected. Comparison of these SM with corresponding sphingomyelinase-generated epidermal Cer and SC Cer revealed that the Cer moieties of SM-1 and SM-3 are equivalent to Cer 2 (NS) and Cer 5 (AS), respectively. Moreover, both Cer 2 and Cer 5 occurred in Gaucher SC, whereas other Cer subfractions did not occur. These results indicate that two epidermal SM, that is, SM-1 and SM-3, are important precursors of two corresponding Cer in mammalian SC, that is, Cer 2 and Cer 5, but other Cer species, including the -hydroxy Cer species, do not derive from SM.

Human epidermis gave two glycolipid bands that migrated faster than glucosylceramide and two bands that migrated like glucosylceramide and galactosylceramide, respectively, on TLC. The two faster migrating glycolipids (GL-I and GL-II), which exhibited alkalilability, were purified by conventional DEAE and silica gel column chromatographies, and further by HPLC on a silica gel column. Structure determination of the two components, named GL-I3 and GL-II3, which were finally purified from GL-I and GL-II, respectively, by HPLC on a reversed phase column, was performed by means of 1H-NMR spectroscopy, fast atom bombardment mass spectrometry, and component analysis involving GLC-mass spectrometry. GL-I3 was determined to be a mixture of glucosyl beta 1-N-(omega-O-linoleoyl)-triacontanoyl- and -dotriacontamonoenoyl-eicosasphingenine, and one of the two components of GL-II3 was determined to be glucosyl beta 1-N-(omega-O-linoleoyl)triacontanoyl-trihydroxyeicosasphingenin e. GL-I3 and GL-II3 were the major components of GL-I and GL-II, respectively, and both the latter contained additional four components, which were heterogeneous as to the ceramide portion. This paper reports the structures of acylglucosylceramides isolated from human epidermis together with 1H-NMR spectra and mass spectra demonstrating their molecular weights. The structure of molecular species containing trihydroxysphingosine having a double bond is novel.

Distinct GlcCer pools segregated to those that were either hydrolysable or nonhydrolysable in differentiated KCs. We assume that the latter pool appears to be LB enriched, and also sequestrates acylGlcCer from other cellular membrane fractions.