Epidermal sphingomyelins are precursors for selected stratum corneum ceramides

Epidermal ceramides (Cer) comprise a heterogeneous family of seven species, including two unique omega -hydroxylated Cer, that are key components of t he stratum corneum (SC) intercellular lamellar membranes responsible for th e epidermal permeability barrier. Although both glucosylceramide (GlcCer) a nd the phospho-sphingolipid sphingomyelin (SM) are potential precursors of SC Cer, based on reported chemical structures of epidermal GlcCer and SC Ce r, it is assumed that all major subfractions of SC Cer are generated from l amellar 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, a re 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 i n epidermal SM, Cer generated from epidermal SM by sphingomyelinase treatme nt, Cer within SC, and Cer that persist in Gaucher SC, where GlcCer cannot generate Cer due to an absence of beta -glucocerebrosidase. Using gas chrom atography-mass spectrometry, fast atom bombardment-mass spectrometry, and n uclear magnetic resonance for Cer characterization, epidermal SM comprise t hree major subfractions with distinctive amide-linked (N-acyl) fatty acid ( FA) compositions: that is, either long-chain FA (SM-1; C22-26), Short-chain FA (SM-2; primarily C-16), and short-chain alpha -hydroxy FA (SM-3; C16-18 ). In contrast, only trace quantities of omega -hydroxy FA were present. Fo r each SM subfraction, the sphingoid base was either sphingosine or sphinga nine, but phytosphingosine was not detected. Comparison of these SM with co rresponding sphingomyelinase-generated epidermal Cer and SC Cer revealed th at 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 c orresponding Cer in mammalian SC, that is, Cer 2 and Cer 5, but other Cer s pecies, including the omega -hydroxy Cer species, do not derive from SM.