Biosynthesis of Acylceramide in Murine Epidermis: Characterization by Inhibition of Glucosylation and Deglucosylation, and by Substrate Specificity

We have investigated the physiological significance of the glucosylation of ceramides and the subsequent deglucosylation of glucosylceramide in the synthetic pathway of acylceramide. In this metabolic pathway using [ 14 C]-serine in organ culture, newborn murine (BALB/c) epidermis synthesizes several types of ceramides, including acylceramide, as analyzed by thin-layer chromatography. When conduritol-B-epoxide, a specific inhibitor of b-glucocerebrosidase, was added to the culture medium, the synthesis of acylceramide was significantly suppressed in concert with a significant increase in acylglucosylceramide. Furthermore, addition of D-threo-1-phenyl2-decanoylamino-3-morpholino-1-propanol, an inhibitor of glucosyltransferase, also specifically abolished the synthesis of acylceramide whereas non-acylated ceramides were relatively less affected. We further determined whether the physiological substrate of glucosyltransferase is x-hydroxyceramide (C30) or non-x-hydroxylated ceramides. Of those, only non-x-hydroxylated ceramides proved to be good substrates for glucosyltransferase in vitro. Our parallel in vitro study also demonstrated that murine epidermis contains enzymatic activity by which x-hydroxyglucosylceramide or x-hydroxyceramide can be converted to acylglucosylceramide or acylceramide. Collectively, these findings indicate that the majority of acylceramides found in the stratum corneum may be synthesized through a distinct sequence of enzymatic reactions consisting of the glucosylation of ceramides by glucosyltransferase, x-hydroxylation of glucosylceramide, the acylation of x-hydroxyglucosylceramide (possibly by an x-acyltransferase), and the deglucosylation of acylglucosylceramide by b-glucocerebrosidase.