To clarify mechanisms underlying acylceramide deficiency as an causative factor of the permeability barrier disruption seen in the skin of patients with atopic dermatitis (AD), we hypothesized and then demonstrated the presence of a novel epidermal enzyme, termed glucosylceramide (GC) deacylase. This enzyme hydrolyzes (acyl)GC at the N-acyl site to yield its lysoform, glucosylsphingosine (GS), instead of the formation of (acyl)ceramides by beta-glucocerebrosidase. Assays of enzymatic activity using [palmitic acid-(14)C] GC as a substrate revealed that extracts from the stratum corneum and from the epidermis (but not from the dermis) of patients with AD have the significantly higher potential to hydrolyze GC at the N-acyl site to release (14)C-labeled free fatty acid than of healthy controls. To determine the in vivo physiologic function of this novel enzyme, we measured the metabolic product GS in the upper stratum corneum. In both the involved and the uninvolved stratum corneum from patients with AD, there were significant increases in the amounts of GS compared with healthy controls and there was a significant inverse correlation with the decreased content of ceramides or ceramide-1 (acylceramide). Thus, collectively these results strongly suggest the physiologic relevance of GC deacylase to the acylceramide deficiency seen in the stratum corneum of patients with AD.