We have previously demonstrated that there is abnormal expression of
sphingomyelin (
SM) deacylase-like
enzyme in the epidermis of patients with
atopic dermatitis (AD), which results in decreased levels of
ceramides in their involved and uninvolved stratum corneum. For quantitation of the expression of
SM deacylase in AD, we synthesized 16-(9-anthroyloxy) hexadecanoylsphingosylphosphorylcholine or [
palmitic acid-14C] SM and used them as substrates to directly measure the activity of
SM deacylase by detecting the release of labeled
free fatty acid. Direct enzymatic measurements demonstrated that stratum corneum from lesional forearm skin (volar side) of AD patients has an extremely high
SM deacylase activity that is at least five times higher than in the stratum corneum from healthy controls. In stratum corneum from nonlesional skin of AD patients,
SM deacylase activity is still at least three times higher than in healthy controls. In contrast, stratum corneum from
contact dermatitis patients shows levels of
SM deacylase similar to healthy controls. In extracts of whole epidermis biopsies from AD patients,
SM deacylase activities are significantly (3-fold) increased over healthy controls in the particulate fraction, whereas there is no significant difference in the activity of
sphingomyelinase between AD and healthy control. In peripheral blood lymphocytes of AD patients, there is no increase in activity compared with healthy controls, indicating a possibility that the high expression of
SM deacylase is highly associated with the skin of AD patients. These findings suggest that, in contrast to changes in
sphingolipid metabolism due to aging, the hitherto undiscovered
enzyme SM deacylase, is highly expressed in the epidermis of AD patients, and competes with
sphingomyelinase or
beta-glucocerebrosidase for the common substrate SM or
glucosylceramide, which leads to the
ceramide deficiency of the stratum corneum in AD.