Journal Club – 22.07.08

Sphingomyelin Deacylase, the Enzyme Involved in the Pathogenesis of Atopic Dermatitis, Is Identical to the β-Subunit of Acid Ceramidase

Yasuhiro Teranishi 1,†,‡, Hiroshi Kuwahara 1,†,§, Masaru Ueda 1, Tadashi Takemura 1, Masanori Kusumoto 1,§, Keiji Nakamura 1, Jun Sakai 1, Toru Kimura 1, Yasuji Furutani 1, Makoto Kawashima 2, Genji Imokawa 3,* and Mari Nogami-Itoh 4,*

Abstract: A ceramide deficiency in the stratum corneum (SC) is an essential etiologic factor for the
dry and barrier-disrupted skin of patients with atopic dermatitis (AD). Previously, we reported that
sphingomyelin (SM) deacylase, which hydrolyzes SM and glucosylceramide at the acyl site to yield
their lysoforms sphingosylphosphorylcholine (SPC) and glucosylsphingosine, respectively, instead of
ceramide and/or acylceramide, is over-expressed in AD skin and results in a ceramide deficiency.
Although the enzymatic properties of SM deacylase have been clarified, the enzyme itself remains
unidentified. In this study, we purified and characterized SM deacylase from rat skin. The activities
of SM deacylase and acid ceramidase (aCDase) were measured using SM and ceramide as substrates
by tandem mass spectrometry by monitoring the production of SPC and sphingosine, respectively.
Levels of SM deacylase activity from various rat organs were higher in the order of skin > lung >
heart. By successive chromatography using Phenyl-5PW, Rotofor, SP-Sepharose, Superdex 200 and
Shodex RP18-415, SM deacylase was purified to homogeneity with a single band of an apparent
molecular mass of 43 kDa with an enrichment of > 14,000-fold. Analysis by MALDI-TOF MS/MS
using a protein spot with SM deacylase activity separated by 2D-SDS-PAGE allowed its amino acid
sequence to be determined and identified as the β-subunit of aCDase, which consists of α- and
β-subunits linked by amino bonds and a single S-S bond. Western blotting of samples treated with
2-mercaptoethanol revealed that, whereas recombinant human aCDase was recognized by antibodies
to the α-subunit at ~56 kDa and ~13 kDa and the β-subunit at ~43 kDa, the purified SM deacylase was
detectable only by the antibody to the β-subunit at ~43 kDa. Breaking the S-S bond of recombinant
human aCDase with dithiothreitol elicited the activity of SM deacylase with ~40 kDa upon gel chromatography. These results provide new insights into the essential role of SM deacylase expressed as an aCDase-degrading β-subunit that evokes the ceramide deficiency in AD skin.

Keywords: atopic dermatitis; ceramide; ceramide deficiency; barrier function; water reservoir faction;
stratum corneum; sphingomyelin deacylase; sphingosylphosphorylcholine; acid ceramidase