Lysosomal ceramidase that hydrolyzes sphingolipid ceramides into sphingosine and free fatty acids at acidic pH (By similarity). Ceramides, sphingosine, and its phosphorylated form sphingosine-1-phosphate are bioactive lipids that mediate cellular signaling pathways regulating several biological processes including cell proliferation, apoptosis and differentiation (By similarity). Has a higher catalytic efficiency towards C12-ceramides versus other ceramides (By similarity). Also catalyzes the reverse reaction allowing the synthesis of ceramides from fatty acids and sphingosine (By similarity). For the reverse synthetic reaction, the natural sphingosine D-erythro isomer is more efficiently utilized as a substrate compared to D-erythro-dihydrosphingosine and D-erythro-phytosphingosine, while the fatty acids with chain lengths of 12 or 14 carbons are the most efficiently used (By similarity). Has also an N-acylethanolamine hydrolase activity (By similarity). By regulating the levels of ceramides, sphingosine and sphingosine-1-phosphate in the epidermis, mediates the calcium-induced differentiation of epidermal keratinocytes (By similarity). Also indirectly regulates tumor necrosis factor/TNF-induced apoptosis (By similarity). By regulating the intracellular balance between ceramides and sphingosine, in adrenocortical cells, probably also acts as a regulator of steroidogenesis (By similarity). an N-acylsphing-4-enine + H2O = a fatty acid + sphing-4-enine H2O + N-dodecanoylsphing-4-enine = dodecanoate + sphing-4-enine H2O + N-tetradecanoylsphing-4-enine = sphing-4-enine + tetradecanoate H2O + N-hexadecanoylsphing-4-enine = hexadecanoate + sphing-4-enine H2O + N-octadecanoylsphing-4-enine = octadecanoate + sphing-4-enine H2O + N-dodecanoyl-(4R)-hydroxysphinganine = (4R)-hydroxysphinganine + dodecanoate H2O + N-(dodecanoyl)-sphinganine = dodecanoate + sphinganine H2O + N-(acetyl)-sphing-4-enine = acetate + sphing-4-enine H2O + N-(hexanoyl)sphing-4-enine = hexanoate + sphing-4-enine H2O + N-octanoylsphing-4-enine = octanoate + sphing-4-enine H2O + N-(9Z-octadecenoyl)-sphing-4-enine = (9Z)-octadecenoate + sphing-4-enine H2O + N-dodecanoylethanolamine = dodecanoate + ethanolamine Lipid metabolism; sphingolipid metabolism. Heterodimer; disulfide-linked. The heterodimer is composed of the disulfide-linked alpha and beta chains produced by autocatalytic cleavage of the precursor. Secretion is extremely low and localization to lysosomes is mannose-6-phosphate receptor-dependent. N-glycosylated. Proteolytically cleaved into two chains alpha and beta that remain associated via a disulfide bond (PubMed:29692406). Cleavage gives rise to a conformation change that activates the enzyme. The same catalytic Cys residue mediates the autoproteolytic cleavage and subsequent hydrolysis of lipid substrates. The beta chain may undergo an additional C-terminal processing (By similarity). Belongs to the acid ceramidase family.