Catalyzes the cleavage of the N-acyl linkage of the ceramides (Cers) to yield sphingosine (Sph) and free fatty acid at an optimal pH of 8-9. Also catalyzes the synthesis of Cers from Sph and fatty acid. an N-acylsphing-4-enine + H2O = a fatty acid + sphing-4-enine Binds 1 zinc ion per subunit. Inhibited by EDTA, EGTA and D/L-sphinganine D-erythro-sphingosine. L-erythro-sphingosine is a less powerful inhibitor. Stimulated by glycerophospholipids: cardiolipin is the most effective, followed by phosphatidic acid, phosphatidylethanolamine and phosphatidylglycerol, whereas phosphatidylcholine, lysophosphatidic acid and diacylglycerol are less effective. Optimum pH is 7.5-9.5. Homodimer. Alternate N-termini have been proposed by different authors. It either starts from Asp-25 (PubMed:10593963) or from Leu-27 (PubMed:12821326). Belongs to the neutral ceramidase family.