Converts sphingomyelin to ceramide (PubMed:9660788, PubMed:8706124). Exists as two enzymatic forms that arise from alternative trafficking of a single protein precursor, one that is targeted to the endolysosomal compartment, whereas the other is released extracellularly. However, in response to various forms of stress, lysosomal exocytosis may represent a major source of the secretory form (By similarity). In the lysosomes, converts sphingomyelin to ceramide. Plays an important role in the export of cholesterol from the intraendolysosomal membranes. Also has phospholipase C activities toward 1,2-diacylglycerolphosphocholine and 1,2-diacylglycerolphosphoglycerol (PubMed:27435900). Modulates stress-induced apoptosis through the production of ceramide (PubMed:8706124). When secreted, modulates cell signaling with its ability to reorganize the plasma membrane by converting sphingomyelin to ceramide. Secreted form is increased in response to stress and inflammatory mediators such as IL1B, IFNG or TNF as well as upon infection with bacteria and viruses. Produces the release of ceramide in the outer leaflet of the plasma membrane playing a central role in host defense. Ceramide reorganizes these rafts into larger signaling platforms that are required to internalize P. aeruginosa, induce apoptosis and regulate the cytokine response in infected cells. In wounded cells, the lysosomal form is released extracellularly in the presence of Ca(2+) and promotes endocytosis and plasma membrane repair. (Microbial infection) Secretion is activated by bacteria such as P. aeruginos, this activation results in the release of ceramide in the outer leaflet of the plasma membrane which facilitates the infection. a sphingomyelin + H2O = an N-acylsphing-4-enine + H(+) + phosphocholine H2O + N-(octadecanoyl)-sphing-4-enine-1-phosphocholine = H(+) + N-octadecanoylsphing-4-enine + phosphocholine a 1,2-diacyl-sn-glycero-3-phosphocholine + H2O = a 1,2-diacyl-sn-glycerol + H(+) + phosphocholine 1,2-dihexadecanoyl-sn-glycero-3-phosphocholine + H2O = 1,2-dihexadecanoyl-sn-glycerol + H(+) + phosphocholine Binds 2 Zn(2+) ions per subunit (PubMed:27435900). Hydrolysis of liposomal sphingomyelin is stimulated by incorporation of diacylglycerol (DAG), ceramide and free fatty acids into the liposomal membranes. Phosphatidylcholine hydrolysis is inhibited by incorporation of cholesterol, ceramide, DAG, monoacylglycerol and fatty acids. Monomer (PubMed:27435900). Interacts with SORT1; the interaction is required for SMPD1 targeting to lysosomes (By similarity). The secreted form is induced in a time- and dose-dependent by IL1B and TNF as well as stress and viral infection. This increase of the secreted form seems to be due to exocytosis of the lysosomal form and is Ca(2+)-dependent. Secretion is dependent of phosphorylation at Ser-506. Secretion is induced by inflammatory mediators such as IL1B, IFNG or TNF as well as infection with bacteria and viruses. Proteolytically processed. Mature lysosomal form arises from C-terminal proteolytic processing of pro-sphingomyelin phosphodiesterase. Both lysosomal and secreted forms are glycosylated but they show a differential pattern of glycosylation. Phosphorylated at Ser-506 by PRKCD upon stress stimuli. Phosphorylation is required for secretion. Mutants infected with Pseudomonas aeruginosa die within 7 days whereas all wild-type mice survive the infection (PubMed:12563314). Mutants are defective in radiation-induced apoptosis (PubMed:8706124). There are two types of sphingomyelinases: ASM (acid), and NSM (neutral). Belongs to the acid sphingomyelinase family.