Sphingomyelin synthases synthesize the sphingolipid, sphingomyelin, through transfer of the phosphatidyl head group, phosphatidylcholine, on to the primary hydroxyl of ceramide. The reaction is bidirectional depending on the respective levels of the sphingolipid and ceramide. Golgi apparatus SMS1 directly and specifically recognizes the choline head group on the substrate, requiring two fatty chains on the choline-P donor molecule in order to be recognized efficiently as a substrate. Major form in macrophages. Required for cell growth in certain cell types. Suppresses BAX-mediated apoptosis and also prevents cell death in response to stimuli such as hydrogen peroxide, osmotic stress, elevated temperature and exogenously supplied sphingolipids. May protect against cell death by reversing the stress-inducible increase in levels of proapoptotic ceramide (By similarity). a 1,2-diacyl-sn-glycero-3-phosphocholine + an N-acylsphing-4-enine = a 1,2-diacyl-sn-glycerol + a sphingomyelin Belongs to the sphingomyelin synthase family. Unusual initiator. The initiator methionine is coded by a non-canonical CTG leucine codon. Unusual initiator. The initiator methionine is coded by a non-canonical CTG leucine codon.