Sphingomyelin synthases (SM synthase or SMS) synthesize the sphingolipid sphingomyelin (SM) through transfer of the phosphatidyl head group of 1,2-diacyl-sn-glycero-3-phosphocholine (phosphatidylcholine, PC) on to the primary hydroxyl of ceramide (N-acylsphingoid base), yielding 1,2-diacyl-sn-glycerol (diacylglycerol, DAG) as a side product. Functions as a bidirectional lipid cholinephosphotransferases capable of converting PC and ceramide to SM and DAG and vice versa depending on the respective levels of ceramide and DAG as phosphocholine acceptors, respectively. a 1,2-diacyl-sn-glycero-3-phosphocholine + an N-acylsphing-4-enine = a 1,2-diacyl-sn-glycerol + a sphingomyelin a 1,2-diacyl-sn-glycero-3-phosphocholine + N-acyl-15-methylhexadecasphing-4-enine = a 1,2-diacyl-sn-glycerol + N-acyl-15-methylhexadecasphing-4-enine-1-phosphocholine Lipid metabolism; sphingolipid metabolism. Belongs to the sphingomyelin synthase family.