Sphingomyelin phosphodiesterase (sphingomyelinase) that converts sphingomyelin (N-acyl-sphingoid-1-phosphocholine) to ceramide (N-acyl-sphingoid base) and phosphocholine at acidic pH. Displays its enzymatic activity when secreted. May play distinct roles in signaling. a sphingomyelin + H2O = an N-acylsphing-4-enine + H(+) + phosphocholine H2O + N-acyl-15-methylhexadecasphing-4-enine-1-phosphocholine = H(+) + N-acyl-15-methylhexadecasphing-4-enine + phosphocholine Binds 2 Zn(2+) per subunit. Lipid metabolism; sphingolipid metabolism. Only the secreted form is enzymatically active. Preferentially expressed in postembryonic development. Requires Zn(2+) to be fully active. There are two types of sphingomyelinases: asm (acid), and nsm (neutral). Only acid sphingomyelinases have been found in worms. Belongs to the acid sphingomyelinase family.