Comparative Hydrolysis of Sphingomyelin and 2-N-(Hexadecanoyl)-Amino-4-Nitrophenyl-Phosphorylcholine by Normal Human Brain Homogenate at Acid and Neutral pH

Hydrolysis of sphingomyelin and 2-N-(hexade-canoyl)-amino-4-nitrophenyl-phosphorylcholine (HDA-PC), a synthetic analogue of sphingomyelin, by acid and Mg-dependent neutral sphingomyelinases was tested with a homogenate of normal human brain cortex. Results demonstrated quite different substrate specificities for these enzymes. Acid sphingomyelinase, which is neither activated by MgCl₂ nor inhibited by EDTA, hydrolyzed both substrates (the hydrolysis ratio of HDA-PC to sphingomyelin is ?2). In contrast, Mg-dependent neutral sphingomyelinase, which is inhibited by EDTA and reactivated by MgCl₂, hydrolyzed only sphingomyelin (the hydrolysis ratio of HDA-PC to sphingomyelin is ?0-0.05). This synthetic substrate seems to be useful for selective determination of acid sphingomyelinase and for avoiding interference of Mg-dependent neutral sphingomyelinase.