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-(hexadecanoyl)-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 approximately 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 approximately 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.