Sphingomyelinase in normal human spleens and in spleens from subjects with Niemann-Pick disease.

This paper describes the purification and some of the properties of an enzyme from human spleen that catalyzes the hydrolysis of sphingomyelin with the formation of ceramide and phosphoryl choline. The enzyme, which is located in the subcellular particulate fraction that sediments between 700 and 8500 g, is readily made soluble and has been partially purified. Its pH optimum is between 4.5 and 5.0. It is unaffected by divalent cations, chelating agents, and sulfhydryl reagents, but is inhibited by phosphate. The enzyme attacks sphingomyelin and dihydrosphingomyelin, but is inactive toward sphingosine phosphoryl choline, O-acetylsphingomyelin, and lecithin. In some of its properties, the enzyme from human spleen is different from the previously studied sphingomyelinase from rat tissues. The enzyme is absent or markedly reduced in spleens from patients with classical and visceral varieties of Niemann-Pick disease, but is present in normal amounts in the late infantile type of the disease. In the present study another enzyme, this one magnesium-dependent, capable of catalyzing the cleavage of sphingomyelin has been detected in the spleens of patients with the classical form of Niemann-Pick disease. Some implications of these findings for theories of the metabolic defect in Niemann-Pick disease are discussed.