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Phosphatidylethanol (Peth) formation catalyzed by the transphosphatidylation activity of phospholipase D was demonstrated to occur in a rat brain synaptosomal enriched preparation. The optimal pH was determined to be 6.5, and the optimal ethanol concentration was determined to be 0.3-0.4 M with an apparent Km of 0.2 M. Peth formation was barely detectable in the absence of an appropriate activator and several unsaturated fatty acids were found to be effective activators. The concentrations of oleic acid required for maximum activation varied with the concentration of exogenous phosphatidylcholine present in the incubation mixtures. All detergents tested were significantly less active than the unsaturated fatty acids and divalent ions were not required for Peth formation. Phosphatidylcholine was the most effective phosphatidyl donor of the phospholipids tested. Peth forming activity was greatest in the synaptic membrane fraction of the various brain subfractions examined. The 12,000 g-100,000 g particulate fraction of lung, heart, and adipose tissue had activities similar to that of brain.

The accumulation of excessive quantities of sphingomyelin in tissues of patients with Niemann-Pick disease was demonstrated by Klenk in 19341, 2 and has been amply confirmed by other investigators.3-5 A study by Crocker and Mays6 indicated that the rate of biosynthesis of sphingomyelin in tissues from these patients appeared to be essentially normal. These findings suggested that the metabolic lesion in this condition might be of a catabolic nature. We have recently obtained evidence for the presence of a specific enzyme in liver tissue which catalyzes the hydrolysis of sphingomyelin.7 The enzyme was partially purified from rat liver tissue, and the products of the reaction were demonstrated to be phosphorylcholine and ceramide. The most highly purified enzyme preparations did not catalyze the hydrolysis of lecithin or phosphatidylethanolamine. Lecithin, however, appeared to be a competitive inhibitor of the reaction. Similar enzymatic activity could be detected in human liver tissue. The present report describes experiments in which the level of the sphingomyelin-cleaving enzyme in liver and kidney tissue obtained from six patients with the classic infantile form of Niemann-Pick disease8 is compared with the values found in a liver biopsy sample from a normal human adult and liver and kidney tissue of patients with various other disorders. Materials and Methods.-Sphingomyelin labeled with C14 in the methyl carbon atoms of the choline portion of the molecule was synthesized as described previously.7 Samples of liver and kidney tissue were homogenized in 10 vol of 0.25 M sucrose with an all-glass TenBroeck homogenizer and centrifuged at 600 X g for 12 min. Suitable aliquots of the supernatant suspensions were