The inhibition of diacylglycerol-stimulated intracellular phospholipases by phospholipids with a phosphocholine-containing polar group. A possible physiological role for sphingomyelin.

Phosphatidylinositol phosphodiesterase activated by diacylglycerol is substantially inhibited by all phospholipids containing a phosphocholine head group, including phosphatidylcholine, hydrogenated phosphatidylcholine, choline plasmalogen, lysophosphatidylcholine, lysocholine plasmalogen, sphingomyelin and sphingosylphosphocholine. The sphingosine-containing phospholipids are the most inhibitory. Phosphatidic acid does not inhibit, and phosphatidylethanolamine activates the hydrolysis still further. Sphingomyelin is highly inhibitory to a diacylglycerol-stimulated intestinal mucosal phospholipase A2, or a liver lysosomal phospholipase A1 + A2, both hydrolysing a phosphatidylcholine substrate. Sphingomyelin [20% molar (20 mol of sphingomyelin/80 mol of phosphatidylethanolamine)] activates phosphatidylethanolamine hydrolysis by intestinal mucosal phospholipase A2, and then at higher concentrations (40% molar) substantially inhibits the activity. The results are discussed in relation to possible molecular reorganizations brought about in the hydrated phospholipid substrate complex, and in particular the possible stabilizing role of sphingomyelin in the maintenance of membrane structure, and hence in the modulation of phospholipase activity.