Mutual modulation of sphingomyelinase and phospholipase A2 activities against mixed lipid monolayers by their lipid intermediates and glycosphingolipids

Sphingomyelinase activity against pure sphingomyelin monolayers is constant up to a surface pressure of 18 mN/m and falls above it. Sphingomyelinase- and phospholipase A₂-mediated phosphohydrolytic pathways are mutually modulated by the presence of their respective substrates and products. At 15 mN/ m non-substrate lipids such as ceramide at a mole fraction of 0·1 in mixed films with the pure substrate, inhibit the sphingomyelinase activity. Ganglioside GM1, another ceramide-containing complex sphingolipid, also inhibits sphingomyelinase activity, while a chemically related glycosphingolipid such as asialo-GM1 has no effect. The activity is unaltered by dipalmitoylphosphatidylcholine and by an equimolar mixture of its products of hydrolysis by phospholipase A₂, fatty acid and lysoderivative, but it is inhibited by only one of them or by dilauroylphosphatidylcholine. Phospholipase A₂ is inhibited by sphingomyelin, and activated by ceramide and by palmitic acid, one of the products of its own phosphohydrolytic reaction.