REGULATION BY GANGLIOSIDES AND SULFATIDES OF PHOSPHOLIPASE A(2) ACTIVITY AGAINST DIPALMITOYLPHOSPHATIDYLCHOLINE AND DILAUROYLPHOSPHATIDYLCHOLINE IN SMALL UNILAMELLAR BILAYER VESICLES AND MIXED MONOLAYERS

The modulation by gangliosides GM1 and GD1a, and sulfatide (Sulf) of the activity of porcine pancreatic phospholipase A(2) was studied with small unilamellar vesicles of dipalmitoylphosphatidylcholine (L-dpPC) and lipid monolayers of dilauroylphosphatidylcholine (L-dlPC). The presence of Sulf always led to an increase of the maximum rate of the enzymatic reaction, irrespective on whether the vesicles were above, in the range of, or below the bilayer transition temperature. Sulf did not modify the latency period for the reaction that is observed at the bilayer phase transition temperature. Gangliosides inhibited the maximum rate of enzymatic activity against bilayer vesicles in the gel phase but the effect was complex. When the reaction was carried out at a temperature within the range of the bilayer phase transition, the gangliosides inhibited the maximal rate of the reaction in proportion to their content in the bilayer. However, at the same time the latency period observed with vesicles of pure phospholipid at this temperature was shortened in proportion to the mole fraction of gangliosides in the bilayer. At temperatures above the bilayer phase transition, gangliosides stimulated the activity of PLA(2). Preincubation of the enzyme with Sulf or gangliosides did not affect the activity against bilayer vesicles of pure substrate. These glycosphingolipids did not modify the rate or extent of desorption of the enzyme from the interface, nor the pre-catalytic steps for the interfacial activation of PLA(2), or the enzyme affinity for the phospholipid substrate. Also, the activity of the enzyme was not altered irreversibly by glycosphingolipids. Our results indicate that Sulf and gangliosides modulate the catalytic activity of PLA(2) at the interface itself, beyond the initial steps of enzyme adsorption and activation, probably through modifications of the intermolecular organization and surface electrostatics of the phospholipid substrate.