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 t he 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 pre sence of Sulf always led to an increase of the maximum rate of the enz ymatic reaction, irrespective on whether the vesicles were above, in t he range of, or below the bilayer transition temperature. Sulf did not modify the latency period for the reaction that is observed at the bi layer 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 tem perature within the range of the bilayer phase transition, the ganglio sides inhibited the maximal rate of the reaction in proportion to thei r content in the bilayer. However, at the same time the latency period observed with vesicles of pure phospholipid at this temperature was s hortened in proportion to the mole fraction of gangliosides in the bil ayer. At temperatures above the bilayer phase transition, gangliosides stimulated the activity of PLA(2). Preincubation of the enzyme with S ulf or gangliosides did not affect the activity against bilayer vesicl es 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 enzy me 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 o f PLA(2) at the interface itself, beyond the initial steps of enzyme a dsorption and activation, probably through modifications of the interm olecular organization and surface electrostatics of the phospholipid s ubstrate.