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Phospholipase A2 hydrolysis of dioleoylphosphatidylcholine, dimyristoylphosphatidylcholine or their equimolar mixture with sphingomyelin in liposomes exposed to gamma-radiation has been investigated. Stabilization of the gamma-irradiated lipid bilayer to hydrolysis by sphingomyelin has been shown.

Stability of liposomes with varied phospholipid (PL) content towards the action of phospholipase A2 (PLA2) was studied in vitro. Liposomes were prepared of phosphatidylcholine (PC), phosphatidylglycerol (PG), phosphatidylethanolamine (PE), and phosphatidylinositol (PI) and their mixtures with sphingomyelin (SM). PC and PG liposomes were also treated with PLA2 in the presence of a number of proteins such as cytochrome c, cytochrome b5, cytochrome P-450, polylysine, and histone H1. Among all the liposomes studied PG-containing ones were found to be most stable towards PLA2. In SM-containing liposomes, the levels of PG, PI, PE and PC after 20 min of hydrolysis were 65, 62, 55 and 50%, respectively. In PC + PG liposomes, the PC content after treatment with PLA2 was by about 15-20% higher than in control PC liposomes. The addition of all the proteins studied influenced significantly stability of both PC and PG in two-component PL liposomes, but had no effect on liposomes consisting only of PC. The incorporation of the integral membrane protein cytochrome P-450 into PC and PG liposomes caused a decrease in their stability towards PLA2, the decrease depending on the lipid/protein molar ratio.

Phospholipase A2 hydrolysis of neutral and negatively charged lipid membranes modified by positively charged proteins has been studied using liposomes composed of either dioleoylphosphatidylcholine (DOPC) or dioleoylphosphatidylglycerol (DOPG) alone or their equimolar mixture in the presence of cytochrome c, histone H1, cytochrome b5, and polylysine. Twenty minutes after the reaction had been initiated, DOPC hydrolysis was 58%, while that in the equimolar mixture with DOPG was 35%. DOPG hydrolysis was more complete in binary mixtures of liposomes. The same was observed for liposomes in the presence of cytochrome c. Hydrolysis of phospholipids in binary liposomes in the presence of histone H1 was 3 times faster than that in protein-free liposomes. In the presence of polylysine the rate of DOPG hydrolysis was decreased. The results obtained are suggestive of electrostatic interactions between hydrophilic proteins and negatively charged phospholipids, the phospholipase A2 catalytic activity being affected by these interactions.