The effect of phosphatidic acid on the radiation-initiated peroxidation of phosphatidylcholine in liposomes

It was found that the incorporation of anionic dipalmitoyl phosphatidic acid (DPPA) into phosphatidylcholine (PC) liposomes up to 15 molar % was accompanied by the intensification of accumulation of diene conjugates (DC), which are primary products of lipid peroxidation (LPO), if the LPO was initiated by γ-irradiation of a 137Cs source. Monoethyl ester of DPPA, phosphatidylethanol (DPPEt), exerted a lesser influence at the same concentrations. Ca2+2 ions inhibited the DC production not only in liposomes consisting of lipid mixture but in lipid membranes of PC alone as well. It was assumed that the electrostatic repulsion of negatively, charged DPPA and DPPEt resulted in the loosening of polar-side region of membrane hydrophobic layer and in consequence the access of hydroxyl radicals to hydrocarbon chains of PC. This assumption is in good agreement with the results of osmotic behavior of liposomes in hypertonic urea solution.