The effect of photodynamic action on leakage of ions through liposomal membranes that contain oxidatively modified lipids

Singlet oxygen, created in photosensitization, peroxidizes unsaturated fatty acids of the membrane's lipids. This generates alcoholic or aldehyde groups at double bonds' breakage points. In a previous study, we examined the leakage of a K⁺-induced cross-membrane electric potential of liposomes that undergo photosensitization. The question remains to what extent peroxidized lipids can compromise the stability of the membrane. In this study, we studied the effect of the oxidatively modified lipids PGPC and ALDOPC in the membrane on its stability, by monitoring the membrane electric potential with the potentiometric dye DiSC₂(5). As the content of the modified lipids increases the membrane becomes less stable, and even at just 2% of the modified lipids the membrane's integrity is affected, in respect to the leakage of ions through it. When the liposomes that contain the modified lipids undergo photosensitization by hematoporphyrin, the lipid bilayer becomes even more unstable and passage of ions is accelerated. We conclude that the existence of lipids with a shortened fatty acid that is terminated by a carboxylic acid or an aldehyde and more so when photosensitized damage occurs to unsaturated fatty acids in lecithin, add up to a critical alteration of the membrane, which becomes leaky to ions. We measured the integrity of the liposomal membrane when it includes oxidatively modified lipids and when it is exposed to photodynamic action by photosensitization with hematoporphyrin. We demonstrate that the photodynamic process, in which unsaturated lipids of natural lecithin are damaged, is not causing the lipid bilayer to leak, but it strongly accelerates the leakage of ions if a very small fraction of the natural lipids are replaced by the modified lipids.