Effect of membrane composition on lipid oxidation in liposomes.

To study the effect of membrane composition on the oxidation of liposomes, different systems were prepared by adding one component at time to phosphatidylcholine (Epikuron 200). In particular, the effect of cholesterol and its ester, cholesterol stearate, on membrane structure and oxidation was studied. A first screening of the structure and net charge of the different preparation was made by means of z-potential and size measurements. Then the liposomes were oxidized by using a hydrophilic radical initiator, the (2,2-azobis(2-amidinopropane) hydrochloride, AAPH, which thermally decomposes to give a constant radical flux in water. The oxidation of liposomes, monitored by following the absorbance of the primary products of oxidation at 234 nm, was shown to be dependent on the composition of the liposomal bilayer and so on its biophysical properties. In addition, size and z-potential measurements gathered in the time course of the peroxidation reaction, revealed that the oxidation induced a modification of the superficial characteristics of the membrane bilayer so as to change its charge at the shear plane (z-potential). This behaviour was shared by all liposomal preparations independent of the composition. The change in sizes of the different liposomal preparation, instead, followed different trends, being more stable both in control samples and in oxidized ones when cholesterol was present. From the analysis of the results, it can be concluded that cholesterol affects the oxidation induced by hydrophilic radical initiator of model membranes by changing the biophysical properties of the phospholipid bilayer. The rigidity induced by cholesterol at temperatures above the T(m) makes the membrane more resistant to radical attack from an external aqueous phase and this in turn delays the start of the reaction. The decrease of z-potential of the liposomal particles induced by the oxidation process can be an important clue to understand the mechanisms involved in the etiology of important diseases.