Liposomes were prepared from phospholipids extracted from biological membranes. A comparison was made between the peroxidation rate in handshake liposomes and in sonicated liposomes. The smaller sonicated liposomes were more vulnerable to peroxidation, probably because of the smaller radius of curvature, which results in a less dense packing of lipid molecules in the bilayer and a facilitated action of water radicals produced by the X-irradiation. High oxygen enhancement ratios were obtained, especially at low dose rates, suggesting the operation of slowly progressing chain reactions initiated by ionizing radiation. Three compounds were tested for their ability to protect the liposomal membranes against lipid peroxidation. The naturally occurring compounds reduced glutathione (GSH) and vitamin E(alpha-T) and the powerful radiation protector cysteamine (MEA). All three molecules could protect the liposomes against peroxidation. The membrane-soluble compound vitamin E was by far the most powerful. About 50 per cent protection was achieved by using 5 X 10(-6) M alpha-T, 10(-4) M GSH and 5 X 10(-4) M MEA. The fatty acid composition of the lipids altered drastically as a result of the irradiation. Arachidonic acid and docosahexanoic acid were the most vulnerable of the fatty acids. Very efficient protection of these polyunsaturated fatty acids could be obtained with relatively low concentrations of vitamin E built into the membranes.
A comparison has been made between the peroxidation rate as a result of ionizing radiation in liposomes prepared from phospholipids which were extracted from biological membranes, in single component micelles and in micelles of mixed composition. The ease of fatty acid oxidation in the different preparations was studied at a variety of pH values. The damage has been quantified spectrophotometrically in terms of diene conjugation (233 nm) and as the disappearance of fatty acids by gas chromatography. The ease of fatty acid oxidation was in the following order for the liposomal and mixed micelle preparations: 22:6 greater than 20:4 greater than 18:2. For single component micelles the order was reversed: 18:2 greater than 18:3 greater than 20:4 greater than 22:6. The micellar lipid preparations were pH-dependent in their response to radiation, which was demonstrated by a dip in the pH-response curve. Peroxidation of especially 22:6 was promoted when present in mixed micelles with 18:2.
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