Lipid peroxidation in egg phosphatidylcholine liposomes: comparative studies on the induction systems Fe2+/ascorbate and Fe(3+)-chelates/xanthine-xanthine oxidase.

Two typical systems of lipid peroxidation in egg yolk phosphatidylcholine (egg PC) liposomes were compared: an enzymic system involving superoxide (O2) generated by xanthine (X), xanthine oxidase (XO) and Fe(3+)-chelates (Fe(3+)-ADP and Fe(3+)-EDTA), and a non-enzymic system involving ascorbic acid (ASA) and Fe2+. These two systems exhibited a different pH-dependence: the rate in the enzymic system was maximal at pH 8-8.5, whereas that in the non-enzymic system was high below pH 7.4 and low above pH 7.6. The rates of lipid peroxidation differed with the membrane charge, and this charge-dependent phenomenon differed in the two peroxidation systems: in the Fe(3+)-chelates/X-XO-system, the rate was slow in neutrally charged egg PC liposomes and rapid in egg PC liposomes containing negatively charged dicetylphosphate (DCP) or positively charged stearylamine (SA), whereas in the Fe2+/AsA-system, the rate was rapid in neutral egg PC liposomes but no lipid peroxidation occurred in egg PC liposomes charged with DCP or SA. The decomposition rate of the hydroperoxide of PC (PC-OOH) incorporated into dimyristoyl-phosphatidylcholine (DMPC) liposomes differed depending on the membrane charge in the two systems and this charge-dependence of the rates correlated well with that of the initiation rate of lipid peroxidation dependent on membrane charge. In the Fe2+/AsA-system, lipid peroxidation depended on the endogenous presence of PC-OOH, and the amounts of PC-OOH required for initiation of the reaction differed depending on the membrane charge. However, in the Fe(3+)-chelates/X-XO-system, lipid peroxidation occurred very slowly in the absence of PC-OOH, but rapidly in its presence.