The mechanism of Fe2+-initiated lipid peroxidation in liposomes: the dual function of ferrous ions, the roles of the pre-existing lipid peroxides and the lipid peroxyl radical

摘要

pThe mechanism of Fesup2+/sup-initiated lipid peroxidation in a liposomal system was studied. It was found that a second addition of ferrous ions within the latent period lengthened the time lag before lipid peroxidation started. The apparent time lag depended on the total dose of Fesup2+/sup whenever the second dose of Fesup2+/sup was added, which indicates that Fesup2+/sup has a dual function: to initiate lipid peroxidation on one hand and suppress the species responsible for the initiation of the peroxidation on the other. When the pre-existing lipid peroxides (LOOH) were removed by incorporating triphenylphosphine into liposomes, Fesup2+/sup could no longer initiate lipid peroxidation and the acceleration of Fesup2+/sup oxidation by the liposomes disappeared. However, when extra LOOH were introduced into liposomes, both enhancement of the lipid peroxidation and shortening of the latent period were observed. When the scavenger of lipid peroxyl radicals (LOOsupP/sup), iN/i,iN/i′-diphenyl-ip/i-phenylene-diamine, was incorporated into liposomes, neither initiation of the lipid peroxidation nor acceleration of the Fesup2+/sup oxidation could be detected. The results may suggest that both the pre-existing LOOH and LOOsupP/sup are necessary for the initiation of lipid peroxidation. The latter comes initially from the decomposition of the pre-existing LOOH by Fesup2+/sup and can be scavenged by its reaction with Fesup2+/sup. Only when Fesup2+/sup is oxidized to such a degree that LOOsupP/sup is no longer effectively suppressed does lipid peroxidation start. It seems that by taking the reactions of Fesup2+/sup with LOOH and LOOsupP/sup into account, the basic chemistry in lipid peroxidation can explain fairly well the controversial phenomena observed in Fesup2+/sup-initiated lipid peroxidation, such as the existence of a latent period, the critical ratio of Fesup2+/sup to lipid and the required oxidation of Fesup2+/sup./p