A marked stimulation of Fe2+-initiated lipid peroxidation in phospholipid liposomes by a lipophilic aluminum complex, aluminum acetylacetonate.

In the present study, the efficacy of a lipophilic Al complex, aluminum acetylacetonate, as a stimulator of Fe2+-initiated lipid peroxidation in phospholipid liposomes was examined, and results were compared with those from the liposomes treated with AlCl3. The extent of lipid peroxidation was assessed by the formation of thiobarbituric acid-reactive substances (TBARS). The results indicated that the stimulatory effect of Al complex on Fe2+-initiated lipid peroxidation in phosphatidylcholine liposomes was more effective than that of AlCl3 under the same conditions. The concentration dependence of Al complex on TBARS production showed that the concentration of the complex required to induce half-maximal stimulation of TBARS production was 43 microM. In contrast, the stimulatory effect of AlCl3 was not observed until the AlCl3 concentration is increased above 300 microM. In addition, it was found that there is a linear relationship between the TBARS values and the residual amounts of Fe2+ at an earlier stage (within 2 min after the addition of Fe2+) of the lipid peroxidation in PC liposomes with different concentrations of Al complex, suggesting that Fe2+ oxidation process is closely related to the stimulatory effect of Al complex. The stimulatory effect of Al complex upon the lipid peroxidation completely disappeared by treatment of Al complex-treated liposomes with Triton X-100. The results of fluorescence anisotropy measurements using 12-(9-anthroyloxy)stearic acid-labeled liposomes suggested that treatment of the liposomes with Al complex caused a decrease in their lipid fluidity. Furthermore, it was found that there is a correlation between the extents of the fluorescence anisotropy and the Fe2+ oxidation parameters in the liposomes with different concentrations of Al complex. From these results, it is suggested that the Al effect on Fe2+-initiated lipid peroxidation in the phospholipid liposomes is markedly enhanced by incorporation of Al complex into the liposomal membranes and that an acceleration of Fe2+ oxidation due to a strengthened packing between the acyl chains in the lipid layer may be one possible mechanism for the occurrence of a marked stimulatory effect of Al complex on Fe2+ initiated lipid peroxidation.