Effect of Aluminum Ion on Fe²⁺-Induced Lipid Peroxidation in Phospholipid Liposomes under Acidic Conditions

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The effects of Al³⁺ on Fe²⁺-induced lipid peroxidation in phospholipid liposomes consisting of phosphatidylcholine (PC) and phosphatidylserine (PS) were examined under acidic conditions. The stimulatory effect of Al³⁺ on Fe²⁺-induced lipid peroxidation in the liposomes showed a biphasic response against pH variation, and the maximum stimulation was observed around pH 6.0. In addition, it was found that the stimulatory effect of Al³⁺ on the lipid peroxidation was dependent on the proportion of PS in the liposomes. On the other hand, the lipid peroxidation in PC liposomes was not stimulated by the addition of Al³⁺. From these findings, it is suggested that the Al³⁺ effect on Fe²⁺-induced lipid peroxidation under acidic conditions is largely dependent on the phospholipid composition. Trivalent cations such as Tb³⁺ and Ga³⁺ also stimulated Fe²⁺-induced lipid peroxidation in PC/PS liposomes under acidic conditions, but divalent cations (Zn²⁺ and Mn²⁺) showed no stimulatory effect. The extents of Fell disappearance and Fe³⁺ formation during the reaction were enhanced by the addition of Al³⁺ or Ga²⁺, but Tb³⁺ had no effect on Fe²⁺ disappearance. The results with 1, 6-diphenyl-1, 3, 5-hexatriene (DPH) showed that the fluorescence anisotropy of DPH-labeled PC/PS liposomes under acidic conditions was increased by the addition of Al³⁺. Furthermore, there is a relation between the extents of the fluorescence anisotropy of the complex and TBARS production. In contrast, the fluorescence anisotropy of DPH molecules embedded in PC liposomes was not changed by the addition of Al³⁺. Based on these results, a possible mechanism of the stimulatory effect of Al³⁺ on Fe²⁺-induced lipid peroxidation under acidic conditions is discussed.