Physicochemical Damage to Liposomal Membrane Induced by Iron- or Copper-Mediated Upid Peroxidation

A carboxyfluorescein (CF)-enveloping soybean phosphatidylcholine liposome was used as a model of physicochemical damage of biomembranes. The liposomes were exposed to a metal-chelate complex [2 mM of ferric nitrilotriacetate (FeNTA) or cupric nitrilotriacetate (CuNTA)] plus a reductant (2 mM of ascorbate or various concentrations of reduced glutathione), and CF release from damaged liposomal membranes and the generation of thiobarbituric acid-reactive substances (TBARS) were measured. In the presence of a reducing agent, both FeNTA and CuNTA stimulated markedly CF release and an increase in the TBARS level, while in the absence of a reducing agent both of the chelate complexes showed little CF release and TBARS. The effects of H2O2 addition to the reaction system containing liposome with FeNTA or CuNTA plus ascorbate were also examined. The CF release was slightly increased by the addition of a smaller dose (0.5 mM) of H2O2 and it was inhibited by 8 mM of H2O2. A similar result was obtained in the TBARS test. These results suggest that FeNTA- or CuNTA-mediated lipid peroxidation can damage liposomal membranes physicochemically, and the redox reaction of the chelated metal itself is more important than a Fenton-type reaction in the process.