The antioxidant properties of zinc: interactions with iron and antioxidants

Potential mechanisms underlying zinc’s capacity to protect membranes from lipid oxidation were examined in liposomes. Using lipid oxidation initiators with different chemical and physical properties (transition metals, lipid- or water-soluble azo compounds, ultraviolet radiation c (UVc), superoxide radical anion (O2•−), and peroxynitrite (ONOO−) we observed that zinc only prevented copper (Cu2+)- and iron (Fe2+)-initiated lipid oxidation. In the presence of Fe2+, the antioxidant action of zinc depended directly on the negative charge density of the membrane bilayer. An inverse correlation (r2: 0.96) was observed between the capacity of zinc to prevent iron binding to the membrane and the inhibitory effect of zinc on Fe2+-initiated lipid oxidation. The interaction of zinc with the bilayer did not affect physical properties of the membrane, including rigidification and lateral phase separation known to increase lipid oxidation rates. The interactions between zinc and the lipid- (α-tocopherol) and water- (epicatechin) soluble antioxidants were studied. The inhibition of Fe2+-induced lipid oxidation by either α-tocopherol or epicatechin was increased by the simultaneous addition of zinc. The combined actions of α-tocopherol (0.01 mol%), epicatechin (0.5 μM) and zinc (5–50 μM) almost completely prevented Fe2+ (25 μM)-initiated lipid oxidation. These results show that zinc can protect membranes from iron-initiated lipid oxidation by occupying negatively charged sites with potential iron binding capacity. In addition, the synergistic actions of zinc with lipid and water-soluble antioxidants to prevent lipid oxidation, suggests that zinc is a pivotal component of the antioxidant defense network that protects membranes from oxidation.