A. Ethylene Diamine Tetraacetic Acid
Transition metals will promote oxidative reactions by hydrogen abstraction and by hydroperoxide decomposition reactions that lead to the formation of free radicals. Prooxidative metal reactivity is inhibited by chelators. Chelators that exhibit antioxidative properties inhibit metal-catalyzed reactions by one or more of the following mechanims: prevention of metal redox cycling; occupation of all metal coordination sites thus inhibiting transfer of electrons; formation of insoluble metal complexes; stearic hinderance of interactions between metals and oxidizable substrates (e.g., peroxides). The prooxidative/antioxidative properties of a chelator can often be dependent on both metal and chelator concentrations. For instance, ethylene diamine tetraacetic acid (EDTA) can be prooxidative when EDTA:iron ratios are ≤1 and antioxidative when EDTA:iron is ≥1. The prooxidant activity of some metal-chelator complexes is due to the ability of the chelator to increase metal solubility and/or increase the ease by which the metal can redox cycle.
The most common metals chelators used in foods contain multiple carboxylic acid (e.g., EDTA and citric acid) or phosphate groups (e.g., polyphosphates and phytate). Chelators are typicallywater soluble but many also exhibit some solubility in lipids (e.g., citric acid), thus allowing it to inactivate metals in the lipid phase. Chelator activity is pH dependent with a pH below the pKa of the ionizable groups resulting in protonation and loss of metal binding activity. Chelator activity is also decreased in the presence of high concentrations of other chelatable nonprooxidative metals (e.g., calcium), which will compete with the prooxidative metals for binding sites.
B. Metal-Binding Proteins
The reactivity of prooxidant metals in biological tissues are mainly controlled by proteins. Metal binding proteins in foods include transferrin (blood plasma), phosvitin (egg yolk), lactoferrin (milk), and ferritin (animal tissues). Transferrin, phosvitin, and lactoferrin are structurally similar proteins consisting of a single polypeptide chain with a molecular weight ranging from 76,000–80,000. Transferrin and lactoferrin each bind two ferric ions, whereas phosvitin has been reported to bind three. Ferritin is a multisubunit protein (molecular weight of 450,000) with the capability of chelating up to 4500 ferric ions. Transferrin, phosvitin, lactoferrin, and ferritin inhibit iron-catalyzed lipid oxidation by binding iron in its inactive ferric state and, possibly, by sterically hindering metal/peroxide interactions. Reducing agents (ascorbate, cysteine, and superoxide anion) and low pH can cause the release of iron from many of the iron-binding proteins, resulting in an acceleration of oxidative reactions. Copper reactivity is controlled by binding to serum albumin, ceruloplasmin, and the skeletal muscle dipeptide, carnosine.
C. Phytic Acid
Phytic acid or myoinositol hexaphophate is one of the primary metal chelators in seeds where it can be found at concentrations ranging from 0.8–5.3% (Fig. 2). Phytic acid is not readily digested in the human gastrointestinal tract but can be digested by dietary plant phytases and by phytases originating from enteric microorganisms. Phytate is highly phosphorylated, thus, allowing it to form strong chelates with iron, with the resulting iron chelates having lower reactivity. The antioxidant properties of phytic acid are thought to help minimize oxidation in legumes and cereal grains as well as in foods that may be susceptible to oxidation in the digestive tract. Phytic acid has been cited as a preventative agent in iron-mediated colon cancer. Although phytate may be beneficial toward colon cancer, it should be noted that it can potentially have deleterious health effects because of its ability to dramatically decrease the bioavailability of minerals including iron, zinc, and calcium.
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