Redox Properties and Activity of Iron–Citrate Complexes: Evidence for Redox Cycling

Iron in iron overload disease is present as non-transferrin-bound iron, consisting of iron, citrate, and albumin. We investigated the redox properties of iron citrate by electrochemistry, by the kinetics of its reaction with ascorbate, by ESR, and by analyzing the products of reactions of ascorbate with iron citrate complexes in the presence of H₂O₂ with 4-hydroxybenzoic acid as a reporter molecule for hydroxylation. We report −0.03 V < <i>E</i>°′ > +0.01 V for the (Fe³⁺–cit/Fe²⁺–cit) couple. The first step in the reaction of iron citrate with ascorbate is the rapid formation of mixed complexes of iron with citrate and ascorbate, followed by slow reduction to Fe²⁺–citrate with <i>k</i> = ca. 3 M^–1 s^–1. The ascorbyl radical is formed by iron citrate oxidation of Hasc^– with <i>k</i> = ca. 0.02 M^–1 s^–1; the majority of the ascorbyl radical formed is sequestered by complexation with iron and remains EPR silent. The hydroxylation of 4-hydroxybenzoic acid driven by the Fenton reduction of iron citrate by ascorbate in the presence of H₂O₂ proceeds in three phases: the first phase, which is independent of the presence of O₂, is revealed as a nonzero intercept that reflects the rapid reaction of accumulated Fe²⁺ with H₂O₂; the intermediate oxygen-dependent phase fits a first-order accumulation of product with <i>k</i> = 5 M^–1 s^–1 under aerobic and <i>k</i> = 13 M^–1 s^–1 under anaerobic conditions; the slope of the final linear phase is ca. <i>k</i> = 5 × 10^–2 M^–1 s^–1 under both aerobic and anaerobic conditions. Product yields under aerobic conditions are greater than predicted from the initial concentration of iron, but they are less than predicted for continuous redox cycling in the presence of excess ascorbate. The ongoing formation of hydroxylated product supports slow redox cycling by iron citrate. Thus, when H₂O₂ is available, iron–citrate complexes may contribute to pathophysiological manifestations of iron overload diseases