Labile iron potentiates ascorbate-dependent reduction and mobilization of ferritin iron
Ascorbate mobilizes iron from equine spleen ferritin by two separate processes. Ascorbate alone mobilizes ferritin iron with an apparent Km (ascorbate) ≈1.5 mM. Labile iron >2 μM, complexed with citrate (10 mM), synergises ascorbate-dependent iron mobilization by decreasing the apparent Km (ascorbate) to ≈270 μM and raising maximal mobilization rate by ≈5-fold. Catalase reduces the apparent Km(ascorbate) for both ascorbate and ascorbate+iron dependent mobilization by ≈80%. Iron mobilization by ascorbate alone has a higher activation energy (Ea=45.0±5.5 kJ/mole) than when mediated by ascorbate with labile iron (10 μM) (Ea=13.7±2.2 kJ/mole); also mobilization by iron-ascorbate has a three-fold higher pH sensitivity (pH range 6.0–8.0) than with ascorbate alone. Hydrogen peroxide inhibits ascorbate's iron mobilizing action.
EPR and autochemiluminescence studies show that ascorbate and labile iron within ferritin enhances radical formation, whereas ascorbate alone produces negligible radicals. These findings suggest that iron catalysed single electron transfer reactions from ascorbate, involving ascorbate or superoxide and possibly ferroxidase tyrosine radicals, accelerate iron mobilization from the ferroxidase centre more than EPR silent, bi-dentate two-electron transfers. These differing modes of electron transference from ascorbate mirror the known mono and bidentate oxidation reactions of dioxygen and hydrogen peroxide with di-ferrous iron at the ferroxidase centre. This study implies that labile iron, at physiological pH, complexed with citrate, synergises iron mobilization from ferritin by ascorbate (50–4000 μM). This autocatalytic process can exacerbate oxidative stress in ferritin-containing inflamed tissue.