Labile iron potentiates ascorbate-dependent reduction and mobilization of ferritin iron

Abstract

Ascorbate mobilizes iron from equine spleen ferritin by two separate processes. Ascorbate alone mobilizes ferritin iron with an apparent K ≈1.5mM. Labile iron >2μM, complexed with citrate (10mM), synergises ascorbate-dependent iron mobilization by decreasing the apparent K to ≈270μM and raising maximal mobilization rate by ≈5-fold. Catalase reduces the apparent K for both ascorbate and ascorbate+iron dependent mobilization by ≈80%. Iron mobilization by ascorbate alone has a higher activation energy (E=45.0±5.5kJ/mole) than when mediated by ascorbate with labile iron (10μM) (E=13.7±2.2kJ/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 (50-4000μM). This autocatalytic process can exacerbate oxidative stress in ferritin-containing inflamed tissue.

Ascorbic acid metabolism and functions: A comparison of plants and mammals

“…Fe deficiency in the green alga Chlamydomonas causes a very large increase in ascorbate and expression of the homologue of the Arabidopsis biosynthesis gene VTC2 , suggesting a role in iron uptake [33] . Ascorbate may also play a role in facilitating Fe uptake and homeostasis in mammals [34] , [35] .…”

Iron, Ferritin, Hereditary Ferritinopathy, and Neurodegeneration

“…Several alternative native ferritin iron release mechanisms that do not require complex cellular responses, but are instead more direct pathways centered on ferritin interaction with locally available cytosolic effector molecules, have been proposed from in vitro studies (Boyer et al, 1988;Crichton, 2009;Melman et al, 2013;Badu-Boateng et al, 2017;Johnson et al, 2017;Koochana et al, 2018). Most of the latter do however require (1) reduction of the iron mineral stored in the interior of the ferritin, (2) an iron chelator, and (3) a pathway through which the chelator and iron-chelator complex can pass through the shell.…”

Responses of common bean ( Phaseolus vulgaris L.) plants to treatment with industrial ceramic-waste water sludge

“…In this respect, AsA is known to act for detoxification of radicals via one electron reduction with formation of the unreactive and "harmless" monodehydroascorbate (MDHA) radical, for removal of H 2 O 2 and for photoprotection of photosynthesis by violaxanthin de-epoxidase (VDE) enzyme. This enzyme uses ascorbate as a reductant, as well as for reduction of Fe 3+ to facilitate Fe uptake and with glutathione it acts to maintain the cellular redox status (Smirnoff, 2011 andBadu-Boateng et al, 2017). Phenolic compounds also exhibit antioxidant activities through inactivating lipid free radicals or preventing decomposition of hydroperoxides into free radicals in addition to decreasing membrane fluidity (Pokorny et al, 2001 andGaballah et al, 2007).…”