A hypothesis for the basis of the pro-oxidant nature of calcium ions

Abstract

A new hypothesis describing the role of the redox inactive Ca2+ ion in the expression of physiological oxidative damage is described. The hypothesis is based on the optimization of the chelation characteristics of iron complexes for pro-oxidant activity. In a previous investigation it was found that an excess of ligand kinetically hindered the Fenton reaction activity of the FeII/III EDTA complex (Bobier et al. 2003). EDTA, citrate, NTA, and glutamate were selected as models for the coordination sites likely encountered by mobile iron, i.e. proteins. The optimal [EDTA]:[FeIII] ratio for Fenton reaction activity as measured by electrocatalytic voltammetry in a solution was found to be 1:1. An excess of EDTA in the amount of 10:1 [ligand]: [metal] suppresses the Fenton reaction activity to nearly the control. It is expected that the physiological coordination characteristics of mobile Fe would have a very large excess of [ligand]:[metal] and thus not be optimized for the Fenton reaction. Introduction of Ca2+ in to a ratio of 10:10:1 [Ca2+]:[EDTA]:[FeIII] to the system reinvigorated the Fenton reaction activity to nearly the value of the optimal 1:1 [EDTA] :[FeIII] complex. The pH distribution diagrams of Ca2+ in the presence of EDTA and FeII/III indicate that Ca2+ has the ability to uptake excess EDTA without displacing either FeII of FeIII from their respective complexed forms. The similarity in the presence for hard ligand sites albeit with a lower binding constant for Ca2+ accounts for this action.

New Insights of the Fenton Reaction Using Glycerol as the Experimental Model. Effect of O2, Inhibition by Mg2+, and Oxidation State of Fe

“…These are cations of biological importance and can be also found in wastes that are prone to Fenton treatment. Data in literature about this subject are scarce …”

“…Data in literature about this subject are scarce. 30 A higher effect was observed with Mg 2+ (the reaction was slower than with Ca 2+ ). Therefore, a detailed study was carried out with this cation.…”

Mechanistic Study of the ZEA Organic Pollutant Degradation System: Evidence for H2O2, HO•, and the Homogeneous Activation of O2 by FeIIEDTA

“…It was hypothesized in that study that a high [chelate]:[Fe] ratio prevents binding of H 2 O 2 to the metal coordination sphere. A later study confirmed this hypothesis as Fenton reaction activity is recovered when Ca 2+ introduced to solutions of high [EDTA]:[Fe] ratios . The calcium ion binds excess EDTA (β = 10.69) with displacing Fe 2+ (β = 14.32) or Fe 3+ (β = 25.1) from that chelate.…”

“…A later study confirmed this hypothesis as Fenton reaction activity is recovered when Ca 2+ introduced to solutions of high [EDTA]:[Fe] ratios. 45 The calcium ion binds excess EDTA (β ) 10.69) with displacing Fe 2+ (β ) 14.32) or Fe 3+ (β ) 25.1) from that chelate.…”

Quality of Cucumbers Commercially Fermented in Calcium Chloride Brine without Sodium Salts

“…The greater availability of oxygen in combination with higher levels of calcium may also induce oxidative softening. There is some evidence that calcium can act indirectly as a prooxidant by preferentially binding to natural chelators, making them unavailable to bind prooxidant metals such as iron [34]. Further research is needed in two key areas: (1) to determine the influence of oxygen and initial brine acidification on the microbiota of low or no salt fermentation during the first few days after brining; and (2) to investigate the combined effects of oxygen, sodium, and calcium on chemical and biochemical reactions during cucumber fermentation that impact product quality.…”