Model Studies of the Iron-Catalysed Haber-Weiss Cycle and the Ascorbate-Driven Fenton Reaction

Abstract

Complementary hydroxylation assays and stopped-flow e.s.r. techniques have been employed in the investigation of the effect of various iron chelators (of chemical, biological and clinical importance) on hydroxyl-radical generation via the Haber-Weiss cycle and the ascorbate-driven Fenton reaction. Chelators have been identified which selectively promote or inhibit various reactions involved in hydroxyl-radical generation (for example, NTA and EDTA promote all the reactions of both the Haber-Weiss cycle and the ascorbate-driven Fenton reaction, whereas DTPA and phytate inhibit the recycling of iron in these reactions). The biological chelators succinate and citrate are shown to be relatively poor catalysts of the Haber-Weiss cycle, whereas they are found to be effective catalysts of .OH generation in the ascorbate-driven Fenton reaction. It is also suggested that continuous redox-cycling reactions between iron, oxygen and ascorbate may represent an important mechanism of cell death in biological systems.

Mechanism of the antioxidant to pro-oxidant switch in the behavior of dehydroascorbate during LDL oxidation by copper(II) ions

“…7B. From the gradient of the plot given in Figure 7C (after conversion of signal heights to concentration units), 2k 8 was estimated to be ≈ 10 5 M -1 s -1 , which is in reasonable agreement with the values reported in the literature [68,71], thereby supporting the kinetics approach used above to obtain ascorbate radical concentrations from EPR signal intensities.…”

“…7A). This signal is essentially identical to that of the ascorbate radical [68][69][70], which for comparison was generated in the reaction between Cu 2+ and ascorbate (Reaction 6) (Fig. 7B).…”

“…6E), which is a well known reaction [67]. Interestingly, a weak doublet signal, characteristic of the ascorbate radical (a resonance-stablized radical that fails to react with spin traps) [68][69][70], was observed in the dehydroascorbate solution in the absence of both Cu 2+ and DMPO (Fig. 6F), suggesting formation of the radical by a comproportionation reaction between dehydroascorbate and ascorbate, which is known to arise spontaneously from dehydroascorbate (Reaction −8) [45].…”

“…Although the rate constant for the spontaneous disproportionation of ascorbate radicals is high (k 8 ~ 10 5 -10 6 M -1 s -1 in the physiological pH range) [68,71], at very low concentrations the reaction rate will be slow (being proportional to [Asc •− ] 2 ) and in equilibrium with the reverse reaction (Reaction −8).…”

The Preparation of Preserved Shallot Powders and a Pilot Study of the Antioxidative Effect of Their Aqueous Extracts on the Formation of Hydroxyl Radical Species

“…As shown in Figure 1, the level of the MDA-TBA complex is relatively low in the absence of any shallot aqueous extracts, whereas the levels of the complex are greatly increased in the presence of aqueous extracts of all three types of shallot prepared in this study. In other words, in the ribose degradation assay, the aqueous extracts of shallot prepared under current conditions do not exhibit any ascorbate, a reductant known to enhance a Fenton-type reaction [16], causes a much greater increase in the levels of MDA-TBA (data not shown).…”