Iron overload and the predisposition of cells to antioxidant consumption and peroxidative damage

Ability of Ferric Nitrilotriacetate Complex with Three pH-dependent Conformations to Induce Lipid Peroxidation

This study examined the generation of reactive oxygen species (ROS) and the induction of lipid peroxidation by carcinogenic iron(III)-NTA complex (1:1), which has three conformations with two pKa values (pKa1 approximately 4, pKa2 approximately 8). These conformations are type (a) in acidic conditions of pH 1-6, type (n) in neutral conditions of pH 3-9, and type (b) in basic conditions of pH 7-10. The iron(III)-NTA complex was reduced to iron(II) complex under cool-white fluorescent light without the presence of any reducer. The reduction rates of three species of iron(III)-NTA were in the order type (a) >> type (n) > type (b). Iron(III)-NTA-dependent lipid peroxidation was induced in the presence and absence of preformed lipid peroxides (L-OOH) through processes associated with and without photoreduction of iron(III). The order of the abilities of the three species of iron(III)-NTA to initiate the three mechanisms of lipid peroxidation was: (1) type (a) > type (n) > type (b) in lipid peroxidation that is induced L-OOH- and H2O2-dependently and mediated by the photoreduction of iron(III); (2) type (b) > type (n) > type (a) in lipid peroxidation that is induced L-OOH- and H2O2-dependently but not mediated by the photoreduction of iron(III); (3) type (n) > type (b) > type (a) in lipid peroxidation that is induced peroxide-independently and mediated by the photoactivation but not by the photoreduction of iron(III). The rate of lipid peroxidation induced L-OOH-dependently is faster than that induced H2O2-dependently in the mechanism (1), but the rate of lipid peroxidation induced H2O2-dependently is faster than that induced L-OOH-dependently in the mechanism (2). In the lag process of mechanism (3), L-OOH and/or some free radical species, not 1O2, were generated by photoactivation of iron(III)-NTA. These multiple pro-oxidant properties that depend on the species of iron(III)-NTA were postulated to be a principal cause of its carcinogenicity.

Oxygen radicals photo-induced by ferric nitrilotriacetate complex

This study examined the photo-induced generation of reactive oxygen species (ROS) by the carcinogenic iron(III)-NTA complex. Iron(III)-NTA complex (1:1) has three conformations (type (a) in acidic conditions of pH 1-6, type (n) in neutral conditions of pH 3-9, and type (b) in basic conditions of pH 7-10) with two pK(a) values (pK(a1) approximately 4, pK(a2) approximately 8). The iron(III)-NTA complex was reduced to iron(II) under cool-white fluorescent light without the presence of any reducing agent, and the reduction rates of the three conformations of iron(III)-NTA were in the order type (a)>type (n)>type (b) as reported previously (Akai K. et al., Free Radic. Res. 38, 951-962, 2004). ROS generation was investigated by electron paramagnetic resonance (EPR) spectroscopy with a spin-trapping technique. Apparent EPR signals attributed to PBN/*(13)CH(3) and PBN/*OCH(3) spin adducts were observed after incubation of the iron(III)-NTA complex was mixed with alpha-phenyl-tert-butylnitrone (PBN) and (13)C-DMSO in an aerobic condition. The addition of catalase effectively attenuated the PBN adducts, but superoxide dismutase enhanced them. Taken together, these results indicate that the iron(III)-NTA complex is spontaneously reduced to the iron(II)-NTA complex by light under acidic to neutral pH, and in turn transfers an electron to molecular oxygen to form ROS.

Mild iron overload effect on rat liver nuclei

A single injection of iron-dextran significantly increased iron content in plasma, whole liver, cellular cytosol and liver nuclei. In vitro nuclear rate of Fe(3+)-EDTA reduction was not affected by the treatment. Membrane-bound enzymatic activities in the nuclei were measured after iron overload. Both NADPH- and NADH-dependent cytochrome c reductases were slightly decreased after iron overload, but cytochrome P450 was undetectable after 6 h of iron supplementation. The contents of lipid- and water-soluble antioxidants were measured in isolated nuclei from control and iron-overloaded rats. alpha-Tocopherol and beta-carotene co-elutant were decreased by 40% and 83%, respectively after 6 h of treatment. Nuclear glutathione content was not affected. The rate of generation of superoxide anion (O2-), hydrogen peroxide (H2O2) and hydroxyl radical-like species by isolated rat liver nuclei, were decreased by 50%, 40% and 60%, respectively after 6 h of iron supplementation. An identical qualitative response to iron overload was observed with NADPH and NADH. The inactivation of nuclear cytochrome P450, the significant loss in lipid-soluble antioxidants (alpha-tocopherol and beta-carotene) and the decrease in enzyme-dependent oxygen radical generation, suggest that the increase in catalytic active iron induced by iron overload could affect the cellular nuclei functionality.

The effect of iron overload on rat plasma and liver oxidant status in vivo

There is ample evidence implicating reactive oxygen species in a number of human degenerative diseases such as atherosclerosis and haemochromatosis. Although lipid peroxidation underlies many of the toxic effects of oxidative stress, there is a lack of a sensitive and reliable method for its assessment in vivo. To understand the implications of oxidative stress in vivo, we have used dietary iron overload (IO) in the rat. Oxidant status in these animals was determined by assessing depletion of endogenous antioxidants and formation of various lipid peroxidation products, including acylated F2-isoprostanes, a novel class of free-radical-derived prostaglandin-F2-like compounds. IO led to a significant decrease in the concentration of the antioxidants alpha-tocopherol and ascorbic acid in plasma, and alpha-tocopherol, beta-carotene and ubiquinol-10 in liver. Whereas there was no significant lipid peroxidation in plasma, hepatic F2-isoprostane levels were moderately but significantly increased in IO. In addition, IO caused a significant increase in plasma total and high-density lipoprotein cholesterol levels, an effect that was correlated with depletion of plasma ascorbic acid but not alpha-tocopherol. The data demonstrate that IO causes lipid metabolism disturbances and oxidative stress which is associated with substantial depletion of endogenous antioxidants and moderate lipid peroxidative damage.

Ferrous-iron-induced oxidation in chicken liver slices as measured by hemichrome formation and thiobarbituric acid-reactive substances: effects of dietary vitamin E and beta-carotene

Hemichrome formation in chicken liver slices was determined by employing a Heme Protein Spectra Analysis Program (HPSAP) on the visible spectrum of the liver tissue. Relative hemichrome formation (RHF) in liver tissue exposed to ferrous iron for 1 h at 37 degrees C could be predicted according to the general catalytic equation RHF = k.[Fe2+]/(Ap + [Fe2+]), with k = 132 +/- 30, where the factor Ap represents the additive antioxidative potential in the liver tissue. RHF in Fe2+ exposed liver slices incubated at 37 degrees C for 1 h correlated significantly with formation of thiobarbituric acid-reactive substances (TBARS) (r = .77, P < .0001). RHF was found to decrease significantly with increasing vitamin E concentration in liver tissue exposed to ferrous iron (1 h, 37 degrees C). However, the influence of beta-carotene on RHF in ferrous-iron exposed liver slices (1 h, 37 degrees C) was less evident, as the concentration of Fe2+ was found to be decisive for whether beta-carotene acted as an antioxidant or a prooxidant under the conditions in question. Results in the liver slice model system regarding the effect of vitamin E and beta-carotene on iron overload were supported in a subsequent in vivo iron injection experiment with chicks. These observations indicate that RHF is a sensitive marker for ferrous-iron-induced oxidative damage in the present tissue slice system.

Direct evidence for in vivo hydroxyl-radical generation in experimental iron overload: an ESR spin-trapping investigation

Although the hydroxyl radical is often implicated as the species responsible for the initiation of oxidative damage in iron-overload conditions, no ESR evidence for the formation of the radical in vivo has been reported. We have employed a secondary radical-trapping technique in which the hydroxyl radical reacts with dimethyl sulfoxide to form the methyl radical, which is then detected as its adduct of the spin trap N-t-butyl-alpha-phenylnitrone in the bile of animals given an intragastric dose of ferrous sulfate. The identity of this adduct was verified by isotope-substitution techniques. We show that unless measures are taken to inactivate the iron excreted in the bile of treated animals, reactions between iron, oxygen, dimethyl sulfoxide, N-t-butyl-alpha-phenylnitrone, and bile components lead to the formation of artifacts during sample collection.

How to characterize a biological antioxidant

An antioxidant is a substance that, when present at low concentrations compared to those of an oxidizable substrate, significantly delays or prevents oxidation of that substrate. Many substances have been suggested to act as antioxidants in vivo, but few have been proved to do so. The present review addresses the criteria necessary to evaluate a proposed antioxidant activity. Simple methods for assessing the possibility of physiologically-feasible scavenging of important biological oxidants (superoxide, hydrogen peroxide, hydroxyl radical, hypochlorous acid, haem-associated ferryl species, radicals derived from activated phagocytes, and peroxyl radicals, both lipid-soluble and water-soluble) are presented, and the appropriate control experiments are described. Methods that may be used to gain evidence that a compound actually does function as an antioxidant in vivo are discussed. A review of the pro-oxidant and anti-oxidant properties of ascorbic acid that have been reported in the literature leads to the conclusion that this compound acts as an antioxidant in vivo under most circumstances.