Lipid peroxidation stimulated by iron nitrilotriacetate in rat liver

Oxidative stress induces phosphoenolpyruvate carboxykinase expression in H4IIE cells

Oxidative stress is closely associated with diabetes and is a major cause of insulin resistance. Impairment of hepatic insulin action is thought to be responsible for perturbations in hepatic glucose metabolism. In this study, we found that oxidative stress is involved in the dysregulation of gene expression of phosphoenolpyruvate carboxykinase (PEPCK), a key gluconeogenic enzyme, by a mechanism independent of insulin. Elevation of oxidative stress by injection of ferric nitrilotriacetate in rats increased the expression of hepatic PEPCK mRNA. To examine the direct action of oxidative stress on PEPCK expression, we treated H4IIE hepatoma cells with buthionine sulfoximine (BSO), an inhibitor of glutathione synthesis. BSO increased intracellular oxidative stress and the expression of PEPCK mRNA. Inhibition of p38 mitogen-activated protein kinase (p38 MAP kinase), which mediates responses to oxidative stress, suppressed the induction of PEPCK mRNA by BSO. These results suggest that oxidative stress dysregulates hepatic PEPCK expression by an insulin-independent mechanism.

Utility of the TBARS assay in detecting oxidative stress in white sucker (Catostomus commersoni) populations exposed to pulp mill effluent

Recent evidence indicates that contaminant-stimulated free radical production and resulting oxidative damage may be an important mechanism of toxicity in organisms exposed to water-borne contaminants. This study tested the hypothesis that increases in oxidative stress and associated biochemical alterations would be present in white sucker (Catostomus commersoni) living in an environment receiving pulp mill effluent. Consistent increases in liver TBARS and frequent increases in gonadal TBARS were observed immediately downstream of the pulp mill effluent discharge; observable effects were attenuated with distance until they were not significantly different from reference values. Increases detected with the TBARS assay were commensurate with increases detected using the lipid hydroperoxides assay, a mechanistically independent technique. Fish exposed to pulp mill effluent also exhibited significant increases in hepatic free iron and ascorbic acid and a reduced free radical scavenging capacity in the livers of fish downstream of the effluent discharge relative to reference fish. Increases in oxidative stress are not necessarily dependent on increases in lipid substrate or related to reductions in ascorbic acid. TBARS values similar to those observed in fish resident below pulp mill effluent discharges were observed in white sucker 2 h after intraperitoneal injection using 15 mg/kg body weight ferric nitrilotriacetate (Fe(3+) NTA) as a positive control. This study indicates oxidative stress could be a mechanism of toxicity in fish exposed to pulp mill effluent and demonstrates the utility of TBARS in delineating zones of exposure to pulp mill effluent.

Time course study of the influence of acute iron overload on Kupffer cell functioning and hepatotoxicity assessed in the isolated perfused rat liver

This study tested the hypothesis that acute iron overload (500 mg/kg) alters Kupffer cell functioning by promoting free radical reactions associated with the respiratory burst of liver macrophages, assessed in the isolated perfused rat liver under conditions of Kupffer cell stimulation by carbon infusion and inactivation by gadolinium chloride pretreatment. Total serum and hepatic iron levels were markedly enhanced compared with control values 2 to 24 hours after iron treatment. Total liver O2 uptake progressively increased by iron overload reaching a maximum at 6 hours after treatment, an effect that was completely blocked by GdCl3. Concomitantly, carbon-induced GdCl3-sensitive liver O2 uptake was either enhanced by 119% at 2 hours after iron overload, diminished compared with control values at 4 hours, or abolished at 6 hours. Iron-overloaded rats showed a marked increase in liver sinusoidal lactate dehydrogenase efflux at 4 and 6 hours after treatment, an effect that is exacerbated by carbon infusion and reduced (69%-89%) by GdCl3 pretreatment. Both basal and carbon-induced lactate dehydrogenase effluxes returned to control values at 24 hours after iron overload concomitantly with depression of the basal O2 uptake, without development of iron-induced GdCl3-sensitive respiration or Kupffer cell activation by carbon infusion. It is concluded that iron overload induces a derangement in the Kupffer cell functional status represented by early increases in macrophage-dependent respiratory activity, which may contribute to the concomitant liver injury that developed and to the impairment of both hepatic respiration and the macrophage response to particle stimulation observed at later times after treatment.

The prevention of ferric nitrilotriacetate-induced nephro- and hepatotoxicity by methylenedioxybenzene antioxidants

Previously it was shown that methylenedioxybenzenes (MDBs), particularly isosafrole, were highly effective at preventing CCl4-induced liver necrosis in vivo (Z.S. Zhao, P.J. O'Brien, The prevention of CCl4-induced liver necrosis in mice by naturally occurring methylenedioxybenzenes, Toxicol. Appl. Pharmacol., 140 (1996) 411-421), probably as a result of forming metabolic intermediate complexes with cytochrome P450. In the following it was shown that pretreatment of mice with isosafrole also completely prevented ferric nitrilotriacetate (FeNTA)-induced renal necrosis and lipid peroxidation, even though metabolic activation by cytochrome P450 is not involved. The naturally occurring or synthetic MDBs that prevented CCl4 hepatotoxicity also prevented hepatocyte lipid peroxidation. induced by FeNTA, but other cytochrome P450 inhibitors were ineffective. These compounds, in decreasing order of antioxidant effectiveness, were sesamol, 4-t-butyl-methylenedioxybenzene, isosafrole, piperonyl butoxide and 4-bromo-methylenedioxybenzene and safrole, whereas, benzodioxole, 3,4-(methylenedioxy)-toluene and 1,2-(methylenedioxy)-4-nitrobenzene were ineffective. Pre-incubating the hepatocytes with P450 inhibitors decreased the protective effects of isosafrole, suggesting that the catecholic metabolites of MDBs were responsible for the antioxidant activity. A greater inhibition of FeNTA-induced lipid peroxidation by catecholic metabolites was observed. Since cytochrome P450 did not participate in FeNTA-induced hepatocyte or microsomal lipid peroxidation, it is likely that the antioxidant properties of MDBs or their catecholic metabolites also contribute to their in vivo protection against CCl4 or FeNTA-induced hepato- or nephrotoxicity.

Free radical-mediated effects on skeletal muscle protein in rats treated with Fe-nitrilotriacetate

Changes in protein conformation and proteolysis in skeletal muscle of rats were studied by the induction of oxidative stress induced in vivo by ferric nitrilotriacetate (FeNTA) treatment. Useful indices of protein modification, including both protein carbonyl content and fluorescence intensity of protein hydrolysate in skeletal muscle, were increased 3 h following FeNTA treatment to rats. Western blot using anti-dinitrophenyl antibody showed oxidative modification of actin and myosin in myofibril by FeNTA. These results demonstrated that muscle proteins were modified after radical attack induced by an iron overload. Furthermore, oxidative stress induced by iron overloading resulted in enhanced degradation of myofibrillar proteins. It is suggested that muscle proteins which have been modified by oxidative stress undergo rapid removal.

Intensification and depletion of specific bulky renal DNA adducts (I-compounds) following exposure of male F344 rats to the renal carcinogen ferric nitrilotriacetate (Fe-NTA)

The effects of the renal carcinogen ferric nitrilotriacetate (Fe-NTA) on kidney DNA of male F344 rats were studied to determine whether bulky DNA oxidation products (putative intrastrand crosslinks) could be detected by 32P-postlabeling in the target organ of carcinogenesis. Rats (10-11 weeks old) were given a single dose of Fe-NTA (15 mg Fe/kg body weight) i.p. at 3:00 pm. After 5 h, renal DNA from Fe-NTA-treated and vehicle control animals was assayed by 32P-postlabeling. Thin-layer chromatography and quantitative analysis of two labeled nucleotide fractions of increasing polarity, L and C, showed that three spots (L1, L2, and C3) were intensified 3.5- to 4.2-fold in treated animals. L1 consisted of subfractions L1a, L1b, and L1c, which could be resolved chromatographically. L1c, L2, and C3 were identical to DNA oxidation products generated by the Fenton reaction in vitro, while L1a and L1b apparently did not arise by this mechanism. DNA damage and toxicity appeared reduced in younger animals and animals treated in the morning, presumably due to differences in antioxidant defenses. Liver and lung (non-target organs) DNA did not exhibit enhanced L1, L2, and C3 spots. In addition to augmenting renal I-compounds, Fe-NTA reduced the levels of three major polar kidney I-compounds (C4, C5, and C6) to 22-53% of control. This reduction did not appear to arise by direct oxidative DNA damage, resembling the previously documented loss of liver I-compounds induced by numerous hepatocarcinogens. Two of these I-compounds (C4 and C5) have been reported to exhibit positive linear correlations with median lifespan of male F344 rats. The pleiotropic response of kidney I-compound levels to Fe-NTA was consistent with different roles of different types (I and II) of I-compounds in Fe-NTA-mediated renal carcinogenesis. The results strongly support a causal relationship between oxidative DNA lesions and Fe-NTA-mediated carcinogenesis.

Fe(2+)-induced lipid peroxidation kinetics in liposomes: the role of surface Fe2+ concentration in switching the reaction from acceleration to decay

Kinetics of malonyldialdehyde (MDA) accumulation, Fe2+ oxidation, and chemiluminescence (CL) at different initial iron ([Fe2+]) and liposome ([L]) concentrations were measured in liposome suspension. Above certain critical Fe2+ concentrations ([Fe2+]*) the latent period (LP) of LPO development was observed. The method of [Fe2+]* estimation by the dependence of LP value (tau) on [Fe2+] was elaborated. The increase of [L] resulted in decrease of tau and increase of delta MDA as well as SF CL amplitude. [Fe2+]* value changed from 10 to 50 microM with change of [L] from 1 to 4 mg/ml, so that the ratio [Fe2+]*/[L] was kept constant. This may be explained under the assumption that the major part of Fe2+ is bound by the membranes. At concentrations of Fe2+ higher than the critical one, iron chelators (desferrioxamine, o-phenanthroline, and EDTA) and cations (Eu3+, Ca2+, and Fe3+) decreased tau without any essential influence on the CL "slow flash" amplitude (h). Apparently, the only result of iron complexones and cations on LPO is the decrease of Fe2+ ion concentration on the membrane surface. Thus, [Fe2+]* value and surface concentration of Fe2+ are the main parameters determining both kinetics and efficiency of Fe(2+)-induced LPO in membrane systems.

Stimulation of microsomal chemiluminescence by ferritin

The ability of ferritin to catalyze rat liver microsomal chemiluminescence was determined in the absence and presence of the redox cycling agent paraquat, and with either NADPH or NADH as reductant. Microsomal chemiluminescence was used as a index of lipid peroxidation. In the absence of added ferritin, NADPH-dependent microsomal light emission was 4-fold greater than the NADH-dependent reaction, and was not sensitive to superoxide dismutase, catalase or DMSO. Ferritin stimulated NADPH-, but not NADH-dependent chemiluminescence in a time- and concentration-dependent manner. The stimulation by ferritin was completely sensitive to superoxide dismutase, but not to catalase or DMSO, suggesting the requirement for superoxide to mobilize iron from ferritin. An iron ligand was not required for the stimulation by ferritin; the addition of certain ligands such as EDTA, DETAPAC or desferrioxamine resulted in inhibition of the stimulation by ferritin. Paraquat potentiated the effect of ferritin on microsomal chemiluminescence with NADPH as cofactor and was weakly stimulatory with NADH. The potentiation by paraquat plus ferritin was prevented by superoxide dismutase and was further elevated by ligands such as ATP. Chemiluminescence proved to be a more sensitive parameter than production of thiobarbituric acid-reactive components to evaluate the stimulation of oxygen radical production by iron released from ferritin, in the absence or in the presence of paraquat.

Hepatic chemiluminescence and lipid peroxidation in mild iron overload

The effect of iron-overload on both hepatic lipid peroxidation and chemiluminescence was studied in early stages after iron-dextran injection. Total hepatic iron content was markedly elevated over control values 2-6 h after iron dose. A 4-fold increase in light emission was detected after 4-6 h after iron injection. Plasma GOT, GPT and LDH activities were not affected by the treatment suggesting that cell permeability was not affected by necrosis. Increases in the generation of thiobarbituric acid reactive substances (TBARS) and chemiluminescence in liver homogenates, were determined as a function of time after iron administration, in the presence of NADPH as cofactor. Under the same experimental conditions, microsomal cytochrome P-450 content was decreased by 40%, 2 h after iron treatment. To evaluate liver antioxidant defenses, catalase, superoxide dismutase and glutathione peroxidase activities were determined. Glutathione peroxidase activity in the homogenate was not affected by the treatment. Catalase and superoxide dismutase activities declined by 25 and 36%, respectively, compared with control values 4 h after the iron dose. Our data suggest that lipid peroxidation occurs after mild iron overload even though the liver remains functional.

DNA damaging activity of cadmium in Leydig cells, a target cell population for cadmium carcinogenesis in the rat testis

To clarify the mechanism by which Cd initiates rat testicular cancer, the ability of Cd or H2O2 to induce DNA single strand breakage was evaluated in testicular Leydig cells using a simple and rapid DNA precipitation method. Effects of Cd, Fe, Zn and Ca on the oxidant-induced DNA damage and effects of reduced glutathione (GSH) on the genotoxicity caused by the peroxide and/or Fe were also assessed. H2O2 induced strong DNA single strand breakage. Cd alone did not exhibit such a genotoxicity nor did it enhance the peroxide-induced DNA damage. Ca and Fe(II) potentiated the oxidant-induced DNA single strand breakage, while Zn partially protected cells from the oxidative damage of DNA caused by the peroxide. GSH attenuated single strand breaks of DNA brought about by H2O2 and/or Fe. These results suggest that the initiation of carcinogenesis in the rat testis by Cd is triggered by active oxygen species such as H2O2, which is generated by the metal exposure, rather than by a direct genotoxicity of Cd. The oxidant-mediated initiation is clearly a complicated event accomplished by multiple factors.

Role of oxidative stress in single-dose, cadmium-induced testicular cancer

Treatment of rats with a single carcinogenic dose of CdCl2 (i.e., 30 mumol/kg) caused severe hemorrhagic damage in the testis within the first 12 h after the metal. Subsequently, atrophy with calcification developed in the next 2-3 mo. Atrophied tissues regenerated during the 1 yr after exposure. Twelve hours after exposure to the Cd treatment, lipid peroxidation levels, Fe content, and cellular production of H2O2 were remarkably elevated in testicular Leydig cells, the target cell population for Cd carcinogenesis. At the same time, glutathione peroxidase activity rose, glutathione reductase and catalase activities were reduced, and superoxide dismutase activity was unchanged. Xanthine oxidase activity in Leydig cells was also elevated at 6 and 9 h after the Cd treatment. Reduced glutathione in testes was decreased and oxidized glutathione was increased 12 h after exposure to the metal. These facts suggest that the carcinogenic doses of Cd induced oxidative stress while compromising cellular defense mechanisms against such stress. Therefore, active oxygen species such as H2O2 may have an important role in the initiation of carcinogenesis within the target cell population.

Enhanced induction of hepatic lipid peroxidation by ferric nitrilotriacetate in chickens susceptible to fatty liver rupture

1. Two strains of single comb White Leghorn birds, one susceptible to fatty liver rupture (UCD-003) and a normal commercial strain, were injected with iron nitrilotriacetate and the extent of hepatic lipid peroxidation that occurred was estimated by measuring concentrations of malondialdehyde (MDA). 2. Higher concentrations of MDA were found in the livers of the UCD-003 strain than in the normal birds after injection of iron nitrilotriacetate. No differences were found in the activities of glutathione peroxidase, superoxide dismutase and catalase in the livers of untreated birds of either strain. 3. The degree of unsaturation of the fats in the livers of the two strains was similar. However, the UCD-003 birds had a significantly higher content of liver fat than the normal birds. The increased concentrations of liver fat could account for the increased lipid peroxidation in the UCD-003 birds. 4. The increased incidence of liver haemorrhage that occurs in the UCD-003 birds may be caused by the increased susceptibility of these birds to hepatic lipid peroxidation.

A chelator is required for microsomal lipid peroxidation following reductive ferritin-iron mobilisation

In the past, antioxidant and chelator studies have implicated a role for iron-dependent oxidative damage in tissues subjected to ischaemia followed by reperfusion. As ferritin is a major source of iron in non-muscular organs and therefore a potential source of the iron required for oxygen radical chemistry, we have determined conditions under which ferritin iron reduction leads to the formation of a pool of iron which is capable of catalysing lipid peroxidation. Under anaerobic conditions and in the presence of rat liver microsomes, flavin mononucleotide (FMN) catalysed the reduction of ferritin iron as shown by both continuous spectrophotometric measurements of tris ferrozine-Fe(II) complex formation and post-reaction Fe(II) determination. The presence of either ferrozine or citrate was not found to alter the time course or extent of ferritin reduction. In contrast, the addition of air to the reactants after a 20 min period of anaerobic reduction resulted in peroxidation of the microsome suspension (as determined with the 2-thiobarbituric acid test) only in the presence of a chelator such as citrate, ADP or nitrilotriacetic acid. These results support the concept that reduced ferritin iron can mediate oxidative damage during reperfusion of previously ischaemic tissues, provided that chelating agents such as citrate or ADP are present.

Techniques for pharmacological and toxicological studies with isolated hepatocyte suspensions

Since its introduction in 1969, the high-yield preparation of isolated hepatocytes has become a frequently used tool for the study of hepatic uptake, excretion, metabolism and toxicity of drugs and other xenobiotics. Basic preparative methods are now firmly established involving perfusion of the liver with a balanced-saline solution containing collagenase. Satisfactory procedures are available for determining cell yields, for expressing cellular activities and for establishing optimal incubation conditions. Gross cellular damage can be detected by means of trypan blue or by measuring enzyme leakage, and damaged cells can be removed from the preparation. Specialized techniques are available for preparing hepatocyte couplets and suspensions enriched with periportal or perivenous hepatocytes. The isolated hepatocyte preparation is particularly convenient for the study of the kinetics of hepatic drug uptake and excretion because the cells can be rapidly separated from the incubation medium. Isolated liver cells have also proved valuable for investigating drug metabolism since they show many of the features of the intact liver. However, they also show important differences such as losses of membrane specialization, some degree of cell polarity and the capacity to form bile. The many consequences of the hepatic toxicity of xenobiotics including lipid peroxidation, free radical formation, glutathione depletion, and covalent binding to macromolecules are also readily studied with the isolated liver cell preparation. A particular advantage is the ease with which morphological changes as a result of drug exposure can be observed in isolated hepatocytes. However, it must be remembered that the isolation procedure inevitably introduces changes that may make the cells more susceptible than the normal liver to damage by xenobiotic agents. Despite its limitations, the isolated hepatocyte preparation is now firmly established in the armamentarium of the investigator examining the interaction of the liver with xenobiotics.

Reactions of low valent transition-metal complexes with hydrogen peroxide. Are they "Fenton-like" or not? 3. The case of Fe(II) [N(CH2CO2)3](H2O)2-

The reaction of Fe(II)[N(CH2CO2)3](H2O)2- with H2O2 in neutral and slightly acidic solutions was studied. The results indicate that the transient complex formed between these reagents, (nta)(H2O)-Fe2+.O2H-, (where nta = N(CH2CO2-)3) reacts either directly with greater than or equal to 0.175 M 2-propanol or 2-methyl-2-propanol, or decomposes into the corresponding tetra-valent iron complex which then reacts with these alcohols. The nature of the final products in this system containing iron ions and nta depends on the pH, thus indicating that the nature of the transient complexes formed, or their relative yields, depend on the pH. The results prove that free hydroxyl radicals are not the major product of this "Fenton like" reaction under the experimental conditions. The implications of these results on the understanding of biological processes is discussed.

High performance liquid chromatographic analysis of the malondialdehyde content of chicken liver

An assay for the determination of malondialdehyde (MDA) levels in chicken liver is described. The method involves the extraction of the tissues with trichloracetic acid and reaction of the extract with TBA. The reaction mixture is then purified using reverse-phase cartridges and the TBA-MDA reaction products are separated from other TBA-reactive substances by reverse-phase HPLC. The method is rapid, reproducible, easy to perform, and is sensitive in the nanogram (10(-11) mol) range. Application of this technique may be useful to determine the extent of lipid peroxidation occurring in the liver as a result of degenerative liver diseases or poisoning with certain hepatotoxins.

Prevention of hepatocyte injury and lipid peroxidation by iron chelators and alpha-tocopherol in isolated iron-loaded rat hepatocytes

These experiments were performed to characterize the relationship between lipid peroxidation and hepatocyte viability in iron overload. Hepatocytes were isolated from rats with chronic dietary iron overload and the effects of in vitro iron chelation on lipid peroxidation, cell viability and ultrastructure were studied over a 4-hr incubation period. Cell viability was significantly reduced at 3 and 4 hr in iron-loaded hepatocytes compared with controls and was preceded by an increase in iron-dependent lipid peroxidation. Similarly, extensive degenerative ultrastructural changes were observed in iron-loaded hepatocytes compared with controls after 4 hr of incubation. In vitro iron chelation with either deferoxamine or apotransferrin protected against lipid peroxidation, loss of viability and ultrastructural damage in iron-loaded hepatocytes. The addition of an antioxidant, alpha-tocopherol, also protected against lipid peroxidation and preserved cell viability over a 4-hr incubation. The protective effects of iron chelators and alpha-tocopherol support a strong association between iron-dependent lipid peroxidation and hepatocellular injury in iron overload.

Generation of reactive oxygen species and reduction of ferric chelates by microsomes in the presence of a reconstituted system containing ethanol, NAD+ and alcohol dehydrogenase

Many of the toxic metabolic actions of ethanol on the liver have been ascribed to the enhanced cellular production of NADH, which arises as a consequence of the oxidation of ethanol by alcohol dehydrogenase (ADH). Experiments were conducted to evaluate whether NADH generated from a reconstituted system containing ethanol plus NAD+ plus ADH could interact with ferric chelates to promote microsomal lipid peroxidation and generation of a hydroxyl radical (OH)-like species. In the presence of the reconstituted system and iron, microsomes produced.OH as assessed by the oxidation of .OH scavenging agents. This oxidation was inhibited by catalase and competitive.OH scavengers but not by superoxide dismutase. The ADH-dependent microsomal production of.OH was effectively catalyzed by ferric-EDTA and -diethylenetriamine pentaacetic acid (-DTPA), but not by ferric-ATP or -citrate. However, all these ferric chelates were reduced by the microsomes in the presence of the reconstituted system. Hydrogen peroxide (H2O2) was produced in the presence of ADH and appeared to be a limiting factor for the production of.OH. The reconstituted system also catalyzed microsomal lipid peroxidation, and the pattern of effectiveness of ferric chelates was opposite that of catalysis of.OH production. There was little effect by catalase, superoxide dismutase or dimethyl sulfoxide (DMSO) on the ADH-dependent microsomal lipid peroxidation. The reconstituted system was characterized with respect to dependence on NAD+ and ADH; ethanol could be replaced by other alcohols, which are substrates for ADH. Pyrazole, a potent inhibitor of ADH, blocked the ability of the reconstituted system to interact with iron and microsomes to produce reactive oxygen species.(ABSTRACT TRUNCATED AT 250 WORDS)

Thiobarbituric acid-reactive substance formation of rat kidney brush border membrane vesicles induced by ferric nitrilotriacetate

An iron chelate, ferric nitrilotriacetate (Fe3+-NTA), is nephrotoxic and also carcinogenic to the kidney in experimental animals. Iron-promoted lipid peroxidation in the proximal tubules is thought to be responsible for the pathologic process. In the present study, iron-promoted lipid peroxidation, with thiobarbituric acid (TBA) formation as an indication, in the tubular surface was simulated in vitro using rat kidney brush border membrane vesicles and the results were compared with those using linoleate micelles and rat liver microsomal lipid liposomes. Addition of ascorbate, cysteine, or dithiothreitol to the Fe3+-NTA solution resulted in consumption of dissolved oxygen and promoted the lipid peroxidation in the micelles and in the liposomes. In contrast, addition of glutathione to the Fe3+-NTA solution caused only sluggish oxygen consumption and far less peroxidation in these lipid systems. When the brush border membrane vesicles were used for the peroxidation substrate, Fe3+-NTA and glutathione could promote TBA formation at a rate comparable to that elicited by Fe3+-NTA with cysteine or dithiothreitol. Acivicin, a gamma-glutamyl transpeptidase inhibitor, suppressed the peroxidation of the brush border membrane vesicles promoted by Fe3+-NTA and glutathione. These results suggest the following mechanism of proximal tubular cell lipid peroxidation promoted by Fe-NTA: Fe3+-NTA filtered through glomeruli is rapidly reduced by cysteine and Fe2+-NTA starts lipid peroxidation at the site, leading to proximal tubular necrosis. Cysteine is amply supplied by the decomposition of glutathione within the lumen by the action of gamma-glutamyl transpeptidase and dipeptidase situated at the proximal tubular brush border membrane.

Effects of acute and sub-chronic administration of iron nitrilotriacetate in the rat

Parenteral administration of iron nitrilotriacetate (FeNTA) to rats resulted in marked loss in body weight, and increases in liver/and kidney/body weight ratios. Fatalities, due to renal failure, depended on dosage and age of the animals, and were greater (70%) after a single large dose (12 mg iron) than after repeated smaller doses (30%). FeNTA administered subchronically gave rise to an increase in ethane exhalation, and to decreased liver glutathione peroxidase activity, and decreased cytochrome P-450 concentration and benzphetamine N-demethylase activity. It also resulted in severe renal tubular necrosis, with deposition of iron in the tubular cells and loss of brush border alkaline phosphatase activity, resulting in a dose-dependent diuresis, with increased urinary excretion of glucose, iron and lipid peroxidation products, and decreased urine creatinine concentration. NTA alone had none of these effects but slightly decreased the hepatic concentration of iron.

Interaction of ferric complexes with rat liver nuclei to catalyze NADH-and NADPH-Dependent production of oxygen radicals

The production of potent oxygen radicals by microsomal reaction systems has been well characterized. Relatively little attention has been paid to generation of oxygen radicals by liver nuclei, or to the interaction of nuclei with different ferric complexes to catalyze NADH- or NADPH-dependent production of reactive oxygen intermediates. Intact rat liver nuclei were capable of catalyzing an iron-dependent production of .OH as reflected by the oxidation of .OH scavenging agents such as 2-keto-4-thiomethylbutyrate, dimethyl sulfoxide, and t-butyl alcohol. Inhibition of .OH production by catalase implicates H2O2 as the precursor of .OH generated by the nuclei, whereas superoxide dismutase had only a partially inhibitory effect. The production of .OH with either cofactor was striking increased by addition of ferric-EDTA or ferric-diethylenetriamine-pentaacetic acid (DTPA) whereas ferric-ATP and ferric-citrate were not effective catalysts. All these ferric complexes were reduced by the nuclei in the presence of either NADPH or NADH. The pattern of iron chelate effectiveness in catalyzing lipid peroxidation by nuclei was opposite to that of .OH production; with either NADH or NADPH, nuclear lipid peroxidation was increased by the addition of ferric ammonium sulfate, ferric-ATP, or ferric-citrate, but not by ferric-EDTA or ferric-DTPA. NADPH-dependent nuclear lipid peroxidation was insensitive to catalase, superoxide dismutase, or .OH scavengers; the NADH-dependent reaction showed a partial sensitivity (30 to 40%) to these additions. The overall patterns of .OH production and lipid peroxidation by the nuclei are similar to those shown by microsomes, e.g., effect of ferric complexes, sensitivity to antioxidants; however, rates with the nuclei are less than 20% those of microsomes, which reflect the lower activities of NADPH- and NADH-cytochrome c reductase in the nuclei. The potential for nuclei to reduce ferric complexes and catalyze production of .OH-like species may play a role in the susceptibility of the genetic material to oxidative damage under certain conditions since such radicals would be produced site-directed and not exposed to cellular antioxidants.

Interactions between paraquat and ferric complexes in the microsomal generation of oxygen radicals

Transition metals may play a central role in the toxicity associated with paraquat. Studies were carried out to evaluate the interaction of paraquat with several ferric complexes in the promotion of oxygen radical generation by rat liver microsomes. In the absence of added iron, paraquat produced some increase in low level chemiluminescence by microsomes; there was a synergistic increase in light emission in the presence of paraquat plus ferric-ATP or ferric-citrate, but not paraquat plus either ferric-EDTA or ferric-diethylenetriamine pentaacetic acid (ferric-DETAPAC). Synergistic interactions could be observed at a paraquat concentration of 100 microM and a ferric-ATP concentration of 3 microM. In the absence or presence of paraquat, microsomal light emission was not affected by catalase or dimethyl sulfoxide (DMSO), indicating no significant role for hydroxyl radicals. Superoxide dismutase (SOD) did not affect chemiluminescence in the absence of paraquat but produced some inhibition in the presence of paraquat; this inhibition by SOD was most prominent in the absence of added iron and less pronounced in the presence of ferric-ATP or ferric-citrate. Although microsomal chemiluminescence is closely associated with lipid peroxidation, paraquat did not increase malondialdehyde production as reflected by production of thiobarbituric acid-reactive components. However, lipid peroxidation was sensitive to inhibition by SOD in the presence, but not in the absence, of paraquat, analogous to results with chemiluminescence. Paraquat synergistically increased microsomal hydroxyl radical production as measured by the production of ethylene from 2-keto-4-thiomethylbutyrate in the presence of ferric-EDTA or ferric-citrate. The interaction of paraquat with microsomes and ferric complexes resulted in an increase in oxygen radical generation. Various ferric complexes can increase the catalytic effectiveness of paraquat in promoting microsomal generation of oxygen radicals, although, depending on the reaction being investigated, the nature of the ferric complex is important.

Oxygen reduction and lipid peroxidation by iron chelates with special reference to ferric nitrilotriacetate

A certain iron chelate, ferric nitrilotriacetate (Fe3+-NTA) is nephrotoxic and also carcinogenic to the kidney in mice and rats, a distinguishing feature not shared by other iron chelates tested so far. Iron-promoted lipid peroxidation is thought to be responsible for the initial events. We examined its ability to initiate lipid peroxidation in vitro in comparison with that of other ferric chelates. Chelation of Fe2+ by nitrilotriacetate (NTA) enhanced the autoxidation of Fe2+. In the presence of Fe2+-NTA, lipid peroxidation occurred as measured by the formation of conjugated diene in detergent-dispersed linoleate micelles, and by the formation of thiobarbituric acid-reactive substances in the liposomes of rat liver microsomal lipids. Addition of ascorbic acid to Fe3+-NTA solution promoted dose-dependent consumption of dissolved oxygen, which indicates temporary reduction of iron. On reduction, Fe3+-NTA initiated lipid peroxidation both in the linoleate micelles and in the liposomes. Fe3+-NTA also initiated NADPH-dependent lipid peroxidation in rat liver microsomes. Although other chelators used (deferoxamine, EDTA, diethylenetriaminepentaacetic acid, ADP) enhanced autoxidation, reduction by ascorbic acid, or in vitro lipid peroxidation of linoleate micelles or liposomal lipids, NTA was the sole chelator that enhanced all the reactions.

Hepatic injury in chronic iron overload. Role of lipid peroxidation

In both hereditary hemochromatosis and in the various forms of secondary hemochromatosis, there is a pathologic expansion of body iron stores due mainly to an increase in absorption of dietary iron. Excess deposition of iron in the parenchymal tissues of several organs (e.g. liver, heart, pancreas, joints, endocrine glands) results in cell injury and functional insufficiency. In the liver, the major pathological manifestations of chronic iron overload are fibrosis and ultimately cirrhosis. Evidence for hepatotoxicity due to iron has been provided by several clinical studies, however the specific pathophysiologic mechanisms for hepatocellular injury and hepatic fibrosis in chronic iron overload are poorly understood. The postulated mechanisms of liver injury in chronic iron overload include (a) increased lysosomal membrane fragility, perhaps mediated by iron-induced lipid peroxidation, (b) peroxidative damage to mitochondria and microsomes resulting in organelle dysfunction, (c) a direct effect of iron on collagen biosynthesis and (d) a combination of all of the above.

Comparison of the ability of ferric complexes to catalyze microsomal chemiluminescence, lipid peroxidation, and hydroxyl radical generation

The interaction of microsomes with iron and NADPH to generate active oxygen radicals was determined by assaying for low level chemiluminescence. The ability of several ferric complexes to catalyze light emission was compared to their effect on microsomal lipid peroxidation or hydroxyl radical generation. In the absence of added iron, microsomal light emission was very low; chemiluminescence could be enhanced by several cycles of freeze-thawing of the microsomes. The addition of ferric ammonium sulfate, ferric-citrate, or ferric-ADP produced an increase in chemiluminescence, whereas ferric-EDTA or -diethylenetriaminepentaacetic acid (detapac) were inhibitory. The same response to these ferric complexes was found when assaying for malondialdehyde as an index of microsomal lipid peroxidation. In contrast, hydroxyl radical generation, assessed as oxidation of chemical scavengers, was significantly enhanced in the presence of ferric-EDTA and -detapac and only weakly elevated by the other ferric complexes. Ferric-desferrioxamine was essentially inert in catalyzing any of these reactions. Chemiluminescence and lipid peroxidation were not affected by superoxide dismutase, catalase, or competitive hydroxyl radical scavengers whereas hydroxyl radical production was decreased by the latter two but not by superoxide dismutase. Chemiluminescence was decreased by the antioxidants propylgallate or glutathione and by inhibiting NADPH-cytochrome P-450 reductase with copper, but was not inhibited by metyrapone or carbon monoxide. The similar pattern exhibited by ferric complexes on microsomal light emission and lipid peroxidation, and the same response of both processes to radical scavenging agents, suggests a close association between chemiluminescence and lipid peroxidation, whereas both processes can be readily dissociated from free hydroxyl radical generation by microsomes.

Lipid peroxidation of rabbit small intestinal microvillus membrane vesicles by iron complexes

Fe(II)- and Fe(III)-induced lipid peroxidation of rabbit small intestinal microvillus membrane vesicles was studied. Ferrous ammonium sulphate, ferrous ascorbate at a molar ratio of 10:1, and ferric citrate, at molar ratios of 1:1 and 1:20, did not stimulate lipid peroxidation. Ferrous ascorbate, 1:1, induced low stimulation, while ferrous ascorbate, 1:20 gave higher stimulation of lipid peroxidation. These results show that in our experimental system, ascorbate is a promotor rather than an inhibitor of lipid peroxidation. Ferric nitrilotriacetate (at molar ratios of 1:2 and 1:10), at an iron concentration of 200 microM, was by far the most effective in inducing lipid peroxidation. Superoxide dismutase, mannitol and glutathione had no effect, while catalase, thiourea and vitamin E markedly decreased ferrous ascorbate 1:20-induced lipid peroxidation. Ferric nitrilotriacetate-induced lipid peroxidation was slightly reduced by catalase and mannitol, significantly reduced by superoxide dismutase, and completely inhibited by thiourea. Glutathione caused a 100% increase in the ferric nitrilotriacetate-induced lipid peroxidation. These results suggest that Fe(II) in the presence of trace amounts of Fe(III), or an oxidizing agent and Fe(III) in the presence of Fe(II) or a reducing agent, are potent stimulators of lipid peroxidation of microvillus membrane vesicles. Addition of deferoxamine completely inhibited both ferrous ascorbate, 1:20 and ferric nitrilotriacetate-induced lipid peroxidation, demonstrating the requirement for iron for its stimulation. Iron-induced peroxidation of microvillus membrane may have physiological significance because it could already be demonstrated at 2 microM iron concentration.

Delayed, ferrous iron-dependent peroxidation of rat liver microsomes

Measurement of both chemiluminescence (CL) and the formation of 2-thiobarbituric acid-reacting substances (TBAR) has been used to study the delayed, nonenzymatic lipid peroxidation (LP) initiated in rat liver microsomes by ferrous chloride. Following Fe2+ addition, the CL technique revealed a burst of light emission (peak, Phase II) which was preceded by a period of little or no detectable photon production (delay, Phase I) and succeeded by an increased emission (Phase III). Analysis of TBAR indicated a low rate of LP during the delay which increased more than fivefold during a 1-min period and which corresponded to the CL peak. The delay length depended on both the Fe2+ concentration and the microsome concentration; increased Fe2+ yielded longer delays while increased microsome concentration decreased the delay. As reported by others [J. R. Bucher, M. Tien, and S. D. Aust (1983) Biochem. Biophys. Res. Commun. 111, 777-784; J. M. Braughler, L. A. Duncan, and R. L. Chase (1986) J. Biol. Chem. 261, 10282-10289], Fe3+ also decreased the delay. The ferric-nitrilotriacetate (Fe3+-NTA) complex was found to be more efficient than "free" Fe3+ [Fe(NO3)3]; a 100 microM concentration of the 1:1 Fe3+-NTA complex eliminated the delay due to 100 microM Fe2+, whereas 400 microM Fe(NO3)3 reduced the delay from 17.5 to 2.5 min. Incubation under reduced O2 tension demonstrated a requirement for O2 during the delay. The use of antioxidants [butylated hydroxytoluene, (+)-catechin, promethazine, and uric acid] and inhibitors of the Haber-Weiss reaction (mannitol, Tris buffer, dimethyl sulfoxide, catalase, and superoxide dismutase) indicated that the initiating species has characteristics of a weak oxidizing radical capable of either hydrogen or electron abstraction from suitable target molecules. We hypothesize that the delay that is sensitive to the Fe2+:microsome ratio is due to reductive elimination of the initiating species by "free" Fe2+. The nature of the initiating species has yet to be determined; however, the argument is presented that the perferryl ion (Fe3+-O2-.) may possess the characteristics required for the initiator.

A survey of chemicals inducing lipid peroxidation in biological systems

A great number of drugs and chemicals are reviewed which have been shown to stimulate lipid peroxidation in any biological system. The underlying mechanisms, as far as known, are also dealt with. Lipid peroxidation induced by iron ions, organic hydroperoxides, halogenated hydrocarbons, redox cycling drugs, glutathione depleting chemicals, ethanol, heavy metals, ozone, nitrogen dioxide and a number of miscellaneous compounds, e.g. hydrazines, pesticides, antibiotics, are mentioned. It is shown that lipid peroxidation is stimulated by many of these compounds. However, quantitative estimates cannot be given yet and it is still impossible to judge the biological relevance of chemical-induced lipid peroxidation.

Lipid peroxidation and associated hepatic organelle dysfunction in iron overload

Iron overload can have serious health consequences. Since humans lack an effective means to excrete excess iron, overload can result from an increased absorption of dietary iron or from parenteral administration of iron. When the iron burden exceeds the body's capacity for safe storage, the result is widespread damage to the liver, heart and joints, and the pancreas and other endocrine organs. Clear evidence is now available that iron overload leads to lipid peroxidation in experimental animals, if sufficiently high levels of iron are achieved. In contrast, there is a paucity of data regarding lipid peroxidation in patients with iron overload. Data from experiments using an animal model of dietary iron overload support the concept that iron overload results in an increase in an hepatic cytosolic pool of low molecular weight iron which is catalytically active in stimulating lipid peroxidation. Lipid peroxidation is associated with hepatic mitochondrial and microsomal dysfunction in experimental iron overload, and lipid peroxidation may underlie the increased lysosomal fragility that has been detected in homogenates of liver samples from both iron-loaded human subjects and experimental animals. Some current hypotheses focus on the possibility that the demonstrated functional abnormalities in organelles of the iron-loaded liver may play a pathogenic role in hepatocellular injury and eventual fibrosis. The recent demonstration that hepatic fibrosis is produced in animals with long-term dietary iron overload will allow this model to be used to further investigate the relationship between lipid peroxidation and hepatic injury in iron overload.

Prooxidant action of desferrioxamine: Fenton-like production of hydroxyl radicals by reduced ferrioxamine

It is common practice in biochemical research to assume that iron bound to desferrioxamine (DFO) to form ferrioxamine (FOA) has been rendered inactive to subsequent redox chemistry within the range of physiological redox potentials, both in vitro and in vivo. However, plants and microorganisms can make iron metabolically available from ferrioxamine and closely related trihydroxamate siderophores, and at neutral pH, cyclic voltammetry of FOA demonstrates a reversible one-electron reduction at about -0.42 to -0.45 V (vs. normal hydrogen electrode), which is within the range of a number of reducing enzymes. We present evidence for the Fenton-like ability of FOA reduced by paraquat cation radicals to consume H2O2 and produce hydroxyl radicals (OH.) in the process. Similar reactions may explain previously reported potentiation of the oxidizing toxicity of paraquat in rats by high doses of DFO, as well as several other examples of prooxidant actions of DFO in vivo. We present the hypothesis that biphasic antioxidant/prooxidant behavior of DFO as a function of dose may be common with iron-catalyzed oxidizing reactions when mobile strong reducing agents are present. Hence, the real possibility of amplifying oxidizing damage must be considered when planning treatment with DFO, and failure of DFO to inhibit a particular response to oxidizing stress or its enhancement by DFO cannot, by itself, be considered sufficient evidence to rule out an iron-dependent process.