Lipid peroxidation in rat liver microsomes. II. Response of hydroperoxide formation to iron concentration

Different antioxidant effects of polyphenols on lipid peroxidation and hydroxyl radicals in the NADPH-, Fe-ascorbate- and Fe-microsomal systems

Antioxidant and pro-oxidant effects of 14 naturally occurring polyphenols (PP) on rat liver microsomal lipid peroxidation (LP) and hydroxyl radical (*OH) production were studied in NADPH-dependent, 50 microM Fe(2+)-500 microM ascorbate (Fe-AA) or 50 microM Fe(2+) system, respectively. LP determined by the thiobarbituric acid method was inhibited in the NADPH system by flavonols and trans-resveratrol that were more effective than other flavonoids and derivatives of benzoic and cinnamic acid and were mostly more efficient than in the Fe-AA system. Inhibition of LP in the Fe system was higher by one order of magnitude than in the Fe-AA system. *OH production in the NADPH system, measured by formaldehyde production, was decreased by myricetin, fisetin and quercetin, but increased by kaempferol, morin and trans-resveratrol, indicating that z.rad;OH played a minor role in LP, which all of these PP inhibited. None of these PP at up to 40 microM concentration quenched *OH in the Fe-AA system. All tested PP, except trans-resveratrol and gentisic acid, spectrally interacted with Fe(2+) or Fe(3+), indicating formation of complexes or oxidation of PP. In contrast to the NADPH system we found no correlation between Fe(2+) chelation and inhibition of Fe-AA- or Fe-dependent LP indicating that iron chelation did not play a significant role in the two latter systems. It is concluded that the inhibition of LP by PP was apparently due to their hydrogen donating properties rather than chelation of iron.

Age-dependent differences in the stimulation of lipid peroxidation in the heart of rats during immobilization stress

In order to investigate the possible reasons for age-related decrease in myocardium resistance to stress, we carried out a study of lipid peroxidation (LPO) stimulation features in the myocardium of adult (10-12 months) and aged (22-25 months) male Wistar rats during immobilization stress. In our studies of ascorbate-dependent LPO and induced chemiluminescence, we found that immobilization stress is accompanied by decreased efficiency in the induction of free radical processes in the heart of aged rats. An important cause of this phenomenon may be age-dependent changes in the catalytical properties of the cytosolic superoxide dismutase. The pathophysiological consequences of stress-related, age-dependent decreased efficiency of induction of free radical processes in the heart are discussed.

Effect of piperonyl butoxide or allopurinol on cyanide-induced lipid peroxidation in mouse brain

The effect of piperonyl butoxide or allopurinol on cyanide-induced lipid peroxidation was investigated using homogenates from whole brain of mice. The brain homogenate exposed to a low concentration of potassium cyanide (10, 50, or 100 microM) was significantly increased in their concentration of malondialdehyde (MDA) + 4-hydroxyalkenals (4-MDA) as compared to control samples, in a concentration-dependent manner. The increased lipid peroxidation induced by cyanide was inhibited by piperonyl butoxide (1 mM), an inhibitor of mixed function oxidase, but not by allopurinol (0.1 mM), an inhibitor of xanthine oxidase. Furthermore, when a brain homogenate heated at 86 degrees C for 1 min was incubated with or without cyanide at 37 degrees C for 20 min, MDA + 4-MDA levels in the homogenate were not changed between cyanide treatment and untreated. An intraperitoneal injection of piperonyl butoxide (1 g/kg) significantly inhibited cyanide-induced seizures in mice. These results suggest that cyanide-induced seizures may be partly involved in the lipid peroxidation produced by the heat unstable and piperonyl butoxide dependent factors in brain.

Effect of melatonin, piperonyl butoxide, or cobalt chloride on L-cysteine-induced lipid peroxidation in homogenate from whole brain of mice

Brain homogenates exposed to a low concentration of L-cysteine (0.05, 0.1, 0.5, 1.0, and 2.0 mM) were significantly increased in their concentration of malondialdehyde (MDA) + 4-hydroxyalkenals (4-HDA) as compared to control samples, in a concentration-dependent manner. The increased lipid peroxidation by L-cysteine was attenuated or completely abolished by co-incubation with melatonin (2 mM), a potent free radical scavenger, piperonyl butoxide (1 mM), an inhibitor of mixed function oxidase, or cobalt chloride (0.01, 0.02 or 0.05 mM), another inhibitor of mixed function oxidase, but not allopurinol (0.1 mM), an inhibitor of xanthine oxidase. In addition, when a brain homogenate heated at 85 degrees C for 1 min was incubated with or without L-cysteine at 37 degrees C for 20 min, MDA + 4-HDA levels in the homogenate was not changed between L-cysteine and control. These results suggest that L-cysteine-induced lipid peroxidation may be involved in the enzymatic mixed function oxidation systems in brain.

A possible mechanism for initiation of lipid peroxidation by ascorbate in rat liver microsomes

The mechanism by which lipid peroxidation progresses has been known for years, but there is disagreement regarding the mode of its initiation. The aim of this study was to examine: (a) the role of endogenous iron in the initiation of ascorbate-induced lipid peroxidation in microsomal and liposomal membranes; (b) the role of oxygen-free radicals in this process; and (c) the redox state of ascorbate during the course of lipid peroxidation. Ascorbate-induced lipid peroxidation was assessed by measuring hydroperoxide and thiobarbituric acid reactive substances (TBARS) formation in membranes after incubation in Tris-HCl buffer (pH 7.4) for 15 min. To confirm the role of endogenous iron and oxygen-free radicals, the effect of iron chelating agents (EDTA and thiourea) and radical scavengers (benzoate, mannitol, catalase and SOD) on lipid peroxidation was examined. Spectrophotometric measurements and ESR spectra have made it possible to determine ascorbate concentration and its redox state. Ascorbate promoted lipid peroxidation in both rat liver microsomes and liposomes without addition of exogenous iron. Iron chelating agents such as EDTA and thiourea inhibited lipid peroxidation, while SOD, catalase, mannitol and benzoate had no effect. The addition of 5 microM Fe2+ (or Fe3+) to the incubation mixture did not significantly alter hydroperoxide production, but that of TBARS was increased. Lipid peroxidation significantly altered the fatty acid profile in microsomes and liposomes, the most affected being the C20:4 and C22:6 species. Ascorbate in Tris-HCl buffer (pH 7.4) autoxidized very slowly. Its oxidation was catalyzed by Fe3+ ions at a rate determined by incubation time and iron concentration. In contrast, no ascorbate oxidation occurred in the presence of microsomes when lipid peroxidation was proceeding at a maximal rate. Under these conditions a typical ascorbyl radical ESR spectrum signal greater than that arising from ascorbate alone was obtained and the magnitude of this signal was unchanged by variations of microsome or ascorbate concentrations. A ferrous ion ascorbyl radical complex was responsible for this signal. These results suggest that an ascorbate-microsomal iron complex is responsible for the initiation of lipid peroxidation, and that during this process ascorbate remains in its reduced form.

Effect of gamma-irradiated carbohydrates on isolated synaptic membranes

The effect of gamma-irradiated solutions of carbohydrates, mainly glucose, upon Na+, K(+)-ATPase and lipid peroxidation in rat brain synaptosomal membranes was studied. The membrane damage by irradiated glucose was enhanced in the presence of Fe2+ and was diminished when a free-radical scavenger (BHT) or metal chelators (EDTA, EGTA) were present. It is suggested that a key element in the free-radical membrane damage by irradiated carbohydrates is an Fe(2+)-complex of some species of the radiolysis products. Participation of radiotoxins of carbohydrate origin in radiobiological effects is discussed.

The role of an endogenous nonheme iron in microsomal redox reactions

Microsomal membranes contain a nonheme iron which serves in vitro for the peroxidation of unsaturated lipids or the oxidation of several other chemicals. These redox reactions are reviewed in light of a recent identification of two or more iron-binding proteins in the microsomal milieu. Indirect evidence that the microsomal iron might serve in vivo for the synthesis of heme iron is also presented and discussed. Consistent with this, the newly identified iron proteins not only participate in redox reactions but also release their bound iron upon incubation with certain intermediates of heme synthesis.

Redox cycling of iron and lipid peroxidation

Mechanisms of iron-catalyzed lipid peroxidation depend on the presence or absence of preformed lipid hydroperoxides (LOOH). Preformed LOOH are decomposed by Fe(II) to highly reactive lipid alkoxyl radicals, which in turn promote the formation of new LOOH. However, in the absence of LOOH, both Fe2+ and Fe3+ must be available to initiate lipid peroxidation, with optimum activity occurring as the Fe2+/Fe3+ ratio approaches unity. The simultaneous availability of Fe2+ and Fe3+ can be achieved by oxidizing some Fe2+ with hydrogen peroxide or with chelators that favor autoxidation of Fe2+ by molecular oxygen. Alternatively, one can use Fe3+ and reductants like superoxide, ascorbate or thiols. In either case excess Fe2+ oxidation or Fe3+ reduction will inhibit lipid peroxidation by converting all the iron to the Fe3+ or Fe2+ form, respectively. Superoxide dismutase and catalase can affect lipid peroxidation by affecting iron reduction/oxidation and the formation of a (1:1) Fe2+/Fe3+ ratio. Hydroxyl radical scavengers can also increase or decrease lipid peroxidation by affecting the redox cycling of iron.

The response of rat liver lipid peroxidation, antioxidant enzyme activities and glutathione concentration to the thyroid hormone

1. In liver microsomes from hyperthyroid rats NADPH-dependent lipid peroxidation induces a hydroperoxide formation 56% higher than that in euthyroid ones. 2. The addition of 5 microM Fe2+ (or Fe3+) strongly decreases the hydroperoxide level in favour of that of TBA-reactive substances. Higher iron concentrations (30 microM) have no significant effect. 3. In hepatocytes from hyperthyroid rats CCl4-induced lipid peroxidation produces an amount of TBA-reactive substances four times higher than that in those from euthyroid rats. 4. In the liver of hyperthyroid rats a GSH concentration decrease (by about 35%) is found while the opposite occurs in the blood of the same animals where GSH increases 2.5 times. 5. It is shown that in the liver of hyperthyroid rats, besides higher lipid peroxidation, a more active defense mechanism is operating since both glutathione peroxidase and glutathione reductase specific activities are higher than in euthyroid rats.