The L-histidine-mediated enhancement of hydrogen peroxide-induced cytotoxicity is a general response in cultured mammalian cell lines and is always associated with the formation of DNA double strand breaks

Catalase inhibition by nitric oxide potentiates hydrogen peroxide to trigger catastrophic chromosome fragmentation in Escherichia coli

Hydrogen peroxide (H2O2, HP) is a universal toxin that organisms deploy to kill competing or invading cells. Bactericidal action of H2O2 presents several questions. First, the lethal H2O2 concentrations in bacterial cultures are 1000x higher than, for example, those calculated for the phagosome. Second, H2O2-alone kills bacteria in cultures either by mode-one, via iron-mediated chromosomal damage, or by mode-two, via unknown targets, but the killing mode in phagosomes is unclear. Third, phagosomal H2O2 toxicity is enhanced by production of nitric oxide (NO), but in vitro studies disagree: some show NO synergy with H2O2 antimicrobial action, others instead report alleviation. To investigate this "NO paradox," we treated Escherichia coli with various concentrations of H2O2-alone or H2O2+NO, measuring survival and chromosome stability. We found that all NO concentrations make sublethal H2O2 treatments highly lethal, via triggering catastrophic chromosome fragmentation (mode-one killing). Yet, NO-alone is not lethal, potentiating H2O2 toxicity by blocking H2O2 scavenging in cultures. Catalases represent obvious targets of NO inhibition, and catalase-deficient mutants are indeed killed equally by H2O2-alone or H2O2+NO treatments, also showing similar levels of chromosome fragmentation. Interestingly, iron chelation blocks chromosome fragmentation in catalase-deficient mutants without blocking H2O2-alone lethality, indicating mode-two killing. In fact, mode-two killing of WT cells by much higher H2O2 concentrations is transiently alleviated by NO, reproducing the "NO paradox." We conclude that NO potentiates H2O2 toxicity by promoting mode-one killing (via catastrophic chromosome fragmentation) by otherwise static low H2O2 concentrations, while transiently suppressing mode-two killing by immediately lethal high H2O2 concentrations.

Potentiation of hydrogen peroxide toxicity: From catalase inhibition to stable DNA-iron complexes

Hydrogen peroxide (H₂O₂) is unique among general toxins, because it is stable in abiotic environments at ambient temperature and neutral pH, yet rapidly kills any type of cells by producing highly-reactive hydroxyl radicals. This life-specific reactivity follows the distribution of soluble iron, Fe(II) (which combines with H₂O₂ to form the famous Fenton's reagent),Fe(II) is concentrated inside cells, but is virtually absent outside them. Because of the immediate danger of H₂O₂, all cells have powerful H₂O₂ scavengers, the equally famous catalases, which enable cells to survive thousand-fold higher concentrations of H₂O₂ and, in combination with adequate movement of H₂O₂ across membranes, make the killing H₂O₂ concentrations virtually impractical to generate in vivo. And yet, low concentrations of H₂O₂ are somehow used as an efficient biological weapon. Here we review several examples of how cells potentiate H₂O₂ toxicity with other chemicals. At first, these potentiators were thought to simply inhibit catalases, but recent findings with cyanide suggest that potentiators mostly promote the other side of Fenton's reaction, recruiting iron from cell depots into stable DNA-iron complexes that, in the presence of elevated H₂O₂, efficiently break duplex DNA, pulverizing the chromosome. This multifaceted potentiation of H₂O₂ toxicity results in robust and efficient killing.

Endogenous Generation of Singlet Oxygen and Ozone in Human and Animal Tissues: Mechanisms, Biological Significance, and Influence of Dietary Components

Recent studies have shown that exposing antibodies or amino acids to singlet oxygen results in the formation of ozone (or an ozone-like oxidant) and hydrogen peroxide and that human neutrophils produce both singlet oxygen and ozone during bacterial killing. There is also mounting evidence that endogenous singlet oxygen production may be a common occurrence in cells through various mechanisms. Thus, the ozone-producing combination of singlet oxygen and amino acids might be a common cellular occurrence. This paper reviews the potential pathways of formation of singlet oxygen and ozone in vivo and also proposes some new pathways for singlet oxygen formation. Physiological consequences of the endogenous formation of these oxidants in human tissues are discussed, as well as examples of how dietary factors may promote or inhibit their generation and activity.

Reactive oxygen species modify oligosaccharides of glycoproteins in vivo: a study of a spontaneous acute hepatitis model rat (LEC rat)

The Long-Evans Cinnamon (LEC) rat, an animal model of Wilson's disease, spontaneously develops hepatitis as the result of abnormal copper accumulation in liver. The findings of this study show that copper, hydrogen peroxide, and lipid peroxides accumulate to drastically high levels in LEC rat serum in acute hepatitis but not chronic hepatitis. The effect of these reactive oxygen species (ROS) on oligosaccharides of glycoproteins in the LEC rat serum was examined. Lectin blot and lectin ELISA analyses showed that sialic acid and galactose residues of serum glycoproteins including transferrin were decreased in acute hepatitis. Further analyses of oligosaccharide structures of transferrin demonstrated that di-sialylated and asialo-agalacto biantennary sugar chains, but not tri-sialylated sugar chain, exist on transferrin in the acute hepatitis rats. In addition, treatment of non-hepatitis rat serum with copper ions and hydrogen peroxide decreased tri-sialylated sugar chain of the normal transferrin and increased di-sialylated and asialo-agalacto biantennary sugar chains. This is the first evidence to show that ROS result in the cleavage of oligosaccharides of glycoproteins in vivo, and indicate this cleavage of oligosaccharides may contribute the development of acute hepatitis.

Shiga toxin 1 and ricin inhibit the repair of H2O2-induced DNA single strand breaks in cultured mammalian cells

A growing body of evidence suggests that ribosome-inactivating proteins (RIPs) remove adenine moieties not only from rRNA, but also from DNA--an effect leading to DNA damage in cultured cells. We herein report that two distinct RIPs of bacterial (shiga toxin 1, Stx1) and plant (ricin) origin, inhibit the repair of the DNA lesions generated by hydrogen peroxide in cultured human cells. This effect is unrelated either to inhibition of protein synthesis or to depletion of cellular antioxidant defenses and is likely to derive from direct interactions with cellular DNA repair machinery. Therefore, the genotoxicity of these toxins on mammalian cells seems to be a complex phenomenon resulting from the balance between direct (DNA damaging activity), indirect (DNA repair inhibition) effects and the eventual presence of other DNA damaging species. In particular, with regard to Stx1, it could be hypothesized that Stx-producing bacteria increase the risk of transformation of surrounding, inflamed tissues in the course of human infections.

Effects of histidine and imidazolelactic acid on various parameters of the oxidative stress in cerebral cortex of young rats

Histidinemia is an inherited metabolic disorder caused by deficiency of histidase activity, which leads to tissue accumulation of histidine and its derivatives. Affected patients usually present with speech delay and mental retardation, although asymptomatic patients have been reported. Considering that the pathophysiology of the neurological dysfunction of histidinemia is not yet understood and since histidine has been considered a pro-oxidant agent, in the present study we investigated the effect of histidine and one of its derivatives, l-beta-imidazolelactic acid, at concentrations ranging from 0.1 to 10 mM, on various parameters of oxidative stress in cerebral cortex of 30-day-old Wistar rats. Chemiluminescence, total radical-trapping antioxidant potential (TRAP), thiobarbituric acid reactive substances (TBA-RS), and the activities of the antioxidant enzymes superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GSH-Px) were measured in tissue homogenates in the presence of l-histidine or l-beta-imidazolelactic acid. We observed that l-histidine provoked an increase of chemiluminescence and a reduction of TRAP at concentrations of 2.5 mM and higher, while TBA-RS measurement, GSH-Px, CAT and SOD activities were not affected. Furthermore, l-beta-imidazolelactic acid provoked antioxidant effects at high concentrations (5-10 mM) as observed by the reduction of chemiluminescence, although this compound enhanced chemiluminescence at low concentrations (0.5-1 mM). These results suggest that in vitro oxidative stress is elicited by histidine but only at supraphysiological concentrations.

Killing of intraerythrocytic Plasmodium falciparum by lysosomotropic amino acid esters

Esters of amino acids are known to penetrate into cells by simple diffusion. Subsequently, they are hydrolyzed by hydrolases to release the parent amino acid. Due to the abundance of hydrolases in phagolysosomes, amino acids accumulate, there because the rate of influx and hydrolysis exceed the rate of amino acid efflux through specific carriers. The osmotic effect of this accumulation results in the disruption of the organelles. This mechanism has been demonstrated to be responsible for the killing of Leishmania amastigotes by amino acid esters. In this investigation, it is shown that all esters tested, including alcohol esters, N-acetyl esters and the esters of some dipeptides, inhibit the growth of Plasmodium falciparum in culture. Inhibition is time-dependent and, in some cases, ring-stage parasites are more sensitive than trophozoites. Similar to the findings with Leishmania, alcohol esters of Glu, Leu, Met, Phe and Trp are more toxic to Plasmodium whereas Ala, Gly, His and Ile are much less noxious. Esters caused the release of acridine orange that selectively accumulates in the phagolysosome-like food vacuole of the parasite, attesting the ostensible destruction of this organelle by osmotic lysis. The toxicity of the N-acetyl esters is probably associated in part to their ability to inhibit cytosolic proteases. Since excess of amino acids can also inhibit proteolysis, the effect of free amino acids on parasite growth was also tested. Of the 19 odd amino acids tested, only three, namely Cys, His and Trp, were found to be toxic to the parasites at millimolar concentrations and the reasons for their possible specific toxicity are discussed.

Peroxynitrite promotes mitochondrial permeability transition-dependent rapid U937 cell necrosis: survivors proliferate with kinetics superimposable on those of untreated cells

A short term exposure to peroxynitrite promotes a time- and concentration-dependent lethal response in U937 cells. The mode of cell death was necrosis and rapid (within minutes) cell lysis was found to occur via a mechanism involving mitochondrial permeability transition. Apoptosis was not detected in cells exposed to low levels of peroxynitrite, or in cells which survived a treatment with toxic amounts of peroxynitrite, neither after the 60 min exposure nor following increasing time intervals of growth in fresh culture medium. Rather, cells treated with peroxynitrite concentrations which were not immediately lethal, as well as the survivors of treatments with toxic levels of peroxynitrite, proliferated with kinetics superimposable on those observed in untreated cells.

Cellular sites of H2O2-induced damage and their protection by nitroxides

While the exact mechanism of H2O2-induced cytotoxicity is unknown, there is considerable evidence implicating DNA as a primary target. A recent study showed that a cell-impermeable nitroxide protected mammalian cells from H2O2-induced cell killing and suggested that the protection was mediated through cell membrane-bound or extracellular factors. To further define the protective properties of nitroxides, Chinese hamster V79 cells were exposed to H2O2 with or without cell-permeable and impermeable nitroxides and selected metal chelators. EPR spectroscopy and paramagnetic line broadening agents were used to distinguish between intra- and extracellular nitroxide distribution. To study the effectiveness of nitroxide protection, in the absence of a cell membrane, H2O2-mediated damage to supercoiled plasmid DNA was evaluated. Both deferrioxamine and Tempol cross the cell membrane, and inhibited H2O2-mediated cell killing, whereas the cell-impermeable DTPA and nitroxide, CAT-1, failed to protect. Similar protective effects of the chelators and nitroxides were observed when L-histidine, which enhances intracellular injury, was added to H2O2. In contrast, when damage to plasmid DNA was induced (in the absence of a cell membrane), both nitroxides were protective. Collectively, these results do not support a role for membrane-bound or extracellular factors in mediating H2O2 cytotoxicity in mammalian cells.

The role of DNA damage in the cytotoxic response to hydrogen peroxide/histidine

1. Histidine enhances the cytotoxic and clastogenic effects of hydrogen peroxide. In this review, we will focus on two lesions that are generated in the presence of histidine in oxidatively injured cells--namely, DNA single- and double-strand breaks (SSBs and DSBs). 2. Hydrogen peroxide is a potent inducer of DNA SSBs, and histidine modulates the formation of these lesions. This effect has been extensively characterized with the use of purified DNA, and the results obtained have demonstrated that, upon exposure to low or high concentrations of H2O2, histidine reduces or enhances the formation of DNA SSBs, respectively. The protective effect has been ascribed to iron chelation, whereas the enhancing effect is probably the consequence of the formation of a histidine/iron/DNA complex. 3. In cultured cells, histidine potentiates the formation of H2O2-induced DNA SSBs but these lesions are efficiently repaired and do not appear to mediate the cytotoxic response. 4. In the presence of micromolar levels of histidine, H2O2 also induces DNA DSBs, a type of lesion that is not generated by the oxidant alone. The experimental evidence that has been thus far collected would suggest that these DNA DSBs are toxic and are indeed the cause of cell death induced by the cocktail H2O2/histidine.

Low levels of hydrogen peroxide and L-histidine induce DNA double-strand breakage and apoptosis

The results presented in this study demonstrate that L-histidine triggers a lethal response in U937 cells exposed to nontoxic, albeit growth-inhibitory, levels of H2O2. Treatment for 1 h with the cocktail H2O2/L-histidine promotes the formation of a low level of DNA double-strand breaks that are rapidly rejoined, and this process is followed by secondary DNA fragmentation at about 7 h of post-treatment incubation, at which time cells are still viable. The appearance of oligonucleosomal DNA fragments associated with the detection of morphological changes typical of apoptosis strongly suggests that a portion of the cells was undergoing an apoptotic process. The relative level of cells with fragmented chromatin never exceeded 15-20% throughout the 20 h post-treatment incubation. Treatment with high concentrations of H2O2 in the presence of L-histidine was found to trigger necrotic cell death. The results presented in this paper provide further experimental evidence in support of the notion that DNA double-strand breaks mediate the lethal effects of the cocktail H2O2/L-histidine and suggest that this type of DNA lesion can promote both apoptotic and necrotic cell death, depending on the concentration of the oxidant.

Cytotoxic impact of DNA single vs double strand breaks in oxidatively injured cells

Hydrogen peroxide is a potent inducer of DNA single strand breaks (SSBs) in cultured mammalian cells. These lesions, however, are efficiently repaired and do not appear to mediate the cytotoxic response. This inference is based on the observations that a) inhibiting the rate of SSB-removal does not result in an increased cytotoxicity; b) using different experimental conditions it is possible to dissociate the formation of DNA SSBs from the cytotoxic response; c) the induction/loss of the oxidant-resistant phenotype in cell variants characterized by different levels of resistance to the lethal effect of the oxidant does not correlate with resistance to DNA SSB-induction; d) a much larger accumulation of DNA SSBs can be observed following treatment with H2O2 at 4 degrees C, as compared to 37 degrees C, although the opposite is true in terms of cytotoxicity. In the presence of micromolar levels of L-Histidine, H2O2 also induces DNA double strand breaks (DSBs), a type of lesion which we suggest may mediate the lethal event. This conclusion finds experimental support in the following observations: a) DNA DSBs are generated at survival-range concentrations, and a linear correlation exists between the level of this lesion and cytotoxicity; b) this correlation curve overlaps with the curves generated under similar experimental conditions using different cell lines with different sensitivity to the oxidant alone, or different clones derived from the same cell line, some of which showed a high degree of resistance to H2O2. Finally, the formation of DNA DSBs appears to enhance both apoptotic and necrotic cell death.

The effect of hydrogen peroxide/L-histidine-induced DNA single- vs. double-strand breaks on poly(ADP-ribose)polymerase

L-Histidine markedly increases the ability of hydrogen peroxide to induce DNA cleavage and this effect is associated with a 3-aminobenzamide-inhibitable decline in NAD+ levels, an event which very likely reflects an enhanced stimulation of the enzyme poly(ADP-ribose)polymerase. 3-Aminobenzamide slowed down the removal of alkaline elution-detected strand breaks induced by either H2O2 alone (producing only DNA single-strand breaks) or associated with L-histidine (resulting in the formation of both single-strand breaks and DNA double-strand breaks), and the extent of inhibition was similar under the two experimental conditions. 3-Aminobenzamide did not affect the rate of rejoining of DNA double-strand breaks generated by the cocktail H2O2/L-histidine. The above results suggest that these double-strand breaks have hardly any effect on the induction of poly(ADP-ribose)polymerase activity, a conclusion that is consistent with the observation that the activity of this enzyme appears to be basically identical under conditions that abolish the formation of DNA double-strand breaks, in the absence of measurable variations in the level of induction of DNA single-strand breaks (e.g. in the presence of an excess of L-glutamine, a competitive inhibitor of L-histidine uptake). Finally, 3-aminobenzamide did not affect the toxicity of the oxidant, both in the absence and presence of L-histidine.