Effect of ascorbic acid on DNA damage, cytotoxicity, glutathione reductase, and formation of paramagnetic chromium in Chinese hamster V-79 cells treated with sodium chromate(VI).

The effect of pretreatment with ascorbic acid (vitamin C) on chromate-induced DNA damage, cytotoxicity, and enzyme inhibition as well as on the cellular reduction of chromium(VI) was investigated using Chinese hamster V-79 cells. Cellular pretreatment with nontoxic levels of 1 mM ascorbic acid for 24 h prior to exposure resulted in a significant increase (1.7-fold) in cellular levels of this vitamin. Alkaline elution assays demonstrated that this pretreatment decreased cellular levels of Na2CrO4-induced alkali-labile sites while the numbers of DNA-protein crosslinks produced by chromate increased. In colony-forming assays, pretreatment with ascorbic acid enhanced the cytotoxicity of chromate. However, the inhibition of glutathione reductase attributed to Na2CrO4 was attenuated by this pretreatment. Under the same experimental condition, the uptake of chromate in pretreated cells was found to increase. ESR studies revealed that cellular pretreatment with ascorbic acid reduced the level of chromium(V) intermediate and increased the level of chromium(III) complex, indicating that cellular reduction of chromium(VI) to chromium(III) was accelerated by this vitamin. These results suggest that ascorbic acid decreases chromate-induced alkali-labile sites and chromium inhibition of glutathione reductase, but it enhances DNA-protein cross-links and cytotoxicity caused by this metal through its ability to directly reduce chromium(VI).     

Generation of PM2 DNA breaks in the course of reduction of chromium(VI) by glutathione

The carcinogen chromate is efficiently taken up and reduced to chromium(III) compounds by various biological systems. To test the possible DNA damage induced in the course of chromium(VI) reduction, we used a combination of chromate with the reductant glutathione (GSH) as well as a green complex of chromium(V), which is formed in the reaction of chromate with GSH. The combination of chromate and glutathione was found to cause single-strand breaks in supercoiled circular DNA of the bacteriophage PM2. The green chromium(V) complex Na4(GSH)4Cr(V).8H2O, prepared from chromate and glutathione, also cleaved supercoiled PM2 DNA. No DNA-degrading effects were observed with either chromate or the final product of the reaction with GSH, a purple anionic chromium(III) GSH complex. The nature of the buffering agents revealed a strong influence on the extent of DNA strand breaks produced by chromate and GSH. A variation of the GSH concentration in the reaction with chromate and PM2 DNA, performed in sodium phosphate-buffered solutions showed an initial increase in the number of strand breaks at GSH concentrations up to 1 mM followed by a decline at higher GSH concentrations. Since neither chromate, when administered individually, nor the final product of chromium(VI) reduction, the purple chromium(III) GSH complex, produced any detectable DNA cleavage, the critical steps leading to DNA strand breaks occur in the course of the conversion of chromium(VI) to chromium(III) by GSH, the most abundant intracellular low molecular thiol. Moreover, the demonstration that DNA cleavage is induced in the presence of the chromium(V) complex identifies chromium(V) as the oxidation state of the metal, which is involved in the steps leading to DNA-damaging effects of chromate.     

Potentiation of sodium chromate(VI)-induced chromosomal aberrations and mutation by vitamin B2 in Chinese hamster V79 cells.

The effect of vitamin B2, which is capable of reducing chromium(VI) to chromium(V), on chromosomal aberrations and mutation caused by Na2CrO4 was investigated in Chinese hamster V79 cells. Pretreatment with 200 microM vitamin B2 (riboflavin) for 24 h prior to exposure to Na2CrO4 (2.5-5 microM) resulted in an increase of metal-induced chromosomal aberrations and mutation at the HGPRT locus. These and other previous studies suggest that vitamin B2 enhances the clastogenic and mutagenic action of chromate compounds, through its ability to directly reduce chromium(VI) in cells.     

Vitamin B2-enhancement of sodium chromate (VI)--Induced DNA single strand breaks: ESR study of the action of vitamin B2

Incubation of Chinese hamster V-79 cells with Na2CrO4 plus vitamin B2 resulted in an increase of Na2CrO4-induced DNA single strand breaks. Electron spin resonance (ESR) studies showed that vitamin B2 enhanced the formation of both hydroxyl radical and tetraperoxochromate (V) during the reaction of Na2CrO4 with hydrogen peroxide. Furthermore, ESR studies demonstrated that a chromium (V) species with a g value of 1.977 was formed by the reaction of Na2CrO4 with vitamin B2. These results indicate that chromate reacts with vitamin B2 to form chromium (V) species and also suggest that the enhancement effect of vitamin B2 on chromate-induced DNA single strand breaks may result from an increase of chromium (V)-related hydroxyl radical formation.     

Beneficial role of hydrophytes in removing Cr(VI) from wastewater in association with chromate-reducing bacterial strains Ochrobactrum intermedium and Brevibacterium

This study deals with the use of three chromium-resistant bacterial strains (Ochrobactrum intermedium CrT-1, Brevibacterium CrT-13, and CrM-1) in conjunction with Eichornia crassipes for the removal of toxic chromium from wastewater. Bacterial strains resulted in reduced uptake of chromate into inoculated plants as compared to noninoculated control plants. In the presence of different heavy metals, chromium uptake into the plants was 28.7 and 7.15% less at an initial K2CrO4 concentration of 100 and 500 microg ml(-1) in comparison to a metal free chromium solution. K2CrO4 uptake into the plant occurred at different pHs tested, but maximum uptake was observed at pH 5. Nevertheless, the bacterial strains caused some decrease in chromate uptake into the plants, but the combined effect of plants and bacterial strains conduce more removal of Cr(VI) from the solution.     

Protective effect of vitamin E against chromosomal aberrations and mutation induced by sodium chromate in Chinese hamster V79 cells

The effect of vitamin E on chromosomal aberrations and mutation caused by Na2CrO4 was investigated in Chinese hamster V79 cells. Pretreatment with 25 microM alpha-tocopherol succinate (vitamin E) for 24 h prior to chromate exposure (2.5-5 microM) resulted in a decrease of metal-induced chromosomal aberrations. Na2CrO4 (2.5-7.5 microM) induced mutations at the HGPRT locus, but only within a very limited concentration range. This mutagenic response could also be suppressed by pretreatment with vitamin E. These results suggest that vitamin E can protect cells from the clastogenic and mutagenic action of chromate compounds, possibly through its ability to scavenge chromium(V) and/or free radicals.     

The cytotoxicity and genotoxicity of particulate and soluble hexavalent chromium in human lung cells

Hexavalent chromium (Cr(VI)) is a human lung carcinogen. Cr(VI) is a particularly important and dangerous carcinogen, because there is widespread exposure to it both occupationally and to the general public. However, despite the potential for widespread exposure and the fact that the lung is its target organ, there are few reports of the genotoxicity of Cr(VI) in human lung cells. Clearly, in order to better understand this carcinogen, its effects in its target cells need to be evaluated. Accordingly, we determined the cytotoxicity and clastogenicity of both particulate (water-insoluble) and soluble Cr(VI) in primary human bronchial fibroblasts (PHBFs). We used lead chromate (PbCrO(4)) and sodium chromate (Na(2)CrO(4)) as prototypical particulate and soluble Cr(VI) salts, respectively. Both compounds induced concentration-dependent cytotoxicity after a 24h exposure in PHBFs. The relative survival was 87, 46, 26 and 2% after exposure to 0.1, 0.5, 1 and 5 microg/cm(2) PbCrO(4), respectively, and 74, 57, 13 and 0% after exposure to 1, 2.5, 5 and 10 microM Na(2)CrO(4), respectively. Similarly, the amount of chromosome damage increased with concentration after 24h exposure to both compounds. Specifically, 0.1, 0.5 and 1 microg/cm(2) PbCrO(4) damaged 15, 34 and 42% of metaphase cells with the total amount of damage reaching 18, 40 and 66 aberrations per 100 metaphases, respectively. PbCrO(4) (5 microg/cm(2)) induced such profound cell cycle delay that no metaphases were found. Na(2)CrO(4) (1 and 2.5 microM) damaged 18 and 33% of metaphase cells with the total amount of damage reaching 19 and 43 aberrations per 100 metaphases, respectively. Na(2)CrO(4) (5 and 10 microM) induced such profound cell cycle delay that no metaphases were found. Overall the data clearly indicate that Cr(VI) compounds are cytotoxic and genotoxic to human lung cells.     

微生物铬转化和抗性机制与生物修复研究进展

铬(Chromium,Cr)是过渡金属元素,在自然界中以六价[CrO_4~(2-),Cr_2O_7~(2-),Cr(Ⅵ)]和三价[Cr(OH)_3,Cr(Ⅲ)]为主。很多微生物在长期铬胁迫的条件下,进化出了一系列铬转化和抗性机制。微生物对铬的转化包括Cr(Ⅵ)的还原和Cr(Ⅲ)的氧化。微生物的Cr(Ⅵ)还原可以将毒性强的六价铬转化为毒性弱或无毒的三价铬,这类微生物有较强的土壤和水体铬污染治理潜力。Cr(Ⅲ)的氧化也在铬的生物地球化学循环过程中起着至关重要的作用。除了Cr(Ⅵ)的还原,微生物对铬的抗性机制还有:(1)减少摄入;(2)外排;(3)清除胞内氧化压力;(4)DNA修复。本文主要介绍微生物的铬转化和抗性机制,以及其在铬污染生物修复中应用的最新研究进展。     

Intracellular chromium reduction

Two steps are involved in the uptake of Cr(VI): (1) the diffusion of the anion CrO4(2-) through a facilitated transport system, presumably the non-specific anion carrier and (2) the intracellular reduction of Cr(VI) to Cr(III). The intracellular reduction of Cr(VI), keeping the cytoplasmic concentration of Cr(VI) low, facilitates accumulation of chromate from extracellular medium into the cell. In the present paper, a direct demonstration of intracellular chromium reduction is provided by means of electron paramagnetic (spin) resonance (EPR) spectroscopy. Incubation of metabolically active rat thymocytes with chromate originates a signal which can be attributed to a paramagnetic species of chromium, Cr(V) or Cr(III). The EPR signal is originated by intracellular reduction of chromium since: (1) it is observed only when cells are incubated with chromate, (2) it is present even after extensive washings of the cells in a chromium-free medium; (3) it is abolished when cells are incubated with drugs able to reduce the glutathione pool, i.e., diethylmaleate or phorone; and (4) it is abolished when cells are incubated in the presence of a specific inhibitor of the anion carrier, 4-acetamido-4'-isothiocyanatostilbene-2-2'-disulfonic acid.     

Mechanisms of chromium toxicity in mitochondria

The oxygen consumption of isolated rat heart mitochondria was potently depressed in presence of 10-50 microM Na2CrO4 when NAD-linked substrates were oxidized. The succinate stimulated respiration and the oxidation of exogeneous NADH in sonicated mitochondria were not affected by chromate at this concentration range. A rapid and persistent drop (40% in 2 min) in the mitochondrial NADH level was observed after chromate addition (30 microM) under conditions which generally should promote regeneration of NADH. Experiments with bis-(2-ethyl-2-hydroxybutyrato)oxochromate(V) and vanadyl induced reduction of Cr(VI) in presence of excess NADH were performed. These experiments indicated that NADH may be directly oxidized by Cr(V) at physiological pH. The activity of 10 different enzymes were measured after lysis of intact mitochondria pretreated with chromate (1-100 microM). Na2CrO4 at a very low level (3-5 microM) was sufficient for 50% inhibition of alpha-ketoglutarate dehydrogenase. Higher concentrations (20-70 microM) was necessary for similar effect on beta-hydroxybutyrate and pyruvate dehydrogenase. The other enzymes tested were unaffected. Thus, the chromate toxicity in mitochondria may be due to NADH depletion as a result of direct oxidation by Cr(V) as well as reduced formation of NADH due to specific enzyme inhibition.     

Chromium (VI)‐induced oxidative stress,apoptotic cell death and modulation of p53 tumor suppressor gene

Chromium (VI) is a widely used industrial chemical, extensively used in paints, metal finishes, steel including stainless steel manufacturing, alloy cast irons, chrome, and wood treatment. On the contrary, chromium (III) salts such as chromium polynicotinate, chromium chloride and chromium picolinate, are used as micronutrients and nutritional supplements, and have been demonstrated to exhibit a significant number of health benefits in rodents and humans. However, the cause for the hexavalent chromium to induce cytotoxicity is not entirely understood. A series of in vitro and in vivo studies have demonstrated that chromium (VI) induces an oxidative stress through enhanced production of reactive oxygen species (ROS) leading to genomic DNA damage and oxidative deterioration of lipids and proteins. A cascade of cellular events occur following chromium (VI)induced oxidative stress including enhanced production of superoxide anion and hydroxyl radicals, increased lipid peroxidation and genomic DNA fragmentation, modulation of intracellular oxidized states, activation of protein kinase C, apoptotic cell death and altered gene expression. In this paper, we have demonstrated concentration and timedependent effects of sodium dichromate (chromium (VI) or Cr (VI)) on enhanced production of superoxide anion and hydroxyl radicals, changes in intracellular oxidized states as determined by laser scanning confocal microscopy, DNA fragmentation and apoptotic cell death (by flow cytometry) in human peripheral blood mononuclear cells. These results were compared with the concentration-dependent effects of chromium (VI) on chronic myelogenous leukemic K562 cells and J774A.1 murine macrophage cells. Chromium (VI)induced enhanced production of ROS, as well as oxidative tissue and DNA damage were observed in these cells. More pronounced effect was observed on chronic myelogenous leukemic K562 cells and J774A.1 murine macrophage cells. Furthermore, we have assessed the effect of a single oral LD₅₀ dose of chromium (VI) on female C57BL/6Ntac and p53deficient C57BL/6TSG p53 mice on enhanced production of superoxide anion, lipid peroxidation and DNA fragmentation in the hepatic and brain tissues. Chromium (VI)induced more pronounced oxidative damage in p53 deficient mice. This in vivo study highlighted that apoptotic regulatory protein p53 may play a major role in chromium (VI)induced oxidative stress and toxicity. Taken together, oxidative stress and oxidative tissue damage, and a cascade of cellular events including modulation of apoptotic regulatory gene p53 are involved in chromium (VI)induced toxicity and carcinogenesis.     

Role of physiological antioxidants in chromium(VI)-induced cellular injury.

Chromium(VI) compounds are well known to be potent toxic and carcinogenic agents. Because chromium(VI) is easily taken up by cells and is subsequently reduced to the trivalent form, the formation of chromium(III) or other intermediate oxidation states such as chromium(V) and (IV) is believed to play a role in the adverse biological effects of chromium(VI) compounds. Recent in vitro studies have shown that this reduction process generates free radical species such as active oxygen radicals. Furthermore, physiological antioxidants are reported to modify the genotoxic and toxic effects of chromate. This article reviewed the recent in vitro and in vivo studies of the effects of antioxidants including active oxygen scavengers; glutathione; vitamins B2, E, and C, on chromate-induced injury such as DNA lesions; lipid peroxidation; enzyme inhibition; cytotoxicity; mutation; and so on. In addition, the mechanism of action of these antioxidants was discussed with respect to the formation of active oxygen radicals and paramagnetic chromium such as chromium(V) and (III). Such studies may help elucidate the mechanism of chromium(VI) toxicity as well as the mechanism of protection.     

ESR characterization and metallokinetic analysis of Cr(V) in the blood of rats given carcinogen chromate(VI) compounds.

It has been shown that bio-trace metal elements are related to many diseases and the aging process. For many years, carcinogen hexavalent chromium (VI) has been known to be toxic to animals, but its dynamic toxicological mechanism is not sufficiently elucidated. Bioinorganic chemistry in terms of metallokinetic analysis of beneficial or toxic metal ions and their complexes is an important investigation for understanding their biochemical and physiological roles. We have tried to examine the real-time behavior of paramagnetic metal ions and complexes in animals, in which electron spin resonance (ESR) was capable of measuring paramagnetic species in chemical and biological systems. On the basis of our previous results on stable nitroxide spin probes, we have developed the in vivo blood circulation monitoring-electron spin resonance (BCM-ESR) method to analyze time-dependent ESR signal changes due to paramagnetic metal ions and their complexes in real time. When K2Cr2O7 or Na2Cr2O7 in saline was intravenously administered to rats, two ESR signals due to pentavalent chromium(V) were detectable in the circulating blood of rats. Cr(V) detected in the blood was indicated to be in the CrO(O4) and CrO(S2O2) coordination modes after the study on model complexes. From the changes of ESR signal intensities due to Cr(V) in the blood, the metallokinetic parameters were obtained using the pharmacokinetic analysis and the curve-fitting methods. The obtained results are important for understanding carcinogen chromate in terms of the formation of Cr(V) in animals. In addition, we propose the BCM-ESR method, which is useful to analyze the disposition of paramagnetic metal species in the blood of living animals.     

Extra-cellular chromate-reducing activity of the yeast cultures

This paper reports on the experimental data supporting an essential role of extra-cellular reduction in chromate detoxification by baker’s and non-conventional yeasts. A decrease of chromate content in the yeast culture coincides with an increase of Cr(III) content in extra-cellular liquid. At these conditions, cell-bound chromium level was insignificant and a dominant part of extra-cellular Cr(III) species was detected in the reaction with chromazurol S only after mineralization of the cell-free samples. This phenomenon of chromium “disappearance” can be explained by the formation of Cr(III) stable complexes with extra-cellular yeast-secreted components which are “inaccessible” in the reaction with chromazurol S without mineralization. It was shown that increasing sucrose concentration in a growth medium resulted in an increase of chromate reduction. A strong inhibition of chromate reduction by 0.25 mM sodium azide, a respiration inhibitor and a protonophore, testifies that extra-cellular chromate detoxification depends on energetic status of the yeast cells. It was shown that Cr(III)-biochelates produced in extra-cellular medium are of a different chemical nature and can be separated into at least two components by ion-exchange chromatography on anionit Dowex 1x10. A total yield of the isolated Cr(III)-biocomplexes is approximately 65 % (from initial level of chromate) with a relative molar ratio 8:5.     

A bacterial flavin reductase system reduces chromate to a soluble chromium(III)-NAD(+) complex

Biological reduction of carcinogenic chromate has been extensively studied in eukaryotic cells partly because the reduction produces stable chromium(III)-DNA adducts, which are mutagenic. Microbial reduction of chromate has been studied for bioremediation purposes, but little is known about the reduction mechanism. In eukaryotic cells chromate is mainly reduced non-enzymatically by ascorbate, which is usually absent in bacterial cells. We have characterized the reduction of chromate by a flavin reductase (Fre) from Escherichia coli with flavins. The Fre-flavin system rapidly reduced chromate, whereas chemical reduction by NADH and glutathione was very slow. Thus, enzymatic chromate reduction is likely the dominant mechanism in bacterial cells. Furthermore, the end-product was a soluble and stable Cr(III)-NAD(+) complex, instead of Cr(III) precipitate. Since intracellularly generated Cr(III) forms adducts with DNA, protein, glutathione, and ascorbate in eukaryotic cells, we suggest that the produced Cr(III) is primarily complexed to NAD(+), DNA, and other cellular components inside bacteria.     

The interaction of chromium with nucleic acids

Native and denatured calf thymus DNA, and homopolyribonucleotides were compared with respect to chromium and protein binding after an in vitro incubation with rat liver microsomes, NADPH, and chromium(VI) or chromium(III). A significant amount of chromium bound to DNA when chromium(VI) was incubated with the native or the denatured form of DNA in the presence of microsomes and NADPH. For both native and denatured DNA the amount of protein bound to DNA increased with the amount of chromium bound to DNA. Denatured DNA had much higher amounts of chromium and protein bound than native DNA. There was no interaction between chromium(VI) and either form of DNA in the absence of the complete microsomal reducing system. The binding of chrornium(III) to native or denatured DNA was small and relatively unaffected by the presence of microsomes and NADPH. The binding of chromium and protein to polyriboadenylic acid (poly(A)), polyribocytidylic acid (poly(C), polyri-boguanylic acid (poly(G)) and polyribouridylic acid (poly(U)) was determined after incubation with chromium(VI) in the presence of microsomes and NADPH. The magnitude of chromium and protein binding to the ribo-polymers was found to be poly(G) ? poly(A) ? poly(C) ? poly(U). These results suggest that the metabolism of chromium(VI) is necessary in order for chromium to interact significantly with nucleic acids. The metabolically-produced chromium preferentially binds to the base guanine and results in DNA-protein cross-links. These findings are discussed with respect to the proposed scheme for the carcinogenicity of chromium(VI). Keywords: DNA-protein cross-links — Chromium-guanine interaction-Microsomal reduction of chromate     

Addition of DNA to Cr(VI) and cytochrome b5 containing proteoliposomes leads to generation of DNA strand breaks and Cr(III) complexes

Chromium (Cr) is a cytotoxic metal that can be associated with a variety of types of DNA damage, including Cr-DNA adducts and strand breaks. Prior studies with purified human cytochrome b(5) and NADPH:P450 reductase in reconstituted proteoliposomes (PLs) demonstrated rapid reduction of Cr(VI) (hexavalent chromium, as CrO(4)(2-), and the generation of Cr(V), superoxide (O(2)(*-)), and hydroxyl radical (HO(*)). Studies reported here examined the potential for the species produced by this system to interact with DNA. Strand breaks of purified plasmid DNA increased over time aerobically, but were not observed in the absence of O(2). Cr(V) is formed under both conditions, so the breaks are not mediated directly by Cr(V). The aerobic strand breaks were significantly prevented by catalase and EtOH, but not by the metal chelator diethylenetriaminepentaacetic acid (DTPA), suggesting that they are largely due to HO(*) from Cr-mediated redox cycling. EPR was used to assess the formation of Cr-DNA complexes. Following a 10-min incubation of PLs, CrO(4)(2-), and plasmid DNA, intense EPR signals at g=5.7 and g=5.0 were observed. These signals are attributed to specific Cr(III) complexes with large zero field splitting (ZFS). Without DNA, the signals in the g=5 region were weak. The large ZFS signals were not seen, when Cr(III)Cl(3) was incubated with DNA, suggesting that the Cr(III)-DNA interactions are different when generated by the PLs. After 24 h, a broad signal at g=2 is attributed to Cr(III) complexes with a small ZFS. This g=2 signal was observed without DNA, but it was different from that seen with plasmid. It is concluded that EPR can detect specific Cr(III) complexes that depend on the presence of plasmid DNA and the manner in which the Cr(III) is formed.     

Hexavalent chromium reduction by an actinomycete, Arthrobacter crystallopoietes ES 32

Environmental contamination by hexavalent chromium, Cr(VI), presents a serious public health problem. This study assessed the reduction of Cr(VI) by intact cells and a cell-free extract (CFE) of an actinomycete, Arthrobacter crystallopoietes (strain ES 32), isolated from soil contaminated with dichromate. Both intact cells and CFE of A. crystallopoietes, displayed substantial reduction of Cr(VI). Intact cells reduced about 90% of the Cr(VI) added within 12 h and Cr(VI) was almost completely reduced after 24 h. The K _M and V _max of Cr(VI) bioreduction by intact cells were 2.61 μM and 0.0142 μmol/min/mg protein, respectively. Cell-free chromate reductase of the A. crystallopoietes (ES 32) reduced hexavalent chromium at a K _M of 1.78 μM and a V _max of 0.096 μmol/min/mg protein. The rate constant (k) of chromate reduction was inversely related to Cr(VI) concentration and the half-life (t _1/2) of Cr(VI) reduction increased with increasing concentration. A. crystallopoietes produced a periplasmic chromate reductase that was stimulated by NADH. Results indicate that A. crystallopoietes ES 32 can be used to detoxify Cr(VI) in polluted sites, particularly in stressed environments.     

Mechanism of DNA cleavage induced by sodium chromate(VI) in the presence of hydrogen peroxide

Reactivities of chromium compounds with DNA were investigated by the DNA sequencing technique using 32P 5'-end-labeled DNA fragments, and the reaction mechanism was investigated by ESR spectroscopy. Incubation of double-stranded DNA with sodium chromate(VI) plus hydrogen peroxide or potassium tetraperoxochromate(V) led to the cleavage at the position of every base, particularly of guanine. Even without piperidine, the formation of oligonucleotides was observed, suggesting the breakage of the deoxyribose-phosphate backbone. ESR studies using hydroxyl radical traps demonstrated that hydroxyl radical is generated both during the reaction of sodium chromate(VI) with hydrogen peroxide and the decomposition of potassium tetraperoxochromate(V), and that hydroxyl radical reacts significantly not only with mononucleotides but also with deoxyribose 5-phosphate. ESR studies using a singlet oxygen trap demonstrated that singlet oxygen is also generated both by the same reaction and decomposition, and reacts significantly with deoxyguanylate, but scarcely reacts with other mononucleotides. Furthermore, ESR studies suggested that tetraperoxochromate(V) is formed by the reaction of sodium chromate(VI) with hydrogen peroxide. These results indicate that sodium chromate(VI) reacts with hydrogen peroxide to form tetraperoxochromate(V), leading to the production of the hydroxyl radical, which causes every base alteration and deoxyribose-phosphate backbone breakage. In addition, sodium chromate(VI) plus hydrogen peroxide generates singlet oxygen, which subsequently oxidizes the guanine residue. The mechanism by which both hydroxyl radical and singlet oxygen are generated during the reaction of sodium chromate(VI) with hydrogen peroxide was presented. Finally, the possibility that this reaction may be one of the primary reactions of carcinogenesis induced by chromate(VI) is discussed.     

Kinetics and modeling of hexavalent chromium reduction in Enterobacter cloacae

Kinetics of bacterial reduction of toxic hexavalent chromium (chromate: CrO(4) (2-)) was investigated using batch and fedbatch cultures of Enterobacter cloacae strain HO1. In fedbatch cultures, the CrO(4) (2-) feed was controlled on the basis of the rate of pH change. This control strategy has proven to be useful for avoiding toxic CrO(4) (2-) overload. A simple mathematical model was developed to describe the bacterial process of CrO(4) (2-) reduction. In this model, two types of bacterial cells were considered: induced, CrO(4) (2-)-resistant cells and uninduced, sensitive ones. Only resistant cells were assumed to be able to reduce CrO(4) (2-). These fundamental ideas were supported by the model predictions which well approximated all experimental data. In a simulation study, the model was also used to optimize fed-batch cultures, instead of lengthy and expensive laboratory experiments. (c) 1993 John Wiley & Sons, Inc.