Showing papers on "Arsenic published in 2001"

A fern that hyperaccumulates arsenic

Abstract: A hardy, versatile, fast-growing plant helps to remove arsenic from contaminated soils.

Arsenic contamination of groundwater and drinking water in Vietnam: a human health threat.

Abstract: This is the first publication on arsenic contamination of the Red River alluvial tract in the city of Hanoi and in the surrounding rural districts. Due to naturally occurring organic matter in the sediments, the groundwaters are anoxic and rich in iron. With an average arsenic concentration of 159 micrograms/L, the contamination levels varied from 1 to 3050 micrograms/L in rural groundwater samples from private small-scale tubewells. In a highly affected rural area, the groundwater used directly as drinking water had an average concentration of 430 micrograms/L. Analysis of raw groundwater pumped from the lower aquifer for the Hanoi water supply yielded arsenic levels of 240-320 micrograms/L in three of eight treatment plants and 37-82 micrograms/L in another five plants. Aeration and sand filtration that are applied in the treatment plants for iron removal lowered the arsenic concentrations to levels of 25-91 micrograms/L, but 50% remained above the Vietnamese Standard of 50 micrograms/L. Extracts of sediment samples from five bore cores showed a correlation of arsenic and iron contents (r2 = 0.700, n = 64). The arsenic in the sediments may be associated with iron oxyhydroxides and released to the groundwater by reductive dissolution of iron. Oxidation of sulfide phases could also release arsenic to the groundwater, but sulfur concentrations in sediments were below 1 mg/g. The high arsenic concentrations found in the tubewells (48% above 50 micrograms/L and 20% above 150 micrograms/L) indicate that several million people consuming untreated groundwater might be at a considerable risk of chronic arsenic poisoning.

Arsenic in groundwater: Testing pollution mechanisms for sedimentary aquifers in Bangladesh

Abstract: In the deltaic plain of the Ganges-Meghna-Brahmaputra Rivers, arsenic concentrations in groundwater commonly exceed regulatory limits (.50 m gL 21 ) because FeOOH is microbially reduced and releases its sorbed load of arsenic to groundwater. Neither pyrite oxidation nor competitive exchange with fertilizer phosphate contribute to arsenic pollution. The most intense reduction and so severest pollution is driven by microbial degradation of buried deposits of peat. Concentrations of ammonium up to 23 mg L 21 come from microbial fermentation of buried peat and organic waste in latrines. Concentrations of phosphorus of up to 5 mg L 21 come from the release of sorbed phosphorus when FeOOH is reductively dissolved and from degradation of peat and organic waste from latrines. Calcium and barium in groundwater come from dissolution of detrital (and possibly pedogenic) carbonate, while magnesium is supplied by both carbonate dissolution and weathering of mica. The 87 Sr/ 86 Sr values of dissolved strontium define a two-component mixing trend between monsoonal rainfall (0.711 6 0.001) and detrital carbonate (,0.735).

Arsenic contamination of groundwater in Bangladesh

Abstract: A survey of well waters (n=3534) from throughout Bang- ' ladesh, excluding the Chitt;agong Hill Tracts, has shown that water from 27% of the 'shallow' tubewells, that is wells less than 150 m deep, exceeded the Bangladesh standard for arsenic in drinking water (50 flg L -I). 46% exceeded the WHO guideline value of 10 flg L-I. Figures for 'deep' wells (greater than 150 m deep) were 1% and 5%, respectively. Since it is believed that there are a total of some 6-11 million tubewells in Bangladesh, mostly exploiting the depth range 10-50 m, some 1.5-2.5 million wells are estimated to be contaminated with arsenic according to the Bangladesh standard. 35 million people are believed to be exposed to an arsenic concentration in drinking water exceeding 50 flg L-I and 57 million people exposed to a concentration exceeding 10 flg L -I. There is a distinct regional pattern of arsenic contamination with the greatest contamination in the south and south-east of the country and the least contamination in the north-west and in the uplifted areas of north-central Bangladesh. However, there are occasional arsenic 'hot spots' in the generally low-arsenic regions of northern _Bangladesh. In arsenic-contaminated areas, the large degree of well-to-well variation within a village means that it is diffic'ult to predict whether a given well will be contaminated from tests carried out on neighbouring wells. The young (Holocene) alluvial and deltaic deposits are __ m8-s_caffe_c.ted_whereas_the older alluvial_sediments in the ,I tion' hypothesis in which pyrite oxidation in the zone of water table fluctuation is assumed to release arsenic and ultimately to be responsible for the groundwater arsenic problem. There is no evidence to support the proposition that the groundwater arsenic problem is caused by the recent seasonal drawdown of the water table due to a recent increase in irrigation abstraction. Monitoring of groundwaters at two-weekly intervals at a number of sites, and at different depths, has shown some variation with time but there is as yet no convincing evidence for seasonal changes. Dramatic changes in contamination are not expected within such a short timescale. A monitoring programme should be undertaken at a range of sites to monitor possible long-term changes. In the three contaminated areas studied in most detail, the arsenic concentration increases most rapidly between 10-20 m below ground level. \Vhile arsenic is the single greatest problem in Bangladesh groundwaters, other elements of concern from a health point of view, are manganese, boron and uranium. Some 35% of the groundwaters sampled exceeded the WHO guideline value for manganese (0.5 mg L-I). The spatial pattern of the arsenic and manganese problem areas was significantly different and only 33% of shallow well waters complied with the WHO guideline values for both arsenic and manganese.

Validation of an Arsenic Sequential Extraction Method for Evaluating Mobility in Sediments

Abstract: Arsenic (As) mobility and transport in the environment are strongly influenced by arsenic's associations with solid phases in soil and sediment. We have tested a sequential extraction procedure intended to differentiate the following pools of solid phase arsenic: loosely and strongly adsorbed As; As coprecipitated with metal oxides or amorphous monosulfides; As coprecipitated with crystalline iron (oxyhydr)oxides; As oxides; As coprecipitated with pyrite; and As sulfides. Additions of As-bearing phases to wetland and riverbed sediment subsamples were quantitatively recovered by the following extractants of the sequential extraction procedure: As adsorbed on goethite, 1 M NaH2PO4; arsenic trioxide (As2O3), 10 M HF; arsenopyrite (FeAsS), 16 N HNO3; amorphous As sulfide, 1 N HCI, 50 mM Ti-citrate-EDTA, and 16 N HNO3; and orpiment (As2S3), hot concentrated HNO3/H2O2. Wet sediment subsamples from both highly contaminated wetland peat and less As-rich sandy riverbed sediment were used to test the extraction procedure for intra-method reproducibility. The proportional distribution of As among extractant pools was consistent for subsamples of the wetland and for subsamples of the riverbed sediments. In addition, intermethod variability between the sequential extraction procedure and a single-step hot concentrated HNO3/H2O2 acid digestion was investigated. The sum of the As recovered in the different extractant pools was not significantly different than results for the acid digestion.

Copper‐ and arsenate‐induced oxidative stress in Holcus lanatus L. clones with differential sensitivity

Abstract: The biochemical responses of Holcus lanatus L. to copper and arsenate exposure were investigated in arsenate-tolerant and -non-tolerant plants from uncontaminated and arsenic/copper-contaminated sites. Increases in lipid peroxidation, superoxide dismutase (SOD) activity and phytochelatin (PC) production were correlated with increasing copper and arsenate exposure. In addition, significant differences in biochemical responses were observed between arsenate-tolerant and -non-tolerant plants. Copper and arsenate exposure led to the production of reactive oxygen species, resulting in significant lipid peroxidation in non-tolerant plants. However, SOD activity was suppressed upon metal exposure, possibly due to interference with metallo-enzymes. It was concluded that in non-tolerant plants, rapid arsenate influx resulted in PC production, glu-tathione depletion and lipid peroxidation. This process would also occur in tolerant plants, but by decreasing the rate of influx, they were able to maintain their constitutive functions, detoxify the metals though PC production and quench reactive oxygen species by SOD activity.

Adsorption of Arsenate (V) and Arsenite (III) on Goethite in the Presence and Absence of Dissolved Organic Carbon

Abstract: The environmental fate of arsenic (As) is of utmost importance as the public and political debate continues with the USEPA's recent proposal to tighten the As drinking water standard from 50 to 10 μg L -1 . In natural systems, the presence of dissolved organic C (DOC) may compete with As for adsorption sites on mineral surfaces, hence increasing its potential bioavailability. Accordingly, the adsorption of arsenate [As (V)] and arsenite [As (III)] on goethite (a-FeOOH) was investigated in the presence of either a peat humic acid (Hap), a Suwannee River Fulvic Acid (FA) (International Humic Substances Society, St. Paul, MN), or citric acid (CA). Adsorption edges and kinetic experiments were used to examine the effects of equimolar concentrations of organic adsorbates on As adsorption. Adsorption edges were conducted across a pH range of 3 to 11, while the kinetic studies were conducted at pH 6.5 for As (V) and pH 5.0 for As (III). Both Hap and FA decreased As (V) adsorption, while CA had no effect. Humic acid reduced As (V) between pH 6 and 9 by 27%. Fulvic acid inhibited As (V) adsorption between pH 3 and 8 by a maximum of 17%. Arsenite adsorption was decreased by all three organic acids between pH 3 and 8 in the order of CA > FA Hap. The different pH regions in which Hap and FA decreased As (V) adsorption suggest that more than one functional group on these complex organic polymers may be responsible for binding to the α-FeOOH surface. Similarly, the relative surface affinity of the As(III or V) species and that of the competing organic ligand as a function of pH may play a major role in the outcome of As adsorption on α-FeOOH. The results of these experiments suggest that DOC substances are capable of increasing the bioavailability of As in soil and water systems in which the dominant solid phase is a crystalline iron oxide.

Identification of dimethylarsinous and monomethylarsonous acids in human urine of the arsenic-affected areas in West Bengal, India.

Abstract: A speciation technique for arsenic has been developed using an anion-exchange high-performance liquid chromatography/inductively coupled argon plasma mass spectrometer (HPLC/ICP MS). Under optimized conditions, eight arsenic species [arsenocholine, arsenobetaine, dimethylarsinic acid (DMA(V)), dimethylarsinous acid (DMA(III)), monomethylarsonic acid (MMA(V)), monomethylarsonous acid (MMA(III)), arsenite (As(III)), and arsenate (As(V))] can be separated with isocratic elution within 10 min. The detection limit of arsenic compounds was 0.14-0.33 microg/L. To validate the method, Standard Reference Material in freeze-dried urine, SRM-2670, containing both normal and elevated levels of arsenic was analyzed. The method was applied to determine arsenic species in urine samples from three arsenic-affected districts of West Bengal, India. Both DMA(III) and MMA(III) were detected directly (i.e., without any prechemical treatment) for the first time in the urine of some humans exposed to inorganic arsenic through their drinking water. Of 428 subjects, MMA(III) was found in 48% and DMA(III) in 72%. Our results indicate the following. (1) Since MMA(III) and DMA(III) are more toxic than inorganic arsenic, it is essential to re-evaluate the hypothesis that methylation is the detoxification pathway for inorganic arsenic. (2) Since MMA(V) reductase with glutathione (GSH) is responsible for conversion of MMA(V) to MMA(III) in vivo, is DMA(V) reductase with GSH responsible for conversion of DMA(V) to DMA(III) in vivo? (3) Since DMA(III) forms iron-dependent reactive oxygen species (ROS) which causes DNA damage in vivo, DMA(III) may be responsible for arsenic carcinogenesis in human.

History of the Development of Arsenic Derivatives in Cancer Therapy

Abstract: Arsenic is a natural substance that has been used medicinally for over 2,400 years. In the 19th century, it was the mainstay of the materia medica. A solution of potassium arsenite (Fowler's solution) was used for a variety of systemic illnesses from the 18th until the 20th century. This multipurpose solution was also primary therapy for the treatment of chronic myelogenous leukemia until replaced by radiation and cytotoxic chemotherapy. The past 100 years have seen a precipitous decline in arsenic use and, by the mid-1990s, the only recognized indication was the treatment of trypanosomiasis. Much of this decline was due to concerns about the toxicity and potential carcinogenicity of chronic arsenic administration. The rebirth of arsenic therapy occurred in the 1970s when physicians in China began using arsenic trioxide as part of a treatment for acute promyelocytic leukemia (APL). Their accumulated experience showed that a stable solution of arsenic trioxide given by intravenous infusion was remarkably safe and effective both in patients with newly diagnosed APL leukemia and in those with refractory and relapsed APL. The mechanisms of action of arsenic derivatives in this disease and other malignancies are many and include induction of apoptosis, partial cytodifferentiation, inhibition of proliferation, and inhibition of angiogenesis. Molecular studies and ongoing clinical trials suggest that, as a chemotherapeutic agent, arsenic trioxide shows great promise in the treatment of malignant disease.

Introduction: the history of arsenic trioxide in cancer therapy.

Abstract: Although arsenic can be poisonous, and chronic arsenic exposure from industrial or natural sources can cause serious toxicity, arsenic has been used therapeutically for more than 2,400 years. Thomas Fowler's potassium bicarbonate-based solution of arsenic trioxide (As(2)O(3)) was used empirically to treat a variety of disorders, and in 1878, was reported to reduce white blood cell counts in two normal individuals and one with "leucocythemia." Salvarsan, an organic arsenical for treating syphilis and trypanosomiasis, was developed in 1910 by Paul EHRLICH: In the 1930s, arsenic was reported to be effective in chronic myelogenous leukemia. After a decline in the use of arsenic during the mid-20th century, reports from China described a high proportion of hematologic responses in patients with acute promyelocytic leukemia (APL) who were treated with arsenic trioxide. Randomized clinical trials in the U.S. led to FDA approval of arsenic trioxide for relapsed or refractory APL in September 2000.

Solar Oxidation and Removal of Arsenic at Circumneutral pH in Iron Containing Waters

Abstract: An estimated 30−50 million people in Bangladesh consume groundwater with arsenic contents far above accepted limits. A better understanding of arsenic redox kinetics and simple water treatment procedures are urgently needed. We have studied thermal and photochemical As(III) oxidation in the laboratory, on a time scale of hours, in water containing 500 μg/L As(III), 0.06−5 mg/L Fe(II,III), and 4−6 mM bicarbonate at pH 6.5−8.0. As(V) was measured colorimetrically, and As(III) and As(tot) were measured by As(III)/As(tot)-specific hydride-generation AAS. Dissolved oxygen and micromolar hydrogenperoxide did not oxidize As(III) on a time scale of hours. As(III) was partly oxidized in the dark by addition of Fe(II) to aerated water, presumably by reactive intermediates formed in the reduction of oxygen by Fe(II). In solutions containing 0.06−5 mg/L Fe(II,III), over 90% of As(III) could be oxidized photochemically within 2−3 h by illumination with 90 W/m2 UV-A light. Citrate, by forming Fe(III)citrate complexes t...

Determination of total and speciated arsenicin rice by ion chromatography and inductively coupled plasma mass spectrometry

Abstract: A method is presented for arsenic speciation analysis in rice using ion chromatography coupled to inductively coupled plasma mass spectrometry. Several procedures for the extraction of arsenic species from rice were investigated and compared. Treatment of the samples with 2 M trifluoroacetic acid for 6 h at 100°C provided good extraction efficiency. Fortification recoveries were 83, 88, 100, and 93% for arsenite (100 ng As g−1), arsenate (100 ng As g−1), methylarsonic acid (MMA, 50 ng As g−1), and dimethylarsinic acid (DMA, 200 ng As g−1), respectively. The arsenate fortification recovery was calculated using the sum of the increase in arsenate and arsenite concentrations because arsenate was partially reduced to arsenite during the extraction process. Thus the sum of their concentrations is reported in this method as total inorganic arsenic. The sum of the arsenic species determined in NIST SRM 1568a rice flour (0.27 µg g−1 As), compared well with the certified total arsenic value (0.29 µg g−1 As). The speciation results obtained for 5 samples of long grain rice and one sample of wild rice are compared with total arsenic concentrations as determined by ICP-MS. The total arsenic concentrations ranged from 0.11 to 0.34 mg kg−1. The sum of the arsenic species extracted and determined by IC-ICP-MS ranged from 84 to 99% of the measured total arsenic concentrations. Inorganic arsenic accounted for 11–91% of the arsenic detected while DMA accounted for most of the remaining arsenic in the samples.

Genetic toxicology of a paradoxical human carcinogen, arsenic: a review.

Abstract: Arsenic is widely distributed in nature in air, water and soil in the form of either metalloids or chemical compounds. It is used commercially, as pesticide, wood preservative, in the manufacture of glass, paper and semiconductors. Epidemiological and clinical studies indicate that arsenic is a paradoxical human carcinogen that does not easily induce cancer in animal models. It is one of the toxic compounds known in the environment. Intermittent incidents of arsenic contamination in ground water have been reported from several parts of the world. Arsenic containing drinking water has been associated with a variety of skin and internal organ cancers. The wide human exposure to this compound through drinking water throughout the world causes great concern for human health. In the present review, we have attempted to evaluate and update the mutagenic and genotoxic effects of arsenic and its compounds based on available literature.

Role of Charge (Donnan) Exclusion in Removal of Arsenic from Water by a Negatively Charged Porous Nanofiltration Membrane

Abstract: The removal of arsenic from water by a negatively charged "loose" (porous) nanofiltration (NF) membrane was investigated. To better understand the mechanisms of arsenic removal by the polymeric membrane, its surface charge, pore size, and separation behavior for several salt solutions (NaCl, CaCl2, and Na2SO4) were first investigated. The ability of the membrane to remove inorganic arsenic species - As(III) and As(V) - from water was further evaluated. The removal of As(V) increased from 60 to 90% as the arsenic feed water concentration was increased from 10 to 316 μg/L. Compared to As(V), the rejection of the uncharged As(III) species was significantly lower; the rejection of As(III) decreased from 28 to 5% as the arsenic feed concentration was increased from 10 to 316 μg/L. In addition, rejection of As(V) decreased sharply from 85% at pH 8.5 to only 8% at pH 4.5. This behavior is mainly attributed to changes in As(V) speciation with pH (HAsO42- at pH >6.8, H2AsO4- at pH <6.8), and to a lesser extent to ...

Role of Metabolism in Arsenic Toxicity

Abstract: In humans, as in most mammalian species, inorganic arsenic is methylated to methylarsonic acid (MMA) and dimethylarsinic acid (DMA) by alternating reduction of pentavalent arsenic to trivalent and addition of a methyl group from S-adenosylmethionine. The methylation of inorganic arsenic may be considered a detoxification mechanism, as the end metabolites, MMA and DMA, are less reactive with tissue constituents, less toxic, and more readily excreted in the urine than is inorganic arsenic, especially the trivalent form (AsIII, arsenite). The latter is highly reactive with tissue components, due to its strong affinity for sulfhydryl groups. Thus, following exposure to AsV the first step in the biotransformation, i.e. the reduction to AsIII, may be considered a bioactivation. Also, reactive intermediate metabolites of high toxicity, mainly MMAIII, may be formed and distributed to tissues. Low levels of MMAIII and DMAIII have been detected in urine of individuals chronically exposed to inorganic arsenic via drinking water. However, the contribution of MMAIIIand DMAIII to the toxicity observed after intake of inorganic arsenic by humans remains to be elucidated. The major route of excretion of arsenic is via the kidneys. Evaluation of the methylation of arsenic is mainly based on the relative amounts of the different metabolites in urine. On average human urine contains 10-30% inorganic arsenic, 10-20% MMA and 60-80% DMA.

Electrochemical and spectroscopic study of arsenate removal from water using zero-valent iron media.

Abstract: This study investigated the mechanisms involved in removing arsenate from drinking water supplies using zero-valent iron media. Batch experiments utilizing iron wires suspended in anaerobic arsenate solutions were performed to determine arsenate removal rates as a function of the arsenate solution concentration. Corrosion rates of the iron wires were determined as a function of elapsed time using Tafel analysis. The removal kinetics in the batch reactors were best described by a dual-rate model in which arsenate removal was pseudo-first-order at low concentrations and approached zero-order in the limit of high arsenate concentrations. The presence of arsenate decreased iron corrosion rates as compared to those in blank 3 mM CaSO4 background electrolyte solutions. However, constant corrosion rates were attained after approximately 10 days elapsed, indicating that the passivation processes had reached steady state. The cathodic Tafel slopes were the same in the arsenate and the blank electrolyte solutions. This indicates that water was the primary oxidant for iron corrosion and that arsenate did not directly oxidize the iron wires. The anodic Tafel slopes were greater in the arsenate solutions, indicating that arsenate formed complexes with iron corrosion products released at anodic sites on the iron surfaces. Ion chromatography analyses indicated that there was no measurable reduction of As(V) to As(III). X-ray absorption spectroscopy analyses indicated that all arsenic associated with the zero-valent iron surfaces was in the oxidation state. Interatomic arsenic-iron distances determined from EXAFS analyses were consistent with bidentate corner-sharing among arsenate tetrahedra and iron octahedra. Results from this study show that under conditions applicable to drinking water treatment, arsenate removal by zero-valent iron media involves surface complexation only and does not involve reduction to metallic arsenic.

Arsenic in groundwater in the Bengal Delta Plain: slow poisoning in Bangladesh

Abstract: The purpose of this paper is to provide an overview of the problems concerning the widespread occurrences of arsenic in groundwater in Bangladesh, a land with enormous resources of precipitation, s...

Isolation and Characterization of a Novel As(V)-Reducing Bacterium: Implications for Arsenic Mobilization and the Genus Desulfitobacterium

Abstract: Arsenic is the 20th most abundant element in the Earth's crust (56) and is widely distributed throughout nature as a result of weathering, dissolution, fire, volcanic activity, and anthropogenic input (13). The last includes the use of arsenic in pesticides, herbicides, wood preservatives, and dye stuffs as well as production of arsenic-containing wastes during smelting and mining operations (56). In arsenic-enriched environments, a major concern is the potential for mobilization and transport of this toxic element to groundwater and drinking water supplies. In Bangladesh, an estimated 57 million people have been exposed to arsenic through contaminated wells (9). This incident serves as an unfortunate reminder of the toxic consequences of arsenic mobilization and underscores the need to understand the factors controlling the mobility and solubility of arsenic in aquatic systems (60). Coeur d'Alene Lake (CDAL) is the second largest lake in Idaho. As a result of a century of mining along the Coeur d'Alene River, one of two rivers feeding CDAL, lake sediments are highly enriched in trace elements including Ag, As, Cd, Pb, Sb, and Zn (31). Sediment pore waters are also trace element enriched with mean total arsenic and lead concentrations exceeding 160 and 250 μg/liter (28), respectively. Nevertheless, CDAL surface waters comply with current federal drinking water standards (28) (50 and 15 μg/liter for As and Pb [75, 76], respectively). Because residents of Northern Idaho use these waters for recreation and fishing and as a source of drinking water (82), concern remains over the possibility that contaminants could be mobilized from the sediment to the water column. Iron is the dominant metal in the Coeur d'Alene system, constituting approximately 10% of the sediments by dry weight (14, 27). Because iron is exceptionally abundant in CDAL sediments, its transformations are likely to influence the bioavailability and mobility of trace elements such as arsenic (16). Sorption of As onto the surface of insoluble iron oxyhydroxides is well documented, as is the observation that the oxidation state of arsenic influences its propensity to sorb onto iron mineral surfaces (5, 18, 22, 50–52, 62, 63, 65). In arsenic-enriched soils and sediments, an increase in soluble As(III) is commonly observed upon establishment of reducing conditions (1, 8, 52, 53, 69). This observation has been attributed to poor sorption of As(III) onto iron oxyhydroxides (18, 62) and is consistent with the idea that As(III) is more mobile than As(V) in aquatic environments (39). Recent studies, however, suggest that under certain conditions at circumneutral pH, As(III) can sorb to iron oxyhydroxides at least as strongly as As(V) (50, 63, 74). In view of this, alternative mechanisms should be considered to explain why soluble As(III) commonly increases when soils and sediments become anoxic. The occurrence of soluble As in reduced aquatic environments has been attributed to the reductive dissolution of solid-phase iron oxyhydroxides followed by the release of sorbed arsenic (1, 4, 8, 18, 53, 60). In such environments, the reductive dissolution of iron oxyhydroxides is largely mediated by the activity of dissimilatory iron-reducing bacteria (DIRB) (43). Our laboratory has demonstrated that the DIRB Shewanella alga BrY, an organism that cannot respire As(V), mobilizes As(V) from the solid-phase ferric arsenate mineral scorodite by reducing Fe(III) to Fe(II) (14). As(V) reduction is therefore not a prerequisite to arsenic solubilization from FeO(OH)x, even though an increase in soluble As(III) is commonly observed upon the onset of anoxia (1, 8, 52, 53). In anaerobic environments, DIRB-mediated reductive dissolution of iron minerals would likely result in an increase in As(V) concentrations, provided DIRB were not also capable of reducing As(V) (14). However, if dissimilatory arsenate-reducing organisms were also present and active, solubilized As(V) could be readily converted into As(III). Our laboratory has demonstrated that CDAL sediments are highly reduced (16) and support biotic reduction of As(V) (28). We have hypothesized that this results from dissimilatory arsenate reduction. Dissimilatory arsenic-reducing bacteria (DAsRB) couple the reduction of As(V) to the oxidation of an organic compound or H2 and thereby conserve energy for growth (33, 46, 72). Most probable number estimates suggest that the number of cultivable DAsRB in this environment ranges from 103 to 105 cells/g (wet weight) of sediment (28). Several studies of dissimilatory arsenate reduction suggest that this transformation is likely to play a role in mobilizing arsenic from ferric oxyhydroxide minerals (3, 19, 28, 83). Incubation of either sterile As-contaminated sediments or scorodite with a pure culture of the DAsRB Sulfurospirillum arsenophilum MIT-13T resulted in elevated aqueous-phase As(III) concentration (3). When the DAsRB Sulfurospirillum barnesii strain SES-3T was incubated with As(V) that was either coprecipitated with or adsorbed to poorly crystalline iron oxyhydroxide minerals, approximately half of the resulting arsenite was retained in the solid phase and the other half was released into solution (83). The few known DAsRB are remarkable for their phylogenetic diversity and metabolic versatility (see Fig. ​Fig.5A)5A) (Table ​(Table1).1). This suggests that the capacity to couple growth to As(V) reduction may be widespread. If further investigation confirms that this trait is broadly distributed among the Bacteria and Archaea and that such organisms are abundant and active, DAsRB may play essential roles in mediating the reductive portion of the arsenic cycle. TABLE 1 Summary of relevant physiological characteristics of previously published DAsRBa FIG. 5 (A) Unrooted phylogenetic tree showing the positions of currently recognized As(V)-reducing microorganisms (in bold) among the major lines of prokaryotes. The tree is based on an optimized global tree reconstructed from almost full-length SSU rRNA gene ... We hypothesized that the biotic generation of As(III) observed in CDAL sediment microcosms (28) arose from the activity of dissimilatory arsenate-reducing bacteria, and we sought to enrich such organisms from CDAL sediments. From thermodynamic considerations, we hypothesized that formate could serve as both the electron donor and carbon source for As(V) reduction and that energy from this reduction could be conserved for growth. Insofar as no characterized DAsRB have been enriched from CDAL and no characterized DAsRB are capable of growth on formate alone, we proposed that a formate-oxidizing DAsRB recovered from CDAL sediments would be phylogenetically distinct from previously characterized arsenate-reducers. Lastly, we hypothesized that As(V)-reducing organisms enriched from these Fe-rich sediments would also be capable of obtaining energy for growth by dissimilatory Fe(III) reduction. Such an organism could be expected to mediate diagenesis of both arsenic and iron.

Overexpression of glutathione S-transferase II and multidrug resistance transport proteins is associated with acquired tolerance to inorganic arsenic.

Abstract: Recent work shows that long-term exposure to low levels of arsenite induces malignant transformation in a rat liver epithelial cell line. Importantly, these chronic arsenic-exposed (CAsE) cells also develop self-tolerance to acute arsenic exposure. Tolerance is accompanied by reduced cellular arsenic accumulation, suggesting a mechanistic basis for reduced arsenic sensitivity. The present study examined the role of xenobiotic export pumps in acquired arsenic tolerance. Microarray analysis of CAsE cells showed increased expression of the genes encoding for glutathione S-transferase Pi (GST-Pi), multidrug resistance-associated protein genes (MRP1/MRP2, which encode for the efflux transporter Mrp1/Mrp2) and the multidrug resistance gene (MDR1, which encodes for the efflux transporter P-glycoprotein). These findings were confirmed at the transcription level by reverse transcription-polymerase chain reaction and at the translation level by Western-blot analysis. Acquired arsenic tolerance was abolished when cells were exposed to ethacrynic acid (an inhibitor of GST-Pi), buthionine sulfoximine (a glutathione synthesis inhibitor), MK571 (a specific inhibitor for Mrps), and PSC833 (a specific inhibitor for P-glycoprotein) in dose-dependent fashions. MK571, PSC833, and buthionine sulfoximine markedly increased cellular arsenic accumulation. Consistent with a role for multidrug resistance efflux pumps in arsenic resistance, CAsE cells were found to be cross-resistant to cytotoxicity of several anticancer drugs, such as vinblastine, doxorubicin, actinomycin-D, and cisplatin, that are also substrates for Mrps and P-glycoprotein. Thus, acquired tolerance to arsenic is associated with increased expression GST-Pi, Mrp1/Mrp2 and P-glycoprotein, which function together to reduce cellular arsenic accumulation.

Association of blood arsenic levels with increased reactive oxidants and decreased antioxidant capacity in a human population of northeastern Taiwan.

Abstract: Arsenic is a notorious environmental toxicant known as both a carcinogen and an atherogen in human beings, but the pathogenic mechanisms are not completely understood. In cell culture studies, trivalent arsenic enhanced oxidative stress in a variety of mammalian cells, and this association may be closely associated with the development of arsenic-related diseases. To investigate the effect of arsenic exposure on oxidative stress in humans, we conducted a population study to determine the relationships of blood arsenic to reactive oxidants and antioxidant capacity at the individual level. We recruited 64 study subjects ages 42-75 years from residents of the Lanyang Basin on the northeast coast of Taiwan, where arsenic content in well water varies from 0 to > or = 3,000 microg/L. We used a chemiluminescence method, with lucigenin as an amplifier for measuring superoxide, to measure the plasma level of reactive oxidants. We used the azino-diethyl-benzthiazoline sulphate method to determine the antioxidant capacity level in plasma of each study subject. We determined arsenic concentration in whole blood by hydride formation with an atomic absorption spectrophotometer. The average arsenic concentration in whole blood of study subjects was 9.60 +/- 9.96 microg/L (+/- SD) with a range from 0 to 46.50 microg/L. The level of arsenic concentration in whole blood of study subjects showed a positive association with the level of reactive oxidants in plasma (r = +0.41, p = 0.001) and an inverse relationship with the level of plasma antioxidant capacity (r = -0.30, p = 0.014). However, we found no significant association (p = 0.266) between levels of plasma reactive oxidants and antioxidant capacity. Our results also show that the lower the primary arsenic methylation capability, the lower the level of plasma antioxidant capacity (p = 0.029). These results suggest that ingestion of arsenic-contaminated well water may cause deleterious effects by increasing the level of reactive oxidants and decreasing the level of antioxidant capacity in plasma of individuals. Persistent oxidative stress in peripheral blood may be a mechanism underlying the carcinogenesis and atherosclerosis induced by long-term arsenic exposure.