可食植物中砷赋存形态研究进展

砷是环境中普遍存在的一种化学元素,近年来日益严重的砷污染问题在世界范围内受到高度重视,我国目前已经成为世界卫生组织(WHO)列出的砷污染最严重的国家和地区之一.经口摄入是外界砷进入人体并累积产生生物毒性的主要途径.可食植物是人类饮食结构中不可缺少的部分,食品总砷含量并不能完全决定其毒性大小,砷的赋存形态与其生物毒性密切相关,无机砷已被确认为致癌物质.本文主要综述了不同种类可食植物中砷形态的分析方法,以及砷的主要赋存形态及其生物毒性.     

Can arsenic-phytochelatin complex formation be used as an indicator for toxicity in Helianthus annuus?

The formation of arsenic-phytochelatin (As-PC) complexes is thought to be part of the plant detoxification strategy for arsenic. This work examines (i) the arsenic (As) concentration-dependent formation of As-PC complex formation and (ii) redistribution and metabolism of As after arrested As uptake in Helianthus annuus. HPLC with parallel ICP-MS/ES-MS detection was used to identify and quantify the species present in plant extracts exposed to arsenate (As(V)) (between 0 and 66.7 micromol As l-1 for 24 h). At As concentrations below the EC50 value for root growth (22 micromol As l-1) As uptake is exponential, but it is reduced at concentrations above. Translocation between root and shoot seemed to be limited to the uptake phase of arsenic. No redistribution of As between root and shoot was observed after arresting As exposure. The formation of As-PC complexes was concentration-dependent. The amount and number of As-PC complexes increased exponentially with concentration up to 13.7 micromol As l-1. As(III)-PC3 and GS-As(III)-PC2 complexes were the dominant species in all samples. The ratio of PC-bound As to unbound As increased up to 1.3 micromol As l-1 and decreased at higher concentrations. Methylation of inorganic As was only a minor pathway in H. annuus with about 1% As methylated over a 32 d period. The concentration dependence of As-PC complex formation, amount of unbound reduced and oxidized PC2, and the relative uptake rate showed that As starts to influence the cellular metabolism of H. annuus negatively at As concentrations well below the EC50 value determined by more traditional means. Generally, As-PC complexes and PC-synthesis rate seem to be the more sensitive parameters to be studied when As toxicity values are to be estimated.     

超富集植物蜈蚣草中砷化学形态的EXAFS研究

采用同步辐射扩展X射线吸收精细结构(SR EXAFS)技术研究了超富集植物蜈蚣草(Pteris vittata L.)中As的化学形态及其在转运过程中的变化.结果表明,蜈蚣草中的As主要以As(Ⅲ)与O配位的形态存在.As(Ⅴ)被植物吸收后,很快转化为As(Ⅲ),其转化过程主要发生在根部.As(Ⅲ)向地上部转运的过程中价态基本不变.在植物的根部和部分叶柄中存在少量与As-GSH相似的As-S结合方式,但是在As含量最高的羽叶中基本上未发现这种结合方式.与需要提取和分离过程的化学方法相比,采用EXAFS方法研究植物中的砷形态不需经过预分离或化学预处理就可以直接测定植物样品中元素的化学形态,因此可以避免样品预处理过程对As形态的干扰,并获得可靠的砷化学形态方面的信息.     

Dynamics and mechanisms of arsenite oxidation by freshwater lake sediments

There has been increasing concern over As in freshwater environments from sources such as arsenical pesticides, smelters, coal-fired power plants, and erosion caused by intensive land use. Arsenic in the reduced state, As (III) (arsenite), is much more toxic, more soluble and mobile, than when in the oxidized state, As (V) (arsenate). This paper summarizes the dynamics and mechanisms involved in the oxidation of As (III) to As (V) by freshwater lake sediments. Sediments from selected freshwater lakes in southern Saskatchewan oxidize As (III) to As (V) predominantly through an abiotic process. Solution analysis of As (III) and As (V) by colorimetry, and examination of the oxidation state of surface-sorbed As species by X-ray photoelectron spectroscopy, indicate that Mn present in the sediment is the primary electron acceptor in the oxidation of As (III). The transformation of As (III) to As (V) by carbonate and silicate minerals, common in sediments, is not evident. The heat of activation, ΔH_a, for the depletion (oxidation plus sorption) of As (III) by the sediments, varies from 3.3 to 8.5 kcal mole⁻¹, indicating that the process is predominantly diffusion-controlled. The Mn present in a series of particle size fractions ( < 2– > 20 μm) of the sediments may potentially detoxify As (III) in aquatic systems, by converting it to As (V).     

Effectiveness of applying arsenate reducing bacteria to enhance arsenic removal from polluted soils by Pteris vittata L

Arsenic is a common contaminant in soils and water. It is well established that the fern Pteris vittata L. is an As hyperaccumulator and therefore has potential to phyroremediate As-polluted soils. Also, it is accepted that rhizosphere microflora play an enhancing role in plant uptake of metallic elements from soils. Studies showed that hydroponiclly grown P. Vittata accumulated arsenite more than the arsenate form of As apparently because arsenate and phosphate are analogues and therefore its absorption is inhibited by phosphate. The objective of this study was to determine whether addition of five different arsenate-reducing bacteria would enhance arsenic uptake by P. vittata grown in arsenic polluted soils in afield experiment. Results showed that addition of the As reducing bacteria promoted the growth of P. vittata, increased As accumulation, activated soil insoluble As, and reduced As leaching compared to the untreated control. Plant biomass increased by 53% and As uptake by 44%. As leaching was reduced by 29% to 71% depending on the As reducing bacterium. The results in their entirety permitted some insight into the mechanisms by which the arsenate reducing bacteria enhanced the effectiveness of P. vittata to remove As from the polluted soil.     

脂蛋白酯酶在动脉粥样硬化发生发展中的作用

脂蛋白酯酶(lipoprotein lipase, LPL)是调节甘油三酯代谢的关键酶,在动脉粥样硬化(atherosclerosis,As)的发生发展中起重要作用。LPL产生部位的差异决定了其具有促As作用还是抗As作用。其次,不同因素对LPL的调控也会使LPL对As产生相反的作用效果。本文综述了LPL在As发生发展中的作用机制以及不同因素对LPL的调控机制,对于As的防治具有重要意义。     

A subgroup of plant aquaporins facilitate the bi-directional diffusion of As(OH)<Subscript>3</Subscript> and Sb(OH)<Subscript>3</Subscript>across membranes

Background  

Arsenic is a toxic and highly abundant metalloid that endangers human health through drinking water and the food chain. The most common forms of arsenic in the environment are arsenate (As(V)) and arsenite (As(III)). As(V) is a non-functional phosphate analog that enters the food chain via plant phosphate transporters. Inside cells, As(V) becomes reduced to As(III) for subsequent extrusion or compartmentation. Although much is known about As(III) transport and handling in microbes and mammals, the transport systems for As(III) have not yet been characterized in plants.     

Inhibition of microbial arsenate reduction by phosphate

The ratio of arsenite (As(III)) to arsenate (As(V)) in soils and natural waters is often controlled by the activity of As-transforming microorganisms. Phosphate is a chemical analog to As(V) and, consequently, may competitively inhibit microbial uptake and enzymatic binding of As(V), thus preventing its reduction to the more toxic, mobile, and bioavailable form - As(III). Five As-transforming bacteria isolated either from As-treated soil columns or from As-impacted soils were used to evaluate the effects of phosphate on As(V) reduction and As(III) oxidation. Cultures were initially spiked with various P:As ratios, incubated for approximately 48 h, and analyzed periodically for As(V) and As(III) concentration. Arsenate reduction was inhibited at high P:As ratios and completely suppressed at elevated levels of phosphate (500 and 1,000 μM; P inhibition constant (K(i))～20-100 μM). While high P:As ratios effectively shut down microbial As(V) reduction, the expression of the arsenate reductase gene (arsC) was not inhibited under these conditions in the As(V)-reducing isolate, Agrobacterium tumefaciens str. 5B. Further, high phosphate ameliorated As(V)-induced cell growth inhibition caused by high (1mM) As pressure. These results indicate that phosphate may inhibit As(V) reduction by impeding As(V) uptake by the cell via phosphate transport systems or by competitively binding to the active site of ArsC.     

Toxicokinetics and Biotransformation of As(III) and As(V) in Eisenia fetida

Earthworms, Eisenia fetida, were exposed to soils spiked with As(III) and As(V), to understand the response of earthworms to As in terms of both toxicity and accumulation using toxicokinetics, and to explain As metabolism and bioavailability. As(III) showed higher toxicity than As(V), in all toxicity endpoints of burrowing time, survival, growth, and cocoon production. Bioconcentration occurred at both As(III) and As(V) treatments during 28-d exposure. Worms did not show the elimination of As for consecutive 28 d of exposure to clean soils. Biotransformation of As was characterized using HPLC-ICP-MS and XANES, showing the reduction of As in worms regardless of the As species to which worms were exposed. Metabolism of As in worms that formed As-thiol complex is thought to limit the excretion of As, and thus induce bioconcentration in worms. Uptake rates by one-compartment model indicated that pore water was the bioaccessible pool of As, and directly controlled the uptake of As by worms. The study suggests that higher uptake rate and bioaccumulation of As(III) than of As(V) are among the factors that make As(III) more toxic than As(V).     

The ars detoxification system is advantageous but not required for As(V) respiration by the genetically tractable Shewanella species strain ANA-3

Arsenate [As(V); HAsO(4)(2-)] respiration by bacteria is poorly understood at the molecular level largely due to a paucity of genetically tractable organisms with this metabolic capability. We report here the isolation of a new As(V)-respiring strain (ANA-3) that is phylogenetically related to members of the genus Shewanella and that also provides a useful model system with which to explore the molecular basis of As(V) respiration. This gram-negative strain stoichiometrically couples the oxidation of lactate to acetate with the reduction of As(V) to arsenite [As(III); HAsO(2)]. The generation time and lactate molar growth yield (Y(lactate)) are 2.8 h and 10.0 g of cells mol of lactate(-1), respectively, when it is grown anaerobically on lactate and As(V). ANA-3 uses a wide variety of terminal electron acceptors, including oxygen, soluble ferric iron, oxides of iron and manganese, nitrate, fumarate, the humic acid functional analog 2,6-anthraquinone disulfonate, and thiosulfate. ANA-3 also reduces As(V) to As(III) in the presence of oxygen and resists high concentrations of As(III) (up to 10 mM) when grown under either aerobic or anaerobic conditions. ANA-3 possesses an ars operon (arsDABC) that allows it to resist high levels of As(III); this operon also confers resistance to the As-sensitive strains Shewanella oneidensis MR-1 and Escherichia coli AW3110. When the gene encoding the As(III) efflux pump, arsB, is inactivated in ANA-3 by a polar mutation that also eliminates the expression of arsC, which encodes an As(V) reductase, the resulting As(III)-sensitive strain still respires As(V); however, the generation time and the Y(lactate) value are two- and threefold lower, respectively, than those of the wild type. These results suggest that ArsB and ArsC may be useful for As(V)-respiring bacteria in environments where As concentrations are high, but that neither is required for respiration.     

Enhanced arsenate reduction by a CDC25-like tyrosine phosphatase explains increased phytochelatin accumulation in arsenate-tolerant Holcus lanatus

Decreased arsenate [As(V)] uptake is the major mechanism of naturally selected As(V) hypertolerance in plants. However, As(V)-hypertolerant ecotypes also show enhanced rates of phytochelatin (PC) accumulation, suggesting that improved sequestration might additionally contribute to the hypertolerance phenotype. Here, we show that enhanced PC-based sequestration in As(V)-hypertolerant Holcus lanatus is not due to an enhanced capacity for PC synthesis as such, but to increased As(V) reductase activity. Vacuolar transport of arsenite-thiol complexes was equal in both ecotypes. Based on homology with the yeast As(V) reductase, Acr2p, we identified a Cdc25-like plant candidate, HlAsr, and confirmed the As(V) reductase activity of both HlAsr and the homologous protein from Arabidopsis thaliana. The gene appeared to be As(V)-inducible and its expression was enhanced in the As(V)-hypertolerant H. lanatus ecotype, compared with the non-tolerant ecotype. Homologous ectopic overexpression of the AtASR cDNA in A. thaliana produced a dual phenotype. It improved tolerance to mildly toxic levels of As(V) exposure, but caused hypersensitivity to more toxic levels. Arabidopsis asr T-DNA mutants showed increased As(V) sensitivity at low exposure levels and enhanced arsenic retention in the root. It is argued that, next to decreased uptake, enhanced expression of HlASR might act as an additional determinant of As(V) hypertolerance and As transport in H. lanatus.     

刈割对蜈蚣草的砷吸收和植物修复效率的影响

以野生苗移栽的蜈蚣草为试材 ,通过盆栽试验研究了收获次数对蜈蚣草生长、砷吸收和植物修复效率的影响。结果表明 :在 3次收获中 ,随着收获次数的增加 ,不同砷浓度处理之间蜈蚣草生物量的差异逐步缩小 ;不加砷的对照处理中 ,每次收获后的砷吸收速率下降趋势 ,而在 3个加砷处理中 ,第 2次收获和第 3次收获的蜈蚣草的吸砷速率为 6 3～ 75 μg/ (plant· d)、4 4～ 5 5μg/ (plant· d) ,均显著高于第 1次收获时的吸收速率。表明多次收获并没有降低砷的积累速度。由此可见 ,通过适当增加蜈蚣草的收获次数是提高砷修复效率的一种策略     

Arsenic supply characteristics of four cotton-producing soils

Cotton-producing soils in Louisiana average 23 mg kg^?1 arsenic (As) due mainly to use of As-containing agrichemicals. Bioavailability of soil As could become an important management factor if As-tolerant cotton is rotated with other more sensitive crops. The first step in modelling As bioavailability and its uptake by plants is to characterize soil supply of As. We sought to: i) determine the relationships among solution As, resin-exchangeable solid-phase As, and As addition, and ii) examine soil properties that affect these relations. Four diverse surface soils were collected from the cotton-producing areas of the state. Five rates of As from 0 to 200 mg kg^?1 were applied and the soils allowed to equilibrate at 80% of “field capacity” water content for 30 days. Total initial As, As in displaced soil solution, and resin-exchangeable solid-phase As were determined for each treatment. For all soils, the concentration of solution As increased curvilinearly with As addition, conforming to the equation As_sol=ax^c+d. Values of “c” describing the curvilinearity ranged from 1.09 to 3.19. Curvilinearity was negatively related to both initial solution As and diethylenetriaminepentaacetic acid (DTPA)-extractable manganese (Mn) (r²=0.99). DTPA-extractable Mn complexed As from the solution phase. The concentration of resin-exchangeable solid-phase As increased at a decreasing rate with As addition. The relation was described by the equation As_resp=mx^?+n, where values for the curvilinearity, ?, ranged from 0.048 to 0.986. Concentrations approached a point where the adsorption sites for this phase became saturated and any additional As remained in solution. These relationships provide valuable information for modelling As uptake by plant roots.     

Phosphate‐Assisted Phytoextraction in As‐Contaminated Soil

In this paper, the effect of phosphate on As phytoextraction was examined. The effect of phosphate on As dissolution, the As uptake of plants, the internal plant translocation, and phytotoxic effect were investigated. Lupine plants were grown on As‐contaminated soil collected from an industrial site containing 670 mg/kg As and were treated with biammonium phosphate (BAP). Two different BAP application procedures were tested: single‐dose and multiple split additions. BAP was found to be effective in increasing the water‐soluble As concentration in the test soil. As the concentration of water‐soluble As increased, the Lupine plants responded accordingly with an increased As uptake. The As content in the shoots and the translocation factor were the highest when BAP was added in multiple split additions. On the contrary, a single application caused the highest As content in the roots and consequently the lowest translocation factor. In addition, it was established that the single‐application method significantly reduced the plant biomass by twofold, this reduction being an evident phytotoxic symptom. Measurements of the combined biomass production and As content values revealed that the highest As phytoextraction is obtained with BAP applied in multiple doses which is about 14‐fold higher than in the control plants, whereas a single‐dose BAP application increased the phytoextraction rate only 1.6 times. These results demonstrate that significant improvements in the current phosphate‐assisted phytoextraction of As could be achieved.     

Glutathione synthetase promotes the reduction of arsenate via arsenolysis of glutathione

The environmentally prevalent arsenate (As(V)) undergoes reduction in the body to the much more toxic arsenite (As(III)). Phosphorolytic enzymes and ATP synthase can promote the reduction As(V) by converting it into arsenylated products in which the pentavalent arsenic is more reducible by glutathione (GSH) to As(III) than in inorganic As(V). Glutathione synthetase (GS) can catalyze the arsenolysis of GSH (γ-Glu-Cys-Gly) yielding two arsenylated products, i.e. γ-Glu-Cys-arsenate and ADP-arsenate. Thus, GS may also promote the reduction of As(V) by GSH. This hypothesis was tested with human recombinant GS, a Mg(2+) dependent enzyme. GS markedly increased As(III) formation when incubated with As(V), GSH, Mg(2+) and ADP, but not when GSH, Mg(2+) or ADP were separately omitted. Phosphate, a substrate competitive with As(V) in the arsenolysis of GSH, as well as the products of GSH arsenolysis or their analogs, e.g. glycine and γ-Glu-aminobutyrate, decreased As(V) reduction. Replacement of ADP with ATP or an analog that cannot be phosphorylated or arsenylated abolished As(V) reduction, indicating that GS-supported As(V) reduction requires formation of ADP-arsenate. In the presence of ADP, however, ATP (but not its metabolically inert analog) tripled As(V) reduction because ATP permits GS to remove the arsenolysis inhibitory glycine and γ-Glu-Cys by converting them into GSH. GS failed to promote As(V) reduction when GSH was replaced with ophthalmic acid, a GSH analog substrate of GS containing no SH group (although ophthalmic acid did undergo GS-catalyzed arsenolysis), indicating that the SH group of GSH is important for As(V) reduction. Our findings support the conclusion that GS promotes reduction of As(V) by catalyzing the arsenolysis of GSH, thus producing ADP-arsenate, which upon being released from the enzyme is readily reduced by GSH to As(III).     

Mechanisms Controlling Arsenic Uptake in Rice Grown in Mining Impacted Regions in South China

Foods produced on soils impacted by Pb-Zn mining activities are a potential health risk due to plant uptake of the arsenic (As) associated with such mining. A field survey was undertaken in two Pb-Zn mining-impacted paddy fields in Guangdong Province, China to assess As accumulation and translocation, as well as other factors influencing As in twelve commonly grown rice cultivars. The results showed that grain As concentrations in all the surveyed rice failed national food standards, irrespective of As speciation. Among the 12 rice cultivars, “SY-89” and “DY-162” had the least As in rice grain. No significant difference for As concentration in grain was observed between the rice grown in the two areas that differed significantly for soil As levels, suggesting that the amount of As contamination in the soil is not necessarily the overriding factor controlling the As content in the rice grain. The iron and manganese plaque on the root surface curtailed As accumulation in rice roots. Based on our results, the accumulation of As within rice plants was strongly associated with such soil properties such as silicon, phosphorus, organic matter, pH, and clay content. Understanding the factors and mechanisms controlling As uptake is important to develop mitigation measures that can reduce the amount of As accumulated in rice grains produced on contaminated soils.     

Arsenic methylation by a novel ArsM As(III) S‐adenosylmethionine methyltransferase that requires only two conserved cysteine residues

Arsenic (As) biomethylation is an important component of the As biogeochemical cycle that can influence As toxicity and mobility in the environment. Biomethylation of As is catalyzed by the enzyme arsenite (As[III]) S‐adenosylmethionine methyltransferase (ArsM). To date, all identified ArsM orthologs with As(III) methylation activities have four conserved cysteine residues, which are thought to be essential for As(III) methylation. Here, we isolated an As(III)‐methylating bacterium, Bacillus sp. CX‐1, and identified a gene encoding a S‐adenosylmethionine methyltranserase termed BlArsM with low sequence similarities (≤ 39%) to other ArsMs. BlArsM has six cysteine residues (Cys10, Cys11, Cys145, Cys193, Cys195 and Cys268), three of which (Cys10, Cys145 and Cys195) align with conserved cysteine residues found in most ArsMs. BlarsM is constitutively expressed in Bacillus sp. CX‐1. Heterologous expression of BlarsM conferred As(III) resistance. Purified BlArsM methylated both As(III) and methylarsenite (MAs[III]), with a final product of dimethylarsenate (DMAs[V]). When all six cysteines were individually altered to serine residues, only C145S and C195S derivatives lost the ability to methylate As(III) and MAs(III). The derivative C10S/C11S/C193S/C268S was still active. These results suggest that BlArsM is a novel As(III) S‐adenosylmethionine methyltransferase requiring only two conserved cysteine residues. A model of As(III) methylation by BlArsM is proposed.     

Arsenic uptake, translocation and speciation in pho1 and pho2 mutants of Arabidopsis thaliana

Arsenate [As (V)] is taken up by phosphate [P (V)] transporters in the plasma membrane of roots cells, but the translocation of As from roots to shoots is not well understood. Two mutants of Arabidopsis thaliana (L.) [( pho1 , P deficient) and ( pho2 , P accumulator)], with defects in the regulation and translocation of P (V) from roots to shoots, were therefore used in this study to investigate uptake, translocation and speciation of As in roots and shoots of plants grown in soil or nutrient solution. The shoots of the pho2 mutant contained higher P concentrations, but similar or slightly higher As concentrations, in comparison with the wild type. In the pho1 mutant, the P concentration in the shoots was lower, and the As concentration was higher, in comparison with the wild type. Both pho2 and the wild type contained mainly As (III) in roots and shoot (67–90% of total As). Arsenic was likely to be translocated by a different pathway to P (V) in the pho2 and pho1 mutants . Therefore, it is suggested that As (III) is the main As species translocated from roots to shoots in Arabidopsis thaliana.     

Role of ubiquitination in arsenic tolerance in plants

Arsenic (As) is harmful to all living organisms, including humans and plants. To limit As uptake and avoid its toxicity, plants employ systems that regulate the uptake of As from the soil and its translocation from roots to grains. Ubiquitination, a highly conserved post-translational modification (PTM) in all eukaryotes, plays crucial roles in modulating As detoxification mechanisms in budding yeast (Saccharomyces cerevisiae), but little is known about its roles in As tolerance and transport in plants. In this opinion article we review recent findings and suggest that ubiquitination plays a crucial role in regulating As transport in plants. We also propose ideas for future research to explore the importance of ubiquitination for enhancing As tolerance in crops.