Distribution of Arsenic Species in Different Leaf Fractions – An Evaluation of the Biochemical Deposition of Arsenic in Plant Cells

Levels of toxic arsenic species in native terrestrial plants from soils polluted by former mining activities

Ten native terrestrial plants from soils polluted by former mining activities (Mónica mine, NW Madrid, Spain), with high total arsenic concentration levels (up to 3500 μg g(-1)), have been studied to determine the fraction of arsenic present as toxic forms (inorganic and methylated species), which present a higher mobility and therefore the potential risk associated with their reintegration into the environment is high. Roots and aboveground parts were analyzed separately to assess possible transformations from translocation processes. Extractions were carried out with deionized water by microwave-assisted extraction at a temperature of 90 °C and three extraction steps of 7.5 min each. Total extracted arsenic concentrations were determined by inductively coupled plasma atomic emission spectrometry, showing extraction percentages from 9 to 39% (calculated as the ratio between total extracted arsenic (Asext) and total arsenic (AsT) concentrations in plants). Speciation studies, performed by high performance liquid chromatography-photo-oxidation-hydride generation-atomic fluorescence spectrometry, showed the main presence of arsenate (As(v)) (up to 350 μg g(-1)), followed by arsenite (As(iii)), in both plant parts. Monomethylarsonic acid (MMA) and trimethylarsine oxide (TMAO) were also found only in some plants. On the other hand, the use of 0.5 mol L(-1) acetic acid as an extractant led to higher extraction percentages (33-87%), but lower column recoveries, probably due to the extraction of arsenic compounds different to the toxic free ions studied, which may come from biotransformation mechanisms carried out by plants to reduce arsenic toxicity. However, As(v) concentrations increased up to 800 μg g(-1) in acid medium, indicating the probable release of As(v) from organoarsenic compounds and therefore a higher potential risk for the environment.

Mass spectrometric evidence for different complexes of peptides and proteins with arsenic(III), arsenic(V), copper(II), and zinc(II) species

Trivalent and pentavalent arsenic were incubated with sulfur-containing amino acid, peptide and protein solutions both as organic compounds (phenylarsine oxide, phenylarsonic acid, dimethylarsinic acid, monomethylarsonic acid) and as inorganic compounds (arsenite, As(III), and arsenate, As(V)). After incubation of phenylarsine oxide solutions with cysteine and glutathione the mass spectra showed a covalent bond between arsenic and sulfur, which was stable at both acidic and neutral pH values. The mass spectra were dominated by monovalent ions at m/z 272 for cysteine samples and at m/z 458 for glutathione samples. Based on these masses the ionic structures could be ascribed to either fragment ions of the covalent arsenic-sulfur complexes or to other arsenic-bonding sites presumably at the amino group. Interestingly, under the same conditions no interactions of inorganic arsenite or arsenate could be measured. In the presence of added Cu(2+) ions all mass signals caused by a reaction of phenylarsine oxide with glutathione disappeared. In these mass spectra only the oxidised form of glutathione (GSSG) was found because of the redox activity of Cu(II). For the model protein lysozyme, no interactions with arsenic could be detected, whereas definite Cu- and Zn-lysozyme complexes with a stoichiometry of 1:1 and 2:1 for Zn(2+) ions and Cu(2+) ions, respectively, were observed. In contrast, for thioredoxin a bonding of As that depended on the concentration of the disulfide-reducing agent tris(2-carboxyethyl) phosphine was demonstrated. For three different phenylarsonic acids and for dimethylarsinic acid that all contain pentavalent arsenic, complexes with glutathione appeared in the mass spectra, which can be attributed to non-covalent interactions or to a covalent bond caused by an additive reaction. The optimisation of the experimental conditions necessary for the mass spectrometric analysis of the interactions of the arsenic species with peptides and proteins is described and the obtained mass spectra that provide information on the kinds of bonds are discussed.