Arsenic compounds arsenite

The arsenic compounds most commonly found in environmental and biological materials, and in working places, are arsenite and arsenate ions [As(III) and As(V)], monomethylarsonic acid (MMA), dimethylarsinic acid (DMA), arsine, di- and tri-... 

In 1933 Challenger et al. discovered that trimethylarsine was synthesized from inorganic arsenic compounds by molds (93). Recently, McBride and Wolfe (94), have reported the synthesis of dimethylarsine from arsenate by cell extracts of the methanogenic bacterium M. O. H. Methylcobalamin is the alkylating coenzyme for this synthesis which requires reduction of arsenate to arsenite, methylation of arsenite to methylarsonic acid, reduction and methylation of methylarsonic acid to dimethylarsinic acid, and finally a four electron reduction of dimethylarsinic acid to dimethylarsine (Fig. 13). 

Marine algae transform arsenate into nonvolatile methylated arsenic compounds such as methanearsonic and dimethylarsinic acids (Tamaki and Frankenberger 1992). Freshwater algae and macrophytes, like marine algae, synthesize lipid-soluble arsenic compounds and do not produce volatile methylarsines. Terrestrial plants preferentially accumulate arsenate over arsenite by a factor of about 4. Phosphate inhibits arsenate uptake by plants, but not the reverse. The mode of toxicity of arsenate in plants is to partially block protein synthesis and interfere with protein phosphorylation — a process that is prevented by phosphate (Tamaki and Frankenberger 1992). 

Several inorganic arsenic compounds are weak inducers of chromosomal aberrations, sister chromatid exchange, and in vitro transformation of mammalian and piscine cells. However, there is no conclusive evidence that arsenic causes point mutations in any cellular system (Pershagen and Valuer 1979 Belton et al. 1985 Lee et al. 1985 Deknudt et al. 1986 Manna and Mukheijee 1989). Studies with bacteria suggest that arsenite is a comutagen, or may inhibit DNA repair (Belton et al. 1985). 

There are reports indicating that arsenic compounds can damage cellular DNA, thereby acting as genotoxins, as assessed by the comet or single-cell gel assay. Some reports indicate that arsenite possesses this activity (5-7), whereas another indicates that although the methylated trivalent species are genotoxic, arsenite itself is not (8). 

Potassium arsenate (see Arsenic and arsenic compounds) Potassium arsenite (see Arsenic and arsenic compounds) Potassium bis(2-hydroxyethyl)dithiocarbamate Potassium bromate... 

Thermodynamic considerations suggest that in oxygenated seawater, arsenic should exist almost entirely as arsenate (71). It was apparent from the early work on arsenic in seawater, however, that arsenite was also present in significant concentrations and could at times predominate over arsenate (7,8, 72, 73). Marine bacteria (74) and marine phytoplankton (75) were shown to reduce arsenate to arsenite, thereby providing an explanation for the observed As(III)/As(V) ratio in seawater. The compounds MMA and DMA also occur in seawater, generally as minor constituents (9, 34, 71, 76). The concentrations of As(III), MMA, and DMA are positively correlated with primary productivity,... 

As stated in Section I, arsenate and phosphate are very similar. Hence, organisms have difficulty in assimilating phosphate without taking up arsenate, and this will uncouple their metabolism (Section II). They make use of an important difference between arsenate and phosphate to avoid this, i.e., the vastly greater ease of reduction of arsenate to arsenite than of phosphate to phosphite. By itself such a reduction would be no help, since arsenite is intensely toxic, but further processes can follow, such as alkylation to produce organoarsenic compounds (6), or extrusion of arsenite from the organism (e.g., 36, 37), driven by hydrolysis of ATP (38). Extruded arsenite may then be rendered less toxic by oxidation to arsenate by the arsenite oxidase mentioned in section III,A. 

Methylation of arsenic is an important pollution problem because of the widespread use of arsenic compounds in insecticides and because of the presence of arsenate in the phosphate used in household detergents.421 422 After reduction to arsenite, methylation occurs in two steps (Eq. 16-45). Additional reduction steps result in the formation of dimethylarsine, one of the principal products of action of methanogenic bacteria on arsenate. The methyl transfer is shown as occurring through CH3+, with an accompanying loss of a proton from the substrate. However, a CH3 radical may be transferred with formation of a cobalt(II) corrinoid.423... 

The combination of arsenic with dry nascent hydrogen was observed by Vournazos,14 who obtained a mixture of hydrogen and arsine by heating rapidly to 400° C. in a round-bottomed flask a mixture of three parts of powdered arsenic with eight parts of dry sodium formate. The addition of sodium hydroxide or lime to the mixture prevents the formation of sodium oxalate and hence of carbon monoxide. Arsenious oxide, sodium arsenite or arsenic acid may be used in place of arsenic, but the yields are small. The gas is also formed if arsenic vapour is passed over heated sodium formate. Also, if the sulphide or phosphide of arsenic is heated with the formate, hydrides of both components of the arsenic compound are formed but with metallic arsenides the hydride of the non-volatile component is not formed. 

Wildfang, E., Zakharyan, R.A. and Aposhian, H.V. (1998) Enzymatic methylation of arsenic compounds VI. Characterization of hamster liver arsenite and methylarsonic acid methyltransferase activities in vitro. Toxicology and Applied Pharmacology, 152(2), 366-75. 

Zakharyan, R., Wu, Y., Bogdan, G.M. and Aposhian, H.V. (1995) Enzymatic methylation of arsenic compounds Assay, partial purification, and properties of arsenite methyltransferase and monomethylarsonic acid methyltransferase of rabbit liver. Chemical Research in Toxicology, 8(8), 1029-38. 

In a departure from the usual study of arsenic metabolites in mammals, Kuehnelt et at. (1997) found mainly arsenate, arsenite and dimethylarsinic acid, and traces of methylarsonic acid and arsenobetaine in ants living close to an arsenic smelter. The low total arsenic levels, about 12.6mg kg-1, in the ants was suggested as indicative of a protective mechanism which prevents the uptake of arsenic from the high ambient levels. The resistance of micro-organisms to arsenic compounds has been reviewed (Cervantes et at., 1994). 

Most arsenic compounds are colorless powders or crystals. They include arsenites and arsenates of many metals syrupy arsenic acid and arsenic trichloride are liquids.1 All must be considered to be extremely poisonous. The metal itself has not been recognized as a noteworthy hazard.2... 

Three methods for the preparation of aromatic arsonic adds are de- scribed in this review. By far the most widely applicable of these is the Bart reaction, which involves the interaction of a diazonium salt with an inorganic arsenic compound. In the original Bart process 1 and most of its modifications an alkali arsenite is used, as shown in the following equation. 

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