Bacteria arsenate methylation

Through the action of methylcobalamin in bacteria, arsenic(III) is methylated to methyl, and then to dimethylarsinic acid ... 

Islam, S.M., Fukushi, K., Yamamoto, K. (2005). Development of an enumeration method for arsenic methylating bacteria from mixed culture samples. Biotechnol. Lett. 27 1885-90. 

Chiu et al. (53) reported that lowering of soil redox potential increased the ratio of As(III) and promoted arsenic methylation. Methylation of arsenic compounds by yeast and bacteria under oxic conditions plays a significant role, whereas methanogenic bacteria are important under anoxic conditions in releasing volatile arsenic from the soil to the atmosphere (39,41). Woolson and Kearney... 

The classic studies by Challenger (127-129) on microbial methyla-tion of arsenic still provide the basis of our understanding of these processes. Although Challenger s work focused on mycological methyla-tions (he mistakenly believed that bacteria did not methylate arsenic), the scheme he proposed is applicable to other biological systems as well. It is briefly discussed here, together with the confirmatory studies of Cullen and co-workers. 

Bacterial methylation of arsenate by a methanogen was first reported by McBride and Wolfe (135) in 1971, and reports of nonmethanogenic bacterial methylation followed. These transformations are now known to be effected by a range of bacteria, and the mechanisms are likely to be similar to those proposed for fungi (32). 

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 marine facultative anaerobe bacterium Serratia marinoruhm and the yeast Rhodotoruhi rubra both methylate arsenate ion to methylarsonate, but only the latter produces cacodylic acid (258). Human volunteers who ingested 500 fig doses of As as sodium arsenite, sodium methylarsonate, and sodium cacodylate excreted these compounds in their urine (259). Of these three, approximately 75% of the sodium arsenite is methylated, while 13% of methylarsonate is methylated. Rat liver subcellular fractions methylated sodium arsenate in vitro, providing the first direct evidence for possible mammalian methylation independent of symbiotic bacteria (260). Shariatpanahi el al. have reported kinetics studies on arsenic biotransformation by five species of bacteria (261). They found that the As(V)-As(IIl) reduction followed a pattern of two parallel first-order reactions, while the methylation reactions all followed first-order kinetics. Of the five species tested, only the Pseudomonas produced all four metabolites (arsenite, methylarsonate, cacodylate, trimethylarsine) (261). 

The methylation of arsenic is entirely or almost entirely biotic (Frankenberger and Arshad, 2002), 367. Specifically, certain fungi (including yeasts) and bacteria are capable of methylating arsenic ((Bentley and Chasteen, 2002), 257-260 (Cullen and Reimer, 1989), 717-724 Chapter 4). Only limited evidence exists for the chemical (abiotic) methylation of arsenic. As mentioned earlier, some volatile arsines have been produced in the laboratory from photochemical reactions involving As(III), carboxylic acids, and ultraviolet radiation (Guo et al., 2005 McSheehy et al., 2005). 

Demethylation refers to the removal of methyls from organoarsenicals, which may ultimately transform the organoarsenicals into inorganic arsenic. Although exposure to ultraviolet radiation may demethylate arsenic (Cullen and Reimer, 1989), 741, the role of microorganisms in demethylation is especially important. Under sterile conditions, MMA(V) and DMA(V) are very stable in water (Cullen and Reimer, 1989), 749. However, bacteria can demethylate them and other methylarsenic species into inorganic arsenic (Frankenberger and Arshad, 2002), 364 (Cullen and Reimer, 1989), 749 (Santosa et al., 1996), 703. 

In solution under these conditions the major species is [H2As03] (68). The cycle is completed trough the rereduction of new methylated arsenical by a range of nonspecific rednctants (Scheme 6). The organoarsenicals prodnced dnring biomethylation can be reduced by bacteria with lipoic acid and by mammals with the ubiquitons thiolate glntathione, as has been reported in in vitro stndies. ... 

Methane bacteria have been shown to catalyze reactions in which the active methyl group is transferred to acceptors such as arsenate or mercury. When extracts are incubated in a hydrogen atmosphere with methylcobalamin, arsenate, and ATP, a volatile arsine derivative is formed (20). Arsines are difficult and dangerous to work with they are extremely poisonous and are oxidized rapidly in air. Fortunately they have an intense garhc odor so the investigator is warned of their presence. 

The third group of As species in natural freshwaters are the methylated arsenic species. Arsenic can be methylated by bacteria, algae and fungi to form gaseous mono-, di- and tri-methyl arsine (CH3) As) (Baker et al., 1983 Cullen and Reimer, 1989 Maeda et al, 1987), which dissolve in water forming the methylarsenic oxyacids, monomethylarsonic acid (MMAA) and... 

Many chemicals can also exist as various species or states of ionization. For example, nitrogen can exist as nitrate, nitrite, or ammonia, arsenic can exist as arsenate or arsenite, and lead can exist as lead nitrate or lead chloride. The species or ionization state may depend upon abiotic variables such as soil or water pH, amount of dissolved oxygen in the water, and presence of other chemicals. Alternatively, bacteria and fungi may change the species or ionization state of a chemical. For example, bacteria can convert arsenite to arsenate, and add methyl groups to ionic mercury to produce methylmercury. 

Beech, I. B., and Cheung, C. W. S. (1995). Interactions of exopolymers produced by sulphate-reducing bacteria with metal ions. Int. Biodeterior. Biodegrad. 35, 59-72. Bentley, R., and Chasteen, T. G. (2002). Microbial methylation of metalloids arsenic,... 

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