Introduction to arsenic methylation and demethylation

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). 

Initially, the resonating inorganic As(V) is biotically or abiotically reduced to inorganic As(III)  

Biomethylation may also produce more complex alkyl arsenic groups. As(C2H5)(CH3)2 has been found in landfill and sewage gas, and probably also exists in natural gas (Bentley and Chasteen, 2002), 251. As(C2H5)3 may also occur in landfill gases and probably natural gas (Bentley and Chasteen, 2002), 251. Further details on the reduction and methylation biochemistry of arsenic are discussed in Chapter 4. 

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. 

Lehr et al. (2003) found that Mycobacterium neoaurum could demethylate MMA(III) and MMA(V) to inorganic arsenic, but not DMA(V) or trimethylarsine oxide. Their results suggest that at least some MMA(V) reductively demethylates to inorganic As(III), which is a reversal of Reaction 2.13 (see above) in the Challenger mechanism (Lehr et al., 2003), 833. Other mechanisms by which microorganisms demethylate arsenic are largely unknown (Lehr et al., 2003). Chapter 4 presents additional information on the demethylation of arsenic. 

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