Biological Methylation of Arsenic

Biological methylation of arsenic is effected via the transfer of a carbocation, a process known as oxidative... 

Biological methylation of arsenic is effected via the transfer of a carbocation, a process known as oxidative methylation. The methyl transfer agent can vary from simple sulphonium salts, for example, 5 -adenosyl-.S -methionine (67)... 

Figure 3 One step of the reduction/oxidation cycle that results in the biological methylation of arsenic.

The biological cycle of arsenic in the surface ocean involves the uptake of arsenate by plankton, the conversion of arsenate to a number of as yet unidentified organic compounds, and the release of arsenite and methylated species into the seawater. Biological demethylation of the methyl-arsenicals and the oxidation of arsenite by as yet... 

It is now well established that organometallic compounds are formed in the environment from mercury, arsenic, selenium, tellurium and tin and hence were also deduced on the basis of analytical evidence for lead, germanium, antimony and thallium. Biological methylation of tin has been demonstrated by the use of experimental organisms. Methylgermanium and methyllead were widely found in the environment but it is debatable whether germanium and lead are directly methylated by biological activity in natural environment. 

Numerous bacteria, fungi, yeasts, and algae are able to transform arsenic compounds. Among the transforming processes mediated by microorganisms are oxidation (9,10), reduction (11), demethylation (12,13), and methylation reactions (12-16). Biological transformations of arsenic in soil are illustrated in Figure 1 (17). 

Biological Mechanisms and Toxicological Consequences of the Methylation of Arsenic ... 

From a practical point of view, saturation of elimination has important consequences. If the metabolism becomes saturated, the duration of the action of the compound is prolonged. In such a case, correct timing for collection of biological monitoring samples also becomes difficult to assess. Furthermore, saturation of metabolism may also have qualitative effects. For example, it has been argued (but not yet proved) that arsenic compounds cause cancer at high doses at which methylation of inorganic arsenic becomes saturated. ... 

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. 

Historically, the investigations into biological methylation have followed two separate and distinct lines of research whose relationships have only recently become apparent and which remain to be fully developed. The first and older line had its origin in the arsenic rooms of the... 

Bentley, R. and Chasteen, T.G. (2002) Microbial methylation of metalloids arsenic, antimony, and bismuth. Microbiology and Molecular Biology Reviews, 66(2), 250-71. 

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