Arsenic, biomethylation mechanism

Cullen et al. (1994) have proposed a possible mechanism of arsenic methylation after the study in which arsenite, arsenate, monomethyl-arsonate or dimethylarsinic acid were added to the growth medium in the presence of the unicellular alga Polyphsa peniculus. Evidence of arsenic biomethylation by the micro-organism Apiotrichum humicola in the presence of L-methionine-methyl-d3 has come from the same laboratory (Cullen et al., 1995). Their findings point to the role of S-adenosylmethionine, or a related sulfonium compound as possible methyl donors. Arsenic biomethylation and biotransformation has also been demonstrated in a freshwater environment (Kuroiwa et al., 1994). 

Biomethylation is the preferred detoxification mechanism for inorganic arsenicals. 

Fig. 7. Challenger mechanism for biomethylation of arsenic. Solid arrows represent pathways proposed originally dashed arrows represent additional pathways proposed by the present authors.

The mechanisms of biomethylation of arsenic have not been documented. However, Cullen and coworkers have proposed a plausible mechanistic model based on oxidative methylation of arsenic(III) by S-adenosylmethionine and reduction by a thiol such as lipoic acid. 

The mechanism of the biomethylation of arsenic is generally believed to involve a series of reduction/oxidation steps, one cycle of which is shown in Figure 3 (11). 

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