Arsenic detoxification and

Arsenic detoxification and evolution of trimethylarsine gas by a microbial arsenite S-adenosylmethionine methyl-transferase. Proc. Natl Acad. Sci. USA 103 2075-80. 

C Diorio, J Cai, J Marmor, R Shinder, MS DuBow. An Escherichia coli chromosomal ars operon homolog is functional in arsenic detoxification and is conserved in gram-negative bacteria. J Bacteriol 177 2050-2056, 1995. 

Saltikov CW, RA Wildman, DK Newman (2005) Expression dynamics of arsenic respiration and detoxification in Shewanella sp. strain ANA-3. J Bacteriol 187 7390-7396. 

Dimethylarsinic acid is the major metabolite of orally administered arsenic trioxide, and is excreted rapidly in the urine (Yamauchi and Yamamura 1985). The methylation process is true detoxification, since methanearsonates and cacodylates are about 200 times less toxic than sodium arsenite (NAS 1977). The marmoset monkey (Callithrix jacchus), unlike all other animal species studied to date, was not able (for unknown reasons) to metabolize administered As+5 to demethylarsinic acid most was reduced to As+3. Only 20% of the total dose was excreted in urine as unchanged As+5, and another 20% as As+3. The rest was bound to tissues, giving distribution patterns similar to arsenite (Vahter and Marafante 1985). Accordingly, the marmoset, like the rat, may be unsuitable for research with arsenicals. 

Crameri, A., S.A.Raillard, E.Bermudez and W.P.Stenuner. (1998) Molecular evolution of an arsenate detoxification pathway by DNA shuffling. Nature, 391, 288-291. 

Phytoremediation with living plants may be improved through genetic engineering and a thorough understanding of arsenic metabolism and detoxification in plants (Montes-Baydn et al., 2004). As an initial step, Montes-Bay on et al. (2004) studied arsenic metabolism in Brassica juncea (Indian mustard). They found that some of the arsenic was associated with thiol groups in the plant. 

The one exception is the capacity of some micro-organisms to methylate certain metals, e.g. arsenic, antimony and mercury, probably as a kind of detoxification mechanism. Methylcobal-amin is the only organometallic compound known to have a physiological function in life processes. 

FIGURE 72.2. Arsenic detoxification mechanisms (reduction, oxidation, methylation, and resistance) in prokaryotes. (A) Respiratory arsenate reductase (Arr) is involved in the reduction of As(V) by the dissimilatory arsenate respiring organisms. (B) Arsenite oxidase (Aox/Aso) is responsible for oxidation of As(III) by chemoautotrophic or heterotrophic arsenite oxidizers. 

Martin, P., DeMel, S., Shi, J., Gladysheva, T., Gatti, D.L., Rosen, B.P., Edwards, B.F.P. (2001). Insights into the structure, solvation, and mechanism of ArsC arsenate reductase, a novel arsenic detoxification enzyme. Structure 9 1071-81. 

Both oxidation and methylation are microbial transformations involved in the redistribution and global cycling of arsenic. Oxidation involves the conversion of toxic arsenite to less toxic arsenate. Bacterial methylation of inorganic arsenic under anaerobic conditions may be a mechanism of arsenic detoxification. Fungi also transform inorganic and organic arsenic compounds into volatile methylar-sines. However, unlike methylated selenium which is nontoxic, the volatile arsine... 

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