Arsenite arsenate detoxification

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. 

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

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... 

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. 

B. The Place of Arsenite in the Detoxification of Arsenate Arsonates as Analogues of Natural Phosphates or Phosphonates... 

Trivalent arsenic is less abundant in nature, but considerably more toxic. Detoxification of arsenite results in the same methylated species as those arising from arsenate. However, because trivalent arsenic binds tightly to thiol-containing molecules in tissues, it is much less easily detoxified and excreted. 

The arsenic cycle in ocean waters and sediments also has important biological steps (Andreae, 1979). Arsenate, As(V), can be biologically converted into arsenite, As(III), and at least eight different organo-arsenic compounds, all presumably representing detoxification processes mediated by bacteria in... 

Arsenic is notorious as a toxic semi-metal and the trivalent form, arsenite [As(ni)] is considered to be the most toxic (1) as it inactivates the sulfhydryl groups of cysteine residues in proteins (2,3). Organisms have adapted mechanisms to convert arsenite to the less toxic pentavalent form, arsenate [As(V)]. Some of these organisms can use arsenite as their sole source of energy (4) whereas others oxidize it to arsenate as part of a detoxification mechanism (5). 

Since detoxification of arsenite occurs via oxidation to arsenate, understanding the mechanism of arsenite oxidoreductase has centered on the three redox-active centers found in the enzyme. These are a molybdenum center, a [3Fe S] cluster. 

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