The Biochemistry of Arsenite

The relative affinities of various hydroxylic dithiols for arsenite were measured by Zahler and Cleland (33), but proved hard to understand (since seven-membered rings seemed to give the most stable complexes) until Cruse and James (34) showed that an oxygen atom of such a compound acted as a third ligand for arsenic, so that bicyclic complexes were formed. 

Some compounds of this type may have a high affinity for proteins that is not due to their binding to two thiol groups (35). In particular, arsenite also reacts with the molybdenum-pterin cofactor of many enzymes (35a-d). This usually inhibits the enzyme, but in particular cases (35e) the arsenite may be oxidized indeed the enzyme arsenite oxidase contains such a center (35f). 

As stated in Section I, arsenate and phosphate are very similar. Hence, organisms have difficulty in assimilating phosphate without taking up arsenate, and this will uncouple their metabolism (Section II). They make use of an important difference between arsenate and phosphate to avoid this, i.e., the vastly greater ease of reduction of arsenate to arsenite than of phosphate to phosphite. By itself such a reduction would be no help, since arsenite is intensely toxic, but further processes can follow, such as alkylation to produce organoarsenic compounds (6), or extrusion of arsenite from the organism (e.g., 36, 37), driven by hydrolysis of ATP (38). Extruded arsenite may then be rendered less toxic by oxidation to arsenate by the arsenite oxidase mentioned in section III,A. 

An analogous mechanism is seen in the utilization of arsonoacetate by a bacterium as its sole source of energy and carbon (39). Arsenate is the product, and since the bacteria can oxidize arsenite, the original degradation of the arsonoacetate may be reductive and form arsenite, which is then oxidized to the less toxic arsenate. 

for some organisms it is advantageous to oxidize arsenite to arsenate, whereas the reverse process may be beneficial to others, particularly if the arsenite is further metabolized. 

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C. Spontaneous Formation of Esters of Arsenate The Biochemistry of Arsenite... 

This chapter concentrates on arsenite oxidation by NT-26 as well as a number of other recently isolated arsenite-oxidizing bacteria. It will be apparent that all but one of these organisms is phylogenetically distinct from the arsenite oxidizers previously identified. The isolation, phytogeny, physiology, and biochemistry of arsenite oxidation are described. 

GN George, RC Bray. Reaction of arsenite ions with the molybdenum center of milk xanthine oxidase. Biochemistry, 22 1013-1021, 1983. 

Vega, L., Lopez-Duran, R.M., and Rodriguez-Sosa, M., Alteration of antigen presentation by KI-OVA macrophages treated with sodium arsenite, presented at the XXIV National Meeting of the Mexican Society of Biochemistry, Puerto Vallarta, Jalisco, November 3-8, 2002. 

Csanaky, I. and Gregus, Z. (2002) Species variations in the biliary and urinary excretion of arsenate, arsenite and their metabolites. Comparative Biochemistry and Physiology C-Toxicology and Pharmacology, 131(3), 355-65. 

Klemperer, N.S., E.S. Berleth and C.M. Pickart. A novel, arsenite-sensitive E2 of the ubiquitin pathway purification and properties. Biochemistry 28 6035-6041, 1989. 

Methylation of Inorganic oxyarsenic anions occurs in organisms ranging from microbial to mammalian methylated and products include arsenocholine, arsenobetaine, dimethylarsinic acid, and methylarsonic acid arsenite methyltransferase and monomethylarsonic acid methyltransferase use S-adenosylmethionine for the methyl donor Boron biochemistry essentially that of boric acid, which forms ester complexes with hydroxyl groups, preferably those adjacent and cis, in organic compounds. Five naturally occurring boron esters (all antibiotics) synthesized by various bacteria have been characterized Exists as Br Ion in vivo, binds to proteins and amino acids... 

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