Arsenic compounds biotransformation

C. Marine Animals Toxicological Considerations Biotransformation of Marine Arsenic Compounds... 

Zakharyan, R.A. and Aposhian, H.V. (1999) Enzymatic reduction of arsenic compounds in mammalian systems the rate-limiting enzyme of rabbit liver arsenic biotransformation is MMA(V) reductase. Chemical Research in Toxicology, 12(12), 1278-83. 

Toxicity of the various arsenic compounds in mammals extends over a wide range, determined in part by unique biochemical actions of each compound, but also by absorbability and efficiency of biotransformation and disposition. Overall, arsines present the greatest toxic hazard, followed closely by arsenites (inorganic trivalent compounds). Inorganic pentavalent compounds are somewhat less toxic that arsenites, while the organic (methylated)... 

It has also been shown that arsenic is incorporated into marine and freshwater organisms in the form of both water-soluble and lipid-soluble compounds. Recent studies have shown the presence of arsenite [As(III)], arsenate [As(V)], methylarsonic acid, dimethylarsinic acid and arsenobetaine (AB). Methylated arsenicals also appear in the urine and plasma of mammals, including man, by biotransformation of inorganic arsenic compounds. Several methods have been devised to characterize these arsenicals. 

Research on organoarsenic compounds up to 1993 has been previously reviewed and the following works should be consulted by readers requiring detailed information. The review of Cullen and Reimer is an all-encompassing treatise of arsenic compounds (species) in the environment and includes sections on basic chemistry, biotransformations, and environmental behavior. Chemical aspects, including synthesis, are the focus of the review by Edmonds et The review of arsenic in marine organisms, published in 1997adequately summarizes the relevant... 

Marafante, E. and Vahter, M. (1989). Biotransformation of organo-arsenic compounds in mammals, in Vernet, J.P. (ed). Proc. Heavy Metals in the Environment, Vol. 2, p. 162-165, CEC Consultants, Edinburgh. 

Peraza et al. (2003) studied toxicity and metabolism of inorganic arsenic in kidney at low level subcytotoxic concentrations. Human renal proximal tubule epithelial cells (HK-2) were used as model in their study. The authors found that HK-2 cells were capable of biotransforming inorganic arsenic compounds in a pathway involving reduction of arsenate to arsenite. 

Included here are novel arsenic compounds reported in environmental samples over the last five years. Dimethylarsinoylacetate was identified as a naturally occurring arsenical in marine reference materials of mussel, oyster, and lobster hepatopancreas (36). This compound had been proposed as a possible intermediate in the formation of arsenobetaine (31). More recently, however, arsenobetaine was found to degrade to dimethylarsinoylacetate under aerobic microbial conditions (37), and such a biotransformation suggests an alternative hypothesis for the presence of dimethylarsinoylacetate in marine samples. 

Chemical structures of the arsenic compounds in marine organisms have been confirmed in many cases, but very few chemical species of arsenic in freshwater organisms have been found. Arsenic transformation via the freshwater food chain has rarely been reported. This chapter focuses on the toxicity of arsenicals and the biotransformation of arsenic in the freshwater organisms. 

Biotransformation of Inorganic Arsenic Compounds by Freshwater Algae... 

The marine facultative anaerobe bacterium Serratia marinoruhm and the yeast Rhodotoruhi rubra both methylate arsenate ion to methylarsonate, but only the latter produces cacodylic acid (258). Human volunteers who ingested 500 fig doses of As as sodium arsenite, sodium methylarsonate, and sodium cacodylate excreted these compounds in their urine (259). Of these three, approximately 75% of the sodium arsenite is methylated, while 13% of methylarsonate is methylated. Rat liver subcellular fractions methylated sodium arsenate in vitro, providing the first direct evidence for possible mammalian methylation independent of symbiotic bacteria (260). Shariatpanahi el al. have reported kinetics studies on arsenic biotransformation by five species of bacteria (261). They found that the As(V)-As(IIl) reduction followed a pattern of two parallel first-order reactions, while the methylation reactions all followed first-order kinetics. Of the five species tested, only the Pseudomonas produced all four metabolites (arsenite, methylarsonate, cacodylate, trimethylarsine) (261). 

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). 

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