Biotransformation of Marine Arsenic Compounds

The classic studies by Challenger (127-129) on microbial methyla-tion of arsenic still provide the basis of our understanding of these processes. Although Challenger s work focused on mycological methyla-tions (he mistakenly believed that bacteria did not methylate arsenic), the scheme he proposed is applicable to other biological systems as well. It is briefly discussed here, together with the confirmatory studies of Cullen and co-workers. 

According to Challenger (127), arsenate is transformed to trimethyl-arsine by the mold Scopulariopsis brevicaulis, by sequential reduction and oxidative methylation of the arsenic species (Fig. 7). The proposed intermediates in the pathway were MMA, DMA, and TMAO. Although Challenger could not detect these compounds, when they were added to a culture of S. brevicaulis trimethylarsine was formed. Challenger (129) considered that the likely source of methyl groups was S-adeno-sylmethionine (AdoMet), which had previously been identified as an 

The source of the electrons in the reduction of arsenic outlined in Fig. 7 is not known, but a sound model involving reduction by thiols has been proposed (133). A reexamination of Challenger s proposed methylation pathway looked at the effects of adding low levels of As(III), As(V), MMA, and DMA to cultures of two microorganisms including 

brevicaulis (134). By the use of sensitive analytical methodology involving arsine generation, the arsenic intermediates proposed in Fig. 7 were identified in the growth medium. A significant result was the detection of TMAO, rather than trimethylarsine, as the major methylation product. The low concentrations of arsenic employed in these experiments resulted in TMAO being present at less than toxic concentrations, and further detoxification by reducing TMAO to the arsine was considered unnecessary. Whether or not TeMA was produced in these experiments is not known its presence was not reported, but it would not have been detected by the analytical technique used. 

Bacterial methylation of arsenate by a methanogen was first reported by McBride and Wolfe (135) in 1971, and reports of nonmethanogenic bacterial methylation followed. These transformations are now known to be effected by a range of bacteria, and the mechanisms are likely to be similar to those proposed for fungi (32). 

Hanaoka and co-workers (141) have reported several experiments describing the microbial degradation of arsenobetaine. Unspecified microorganisms derived from sediments, algae, mollusc intestine, or suspended sediments were incubated with arsenobetaine under various conditions. Arsenobetaine was degraded to TMAO and DMA in all cases and, for sediments and suspended sediments, further degradation 

When subjected to anaerobic microbial conditions, the trimethyar-sonioriboside 33 was transformed quantitatively into arsenocholine, suggesting that cleavage of the C3—C4 bond of the sugar ring had occurred in an analogous manner to that observed for the dimethylarsinoylribosides (Fig. 8b) (145). 

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

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. 

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. 

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... 

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. 

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). 

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