JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. The National Institute of Environmental Health Sciences (NIEHS) and Brogan & Partners are collaborating with JSTOR to digitize, preserve and extend access to Environmental Health Perspectives.Methylcobalamin (methyl-B12) has been implicated in the biomethylation of the heavy metals (mercury, tin, platinum, gold, and thallium) as well as the metalloids (arsenic, selenium, tellurium and sulfur). In addition, methylcobalamin has been shown to react with lead, but the lead-alkyl product is unstable in water.Details of the kinetics and mechanisms for biomethylation of arsenic are presented, with special emphasis on synergistic reactions between metal and metalloids in different oxidation states. This study explains why synergistic, or antagonistic, processes can occur when one toxic element reacts in the presence of another.The relative importance of biomethylation reactions involving methylcobalamin will be compared to those reactions where S-adenosylmethionine is involved. fore, the biosynthesis of methylarsenic compounds must occur in a reducing environment. However, McBride and Wolfe (3) found that methyl-B12 wascapable of functioning as methyl donor in the biosynthesis of dimethylarsine from arsenate or arsenite in cell extracts of Methanobacillus (M. O.H.). Since the Co-C (r bond has been shown to be susceptible to electrophilic attack (4), nucleophilic attack (5), and homolytic attack by freeradical species (6), it is important to determine how methyl-groups are transferred from methyl-B12 to arsenic. Schrauzer et al. (7) have formulated a mechanism for B12-dependent synthesis of dimethylarsine which involves inorganic arsenic salts functioning as electrophiles. In this communication we review the alternative mechanisms for the biosynthesis of methyl-arsenic compounds with a view to providing a chemical basis for determining which methylated species of arsenic are likely to reach significant concentrations in aqueous systems.
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