Stephen B. Waters

Arsenic (+3 Oxidation State) Methyltransferase and the Methylation of Arsenicals

Metabolic conversion of inorganic arsenic into methylated products is a multistep process that yields mono-, di-, and trimethylated arsenicals. In recent years, it has become apparent that formation of methylated metabolites of inorganic arsenic is not necessarily a detoxification process. Intermediates and products formed in this pathway may be more reactive and toxic than inorganic arsenic. Like all metabolic pathways, understanding the pathway for arsenic methylation involves identification of each individual step in the process and the characterization of the molecules which participate in each step. Among several arsenic methyltransferases that have been identified, arsenic (+3 oxidation state) methyltransferase is the one best characterized at the genetic and functional levels. This review focuses on phylogenetic relationships in the deuterostomal lineage for this enzyme and on the relation between genotype for arsenic (+3 oxidation state) methyltransferase and phenotype for conversion of inorganic arsenic to methylated metabolites. Two conceptual models for function of arsenic (+3 oxidation state) methyltransferase which posit different roles for cellular reductants in the conversion of inorganic arsenic to methylated metabolites are compared. Although each model accurately represents some aspects of enzyme’s role in the pathway for arsenic methylation, neither model is a fully satisfactory representation of all the steps in this metabolic pathway. Additional information on the structure and function of the enzyme will be needed to develop a more comprehensive model for this pathway.

Comprehensive analysis of arsenic metabolites by pH-specific hydride generation atomic absorption spectrometry

In a variety of biological systems, inorganic arsenic (iAs) is metabolized to yield methylated arsenicals that contain arsenic in +5 or +3 oxidation states. Atomic absorption spectrometry (AAS) coupled with a pH-specific generation of arsines has been used for selective analysis of trivalent and pentavalent inorganic, mono-, and dimethylated arsenicals in biological matrices. We have optimized this method to permit simultaneous detection and quantification of all relevant metabolites of iAs, including trimethylarsine oxide (TMAsVO). The optimization includes increasing the density of the chromatographic adsorbent used for cold-trapping of generated arsines and modification of the temperature gradient for release of arsines from the cold trap. These modifications improve the boiling-point separation of arsine, methylarsine, dimethylarsine, and trimethylarsine before the detection by AAS. Arsines from trivalent arsenicals and from TMAsVO are selectively generated at pH 6. At pH 1, arsines are generated from both tri- and pentavalent arsenicals. Thus, the optimized technique permits analysis of arsenite (iAsIII), arsenate (iAsV), monomethylarsonic acid (MAsV), monomethylarsonous acid (MAsIII), dimethylarsinic acid (DMAsV), dimethylarsinous acid (DMAsIII), and TMAsVO. The detection limits range from 0.14 ng As (for TMAsVO) to 0.40 ng As (for iAsV). Calibration curves are linear over the concentration range of 0.5–100 ng As. Recoveries vary between 85 and 124%. The precision of the method in various biological matrices ranges from 1.0 to 14.5%. Using the optimized technique, both trivalent and pentavalent methylated and dimethylated arsenicals, but not TMAsVO, have been detected in cultured primary human hepatocytes exposed to iAsIII. In contrast, TMAsVO was detected as the final product of in vitro methylation of iAsIII by rat AsIII-methyltransferase, cyt19. TMAsVO was also detected in the urine of mice treated with MAsV or DMAsV. Thus, the optimized method improves the efficiency of arsenic speciation analysis in biological matrices, providing a more comprehensive picture of the role of metabolism in the disposition and action of iAs.

Commonalities in Metabolism of Arsenicals

Environmental Context. Health effects associated with inorganic arsenic include various cancers and increased risk of diabetes. Millions of people in Bangladesh and India are at risk through use of contaminated drinking water. When humans ingest inorganic arsenic, it is rapidly converted to methylated metabolites. Although this methylation process is largely understood, the metabolism of other arsenicals (e.g. arsenosugars to dimethylarsenic) is very unclear. Connections among pathways for metabolism of various arsenicals are now being elucidated. Commonalities and differences in these pathways may be important determinants of the risk associated with exposure to these agents. Abstract. Elucidating the pathway of inorganic arsenic metabolism shows that some of methylated arsenicals formed as intermediates and products are reactive and toxic species. Hence, methylated arsenicals likely mediate at least some of the toxic and carcinogenic effects associated with exposure to arsenic. Trimethylarsonium compounds and arsenosugars are two other classes of arsenicals to which humans are routinely exposed and there is evidence that both classes are metabolized to produce methylated arsenicals. Here, we review evidence for production of methylated metabolism and consider the challenges posed in unraveling a complex web for metabolism of arsenicals in humans.

A novel S -adenosylmethionine-dependent methyltransferase from rat liver cytosol catalyzes the formation of methylated arsenicals

Publisher Summary The discovery that inorganic arsenic (As) was converted into methylated metabolites in many species, including humans, elucidating the pathway for As metabolism is a topic of considerable interest. Cyt19, a methyltransferase that catalyzes both monomethylation and dimethylation reactions, is purified from rat liver cytosol and cloned to permit its expression as a recombinant protein. Cyt19 is also expressed in human tissues and is also an arsenic (As) methyltransferase. Future research will probe the mechanism by which cyt19 catalyzes the oxidative methylation of arsenicals and the role of reductants in its function.