Arsenic (+ 3 oxidation state) methyltransferase and the methylation of arsenicals in the invertebrate chordate Ciona intestinalis

David J. Thomas, Gerardo M. Nava, Shi Ying Cai, James L. Boyer, Araceli Hernández-Zavala, H. Rex Gaskins

Research output: Contribution to journalArticle


Biotransformation of inorganic arsenic (iAs) involves methylation catalyzed by arsenic (+ 3 oxidation state) methyltransferase (As3mt) yielding mono-, di-, and trimethylated arsenicals. To investigate the evolution of molecular mechanisms that mediate arsenic biotransformation, a comparative genomic approach focusing on the invertebrate chordate Ciona intestinalis was used. Bioinformatic analyses identified an As3mt gene in the C. intestinalis genome. Constitutive As3mt RNA expression was observed in heart, branchial sac, and gastrointestinal tract. Adult animals were exposed to 0 or 1 ppm of iAs for 1 or 5 days. Steady-state As3mt RNA expression in the gastrointestinal tract was not modulated significantly by 5 days of exposure to iAs. Tissue levels of iAs and its methylated metabolites were determined by hydride generation-cryotrapping-gas chromatography-atomic absorption spectrometry. At either time point, exposure to iAs significantly increased concentrations of iAs and its methylated metabolites in tissues. After 5 days of exposure, total speciated arsenic concentrations were highest in branchial sac (3705 ng/g), followed by heart (1019 ng/g) and gastrointestinal tract (835 ng/g). At this time point, the sum of the speciated arsenical concentrations in gastrointestinal tract and heart equaled or exceeded that of iAs; in branchial sac, iAs was the predominant species present. Ciona intestinalis metabolizes iAs to its methylated metabolites, which are retained in tissues. This metabolic pattern is consistent with the presence of an As3mt ortholog in its genome and constitutive expression of the gene in prominent organs, making this basal chordate a useful model to examine the evolution of arsenic detoxification.

Original languageEnglish (US)
Article numberkfp250
Pages (from-to)70-76
Number of pages7
JournalToxicological Sciences
Issue number1
StatePublished - Oct 15 2009


  • Arsenic
  • Ciona intestinalis
  • Comparative genomics
  • Methyltransferases
  • Toxicogenomics

ASJC Scopus subject areas

  • Toxicology

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