Transition metal ions and selenite modulate the methylation of arsenite by the recombinant human arsenic (+3 oxidation state) methyltransferase (hAS3MT)

This report demonstrates that transition metal ions and selenite affect the arsenite methylation by the recombinant human arsenic (+3 oxidation state) methyltransferase (hAS3MT) in vitro. Co2+, Mn2+, and Zn2+ inhibited the arsenite methylation by hAS3MT in a concentration-dependent manner and the kinetics indicated Co2+ and Mn2+ to be mixed (competitive and non-competitive) inhibitors while Zn2+ to be a competitive inhibitor. However, only a high concentration of Fe2+ could restrain the methylation. UV–visible, CD and fluorescence spectroscopy were used to study the interactions between the metal ions above and hAS3MT. Further studies showed that neither superoxide anion nor hydrogen peroxide was involved in the transition metal ion or selenite inhibition of hAS3MT activity. The inhibition of arsenite methylating activity of hAS3MT by selenite was reversed by 2 mM DTT (dithiothreitol) but neither by cysteine nor by β-mercaptoethanol. Whereas, besides DTT, cysteine can also prevent the inhibition of hAS3MT activity by Co2+, Mn2+, and Zn2+. Free Cys residues were involved in the interactions of transition metal ions or selenite with hAS3MT. It is proposed that the inhibitory effect of the ions (Co2+, Mn2+, and Zn2+) or selenite on hAS3MT activity might be via the interactions of them with free Cys residues in hAS3MT to form inactive protein adducts.

Transition metal ions (Co2+, Mn2+ and Zn2+) and selenite modulate the structure and activity of hAS3MT through their interactions with the enzyme. Co2+ and Mn2+ are mixed (competitive and non-competitive) inhibitors of iAs3+ methylation by hAS3MT while Zn2+ is a competitive inhibitor. However, SeO32- might bind to the non-active sites of the enzyme as a non-competitive inhibitor of iAs3+ methylation by hAS3MT.Figure optionsDownload as PowerPoint slide