Arsenic methylation in perspective

Abstract

The following arsenite methyltransferase activities (U/mg) were measured in untreated mice: liver, 1.42 ± 0.17 (mean ± SEM); kidney, 0.62 ± 0.18; lung, 0.33 ± 0.08; testis, 1.21 ± 0.01. Arsenite methyltransferase metabolites were not detectable using guinea pig liver, kidney, lung or testis cytosol as the source of enzyme. A twofold increase in liver arsenite methyltransferase activity was observed in mice exposed to 28.6 mg sodium arsenite/L drinking water after 24 hr compared to control. Trivalent arsenic species were separated from pentavalent arsenicals in liver homogenates of hamsters injected 15 hr priorly with [⁷³As]arsenate by (CCl₄)-20 mM (DDDC) extraction and both phases analyzed by HPLC. Metabolites of inorganic arsenate were observed in the following concentrations (ng/g liver ± SEM); MMAᴵᴵᴵ, 38.5 ± 2.9; DMAᴵᴵᴵ, 49.9 ± 10.2; arsenite, 35.5 ± 3.0; arsenate, 118.2 ± 8.7; MMAᵛ, 31.4 ± 2.8; and DMAᵛ, 83.5 ± 6.7. Neither in vitro nor in vivo arsenite methyltransferase activity could be detected in S. cerevisiae . Attempting to induce enzyme activity, yeast were grown in 0-100 mM, 8 μCi [⁷³As]Na₂HAsO₄. No methylated metabolites were detected in cell lysate or media under these experimental conditions. The dissociation constants for guinea pig and rabbit cytosolic arsenite binding proteins were determined to be 59.62 ± 11.97 and 120.4 μM Asᴵᴵᴵ, respectively, and the respective specificities, 53.83 ± 3.67% and 59%. Guinea pig arsenite binding protein was fractionated using bioaffinity and size exclusion chromatography to yield partially purified protein(s) with an approximate molecular weight of 115 kDa. Arsenite methyltransferase activity was purified >7000-fold from rabbit liver and identified as 1 cys peroxiredoxin, a conserved family of antioxidant proteins characterized by non-selenium dependent glutathione peroxidase and calcium-independent phospholipase A2 activities. Murine 1 cys peroxiredoxin was cloned and expressed in E. coli and S. cerevisiae and expressed in reticulocyte lysate. Recombinant proteins had neither arsenite methyltransferase nor glutathione peroxidase activities. Antibodies directed against 1-cys peroxiredoxin immunoprecipitated a ∼29 kDa protein from guinea pig kidney cytosol which was identified as 1-cys peroxiredoxin by LC-MS/MS. Under these experimental conditions, guinea pig kidney cytosol did not catalyze the reduction of peroxides