Improving Arsenic Tolerance of Pyrococcus furiosus by Heterologous Expression of a Respiratory Arsenate Reductase

Arsenate is a notorious toxicant that is known to disrupt multiple biochemical pathways. Many microorganisms have developed mechanisms to detoxify arsenate using the ArsC-type arsenate reductase, and some even use arsenate as a terminal electron acceptor for respiration involving arsenate respiratory reductase (Arr). ArsC-type reductases have been studied extensively, but the phylogenetically unrelated Arr system is less investigated and has not been characterized from Archaea Here, we heterologously expressed the genes encoding Arr from the crenarchaeon Pyrobaculum aerophilum in the euryarchaeon Pyrococcus furiosus, both of which grow optimally near 100°C. Recombinant P. furiosus was grown on molybdenum (Mo)- or tungsten (W)-containing medium, and two types of recombinant Arr enzymes were purified, one containing Mo (Arr-Mo) and one containing W (Arr-W). Purified Arr-Mo had a 140-fold higher specific activity in arsenate [As(V)] reduction than Arr-W, and Arr-Mo also reduced arsenite [As(III)]. The P. furiosus strain expressing Arr-Mo (the Arr strain) was able to use arsenate as a terminal electron acceptor during growth on peptides. In addition, the Arr strain had increased tolerance compared to that of the parent strain to arsenate and also, surprisingly, to arsenite. Compared to the parent, the Arr strain accumulated intracellularly almost an order of magnitude more arsenic when cells were grown in the presence of arsenite. X-ray absorption spectroscopy (XAS) results suggest that the Arr strain of P. furiosus improves its tolerance to arsenite by increasing production of less-toxic arsenate and nontoxic methylated arsenicals compared to that by the parent.IMPORTANCE Arsenate respiratory reductases (Arr) are much less characterized than the detoxifying arsenate reductase system. The heterologous expression and characterization of an Arr from Pyrobaculum aerophilum in Pyrococcus furiosus provides new insights into the function of this enzyme. From in vivo studies, production of Arr not only enabled P. furiosus to use arsenate [As(V)] as a terminal electron acceptor, it also provided the organism with a higher resistance to arsenate and also, surprisingly, to arsenite [As(III)]. In contrast to the tungsten-containing oxidoreductase enzymes natively produced by P. furiosus, recombinant P. aerophilum Arr was much more active with molybdenum than with tungsten. It is also, to our knowledge, the only characterized Arr to be active with both molybdenum and tungsten in the active site.

Analysis of arsenical metabolites in biological samples

Quantitation of iAs and its methylated metabolites in biological samples provides dosimetric information needed to understand dose-response relations. Here, methods are described for separation of inorganic and mono-, di-, and trimethylated arsenicals by thin layer chromatography. This method has been extensively used to track the metabolism of the radionuclide [(73)As] in a variety of in vitro assay systems. In addition, a hydride generation-cryotrapping-gas chromatography-atomic absorption spectrometric method is described for the quantitation of arsenicals in biological samples. This method uses pH-selective hydride generation to differentiate among arsenicals containing trivalent or pentavalent arsenic.

In vitro assays of inorganic arsenic methylation

Inorganic arsenic is extensively metabolized to produce mono-, di-, and trimethylated products. The formation of these metabolites produces a variety of intermediates that differ from inorganic arsenic in terms of patterns of distribution and retention and in toxic effects. In order to elucidate the pathway for arsenic methylation, it was necessary to develop a reliable in vitro assay system in which the formation of methylated metabolites could be monitored. Here, in vitro assay system that uses the postmicrosomal supernate from rat liver is used as the source of the enzymatic activity that catalyzes methylation reactions. This system can be used to study the requirements for methylation reactions (e.g., identifying the donor of methyl groups) and for screening of compounds as potential activators or inhibitors of arsenic methylation.