Arsenic Methytransferase is Involved in Arsenosugar Biosynthesis by Providing DMA

摘要

Arsenic is an ubiquitous toxic element in the environment, and organisms have evolved different arsenic detoxification strategies. Studies on arsenic biotransformation mechanisms have mainly focused on arsenate (As(V)) reduction, arsenite (As(III)) oxidation, and arsenic methylation； little is known, however, about the pathway for the biosynthesis of arsenosugars, which are significant arsenic transformation products. Here, the involvement of As(III) S-Adenosylmethionine methyltransferase (ArsM) in arsenosugar synthesis is demonstrated for the first time. Synechocystis sp. PCC 6803 incubated with As(III) or monomethylarsonic acid ¢MMA¢V)) produced dimethylarsinic add (DMA(V)) and arsenosugars, as detennined by high performance Iiquid chromatography-inductively coupled plasma, mass spectrometry (HPLC/ICPMS). Arsenosugars were also detected in the cells when they were exposed to DMA(V). A mutant strain Synechocystis ΔarsM was constructed by disrupting arsM in Synechocystis sp. PCC 6803. Methhylation of arsenic species was not observed in the mutant strain after exposure to arsenite or MMA(V)； when Synechocystis ΔarsM was incubated with DMA(V), arsenosugars were detected in the cells. These results surest that ArsM is a required enzyme for die methylation of inorganic arsenicals, but not required for the synthesis of arsenosugars from DMA, and that DMA is the precursor of arsenosugar biosynthesis. The findings will stimulate more studies on the biosyntiesis of complex organoarsenicals, and lead to a better understanding of the bioavailability and function of the organoarsenicals in biological systems.