Key Laboratory of Urban Environment and Health, Institute of Urban Environment, Chinese Academy of Sciences , Xiamen 361021, China.
Institute of Chemistry, University of Graz , Graz, Austria.
Environ Sci Technol. 2017 Feb 7;51(3):1224-1230. doi: 10.1021/acs.est.6b04952. Epub 2017 Jan 27.
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 acid (DMA(V)) and arsenosugars, as determined by high performance liquid 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. Methylation 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 suggest that ArsM is a required enzyme for the 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 biosynthesis of complex organoarsenicals, and lead to a better understanding of the bioavailability and function of the organoarsenicals in biological systems.
砷是环境中普遍存在的有毒元素，生物已经进化出不同的砷解毒策略。砷生物转化机制的研究主要集中在砷酸盐（As(V)）还原、亚砷酸盐（As(III)）氧化和砷甲基化上；然而，对于砷糖的生物合成途径，即重要的砷转化产物，知之甚少。本文首次证明了 As(III) S-腺苷甲硫氨酸甲基转移酶（ArsM）参与了砷糖的合成。用 As(III)或一甲基砷酸（MMA(V)）孵育的集胞藻 PCC 6803 用高效液相色谱-电感耦合等离子体质谱法（HPLC/ICPMS）检测到产生了二甲基砷酸（DMA(V)）和砷糖。当细胞暴露于 DMA(V)时，也检测到了砷糖。通过在集胞藻 PCC 6803 中敲除 arsM 构建了突变株 Synechocystis ΔarsM。突变株在暴露于亚砷酸盐或 MMA(V)后，未观察到砷物种的甲基化；当 Synechocystis ΔarsM 用 DMA(V)孵育时，细胞中检测到了砷糖。这些结果表明，ArsM 是无机砷甲基化所必需的酶，但不是从 DMA 合成砷糖所必需的酶，DMA 是砷糖生物合成的前体。这些发现将促进对复杂有机砷化合物生物合成的更多研究，并深入了解生物体系中有机砷的生物利用度和功能。
