A new metabolic pathway of arsenite: arsenic-glutathione complexes are substrates for human arsenic methyltransferase Cyt19
A new metabolic pathway of arsenite: arsenic-glutathione complexes are substrates for human arsenic methyltransferase Cyt19
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DOI：
10.1007/s00204-004-0620-x
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发表时间：
2005-04-01
影响因子：
6.1
通讯作者：
Hirano, S
中科院分区：
文献类型：
--
作者：
Hayakawa, T;Kobayashi, Y;Hirano, S
砷的代谢通常被认为是通过重复还原和氧化甲基化进行的;后者是由砷甲基转移酶（Cyt 19）介导的。无机砷如亚砷酸盐（iAs（III））和砷酸盐（iAs（V））在人体尿液中的主要代谢产物是甲基胂酸（MMA（V））和二甲基胂酸（DMA（V））。另一方面，在大鼠胆汁中，iAs（III）的主要代谢产物据报道是砷-谷胱甘肽（As-GSH）复合物。在本研究中，我们调查是否这些AsGSH复合物的砷甲基转移酶的底物，通过使用人类重组Cyt 19。高效液相色谱-电感耦合等离子体质谱分析表明，砷三谷胱甘肽（ATG）产生nonenzymatically从iAs（III）时，GSH是在2 mM或更高的浓度。人重组Cyt 19催化从腺苷-L-甲硫氨酸的甲基转移到砷，并产生单甲基和二甲基砷。只有当ATG存在于反应混合物中时，Cyt 19才能催化砷的甲基化。此外，单甲基胂二谷氨酸（MADG）是Cyt 19进一步甲基化为二甲基胂谷胱甘肽（DMAG）的底物。另一方面，MADG的水解产物甲基亚胂酸（MMA（III））没有被Cyt 19甲基化为二甲基砷。这些结果表明，As-GSH复合物如ATG和MADG被Cyt 19分别转化为MADG和DMAG。当GSH浓度低于1 mM时，MADG和DMAG在溶液中均不稳定，分别被水解和氧化为MMA（V）和DMA（V）。Cyt 19将iAs（III）代谢为甲基化砷是通过ATG和MADG，而不是通过iAs（III）和MMA（III）的氧化甲基化。
The metabolism of arsenic is generally accepted to proceed by repetitive reduction and oxidative methylation; the latter is mediated by arsenic methyltransferase (Cyt19). In human urine, the major metabolites of inorganic arsenicals such as arsenite ( iAs(III)) and arsenate (iAs(V)) are monomethylarsonic acid (MMA(V)) and dimethylarsinic acid (DMA(V)). On the other hand, in rat bile, the major metabolites of iAs(III) have been reported to be arsenic-glutathione (As-GSH) complexes. In the present study we investigate whether these AsGSH complexes are substrates for arsenic methyltransferase by using human recombinant Cyt19. Analyses by high-performance liquid chromatography-inductively coupled plasma mass spectrometry suggested that arsenic triglutathione (ATG) was generated nonenzymatically from iAs(III) when GSH was present at concentrations 2 mM or higher. Human recombinant Cyt19 catalyzed transfer of a methyl group from Sadenosyl-L- methionine to arsenic and produced monomethyl and dimethyl arsenicals. The methylation of arsenic was catalyzed by Cyt19 only when ATG was present in the reaction mixture. Moreover, monomethylarsonic diglutathione (MADG) was a substrate of Cyt19 for further methylation to dimethylarsinic glutathione (DMAG). On the other hand, monomethylarsonous acid (MMA(III)), a hydrolysis product of MADG, was not methylated to dimethyl arsenical by Cyt19. These results suggest that As-GSH complexes such as ATG and MADG were converted by Cyt19 to MADG and DMAG, respectively. Both MADG and DMAG were unstable in solution when the GSH concentration was lower than 1 mM, and were hydrolyzed and oxidized to MMA(V) and DMA(V), respectively. Metabolism of iAs(III) to methylated arsenicals by Cyt19 was via ATG and MADG rather than by oxidative methylation of iAs(III) and MMA(III).