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Investigation of the Biotransformation of a DMA in Mouse Cecum Samples Using IC-ICP-MS and LC-ESI-MS/MS Detection
Citation:
KUBACHKA, K., S. CONKLIN, JOHN T. CREED, AND D. J. THOMAS. Investigation of the Biotransformation of a DMA in Mouse Cecum Samples Using IC-ICP-MS and LC-ESI-MS/MS Detection. Presented at 2008 Winter Conference on Plasma Spectrochemistry, Tennecula, CO, January 06 - 12, 2008.
Impact/Purpose:
Develop analytical approaches to quantify reactive intermediates within the metabolic pathway that biotransforms inorganic arsenic to DMA(V). The ability to quantify these species will enhance the mode of action (within NHEERL) and the bioavailability / bioaccessibility research (within NERL).
Description:
Recent arsenic metabolism studies have begun to indicate the presence of sulfur analogs of the more common arsenic oxides in biological systems. An emerging area of research is how and where these arsenic species are formed in the metabolic pathway. The authors have previously indicated that the microflora in the cecum of a mouse is able to biotransform an arsenosugar oxide (a tri-alkyl substituted arsenic oxide) to its corresponding sulfide [1]. The initial mechanism of the aforementioned conversion theorized that the bacteria produced H2S as a waste product which then facilitated the conversion. Subsequent research using H2S in an ideal solution indicated that the rate of conversion was influenced by the degree of substitution of the arsenic oxide. For instance, the reaction rate for tetramethylarsine oxide (TMAO, tri-alkyl substituted) was faster than dimethylarsinic acid (DMA, di-alkyl substituted) which was faster than monomethylarsinic acid (MMA, mono-alkyl substituted) [2]. The conversion of DMA to dimethylthioarsinic acid (DMTA) in these ideal solutions raised the question if a similar conversion would be observed in the anaerobically incubated cecal samples from a mouse. In this experiment, DMA was added at various concentrations to the cecum contents of a mouse, which were then incubated at 37 °C. The DMA spiked cecum samples were incubated for various periods and then flash frozen at -80 °C to minimize additional unwanted activity and subsequent transformation. An outline of the study design is given below: Cecum Samples (cecum + buffer) Time (Hours at 37 °C) Arsenic (ng/g) 0 1 6 18 24 0 X X X 20 X X X X X 200 X X X X X 1000 X X Controls (cecum, no buffer) Time (Hours) Arsenic (ng/g) 0 24 0 X X 20 X X 200 X X 1000 X X The fortified samples, blanks and controls were analyzed for arsenic metabolites via speciation with IC-ICP-MS, utilizing arsenic specific detection at m/z 75. Molecular characterization was also conducted using LC-ESI-MS/MS. The metabolites detected were: DMA, DMTA, dimethyldithioarsinic acid (DMDTA), and trimethylarsine sulfide (TMAS). Both elemental and molecular data will be presented to support these identifications. The presence of TMAS was not expected and thorough analysis of controls and blanks lead the authors to believe it is not an artifact of sample contamination. Its presence could possibly be explained by DMA being methylated to TMAO, which could then be sulfurylated to TMAS. Finally, the conversion from DMA to the corresponding sulfur analogs will be presented with respect to incubation times. This data set indicates that sulfur analogs of arsenic oxides may be produced in the gastrointestinal tract. The data set shows evidence of novel biotransformation pathways of DMA in the gastrointestinal tract, and should prove useful in further understanding the arsenic metabolism pathways.