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Widespread Occurrence of the Highly Toxic Dimethylated Monothioarsenate in Rice


Arsenic is ubiquitous in soils globally and occurring at elevated concentrations in irrigation water in south and south-east Asia, threatening the production of food crops. Paddy rice accumulates more arsenic in the grain than any other cereal because arsenite is mobilized from soil minerals under anaerobic conditions in flooded paddy fields. Since inorganic arsenic is classified as a carcinogen, arsenic accumulation in rice is a global concern for human health resulting in regulation for maximal tolerable concentrations for inorganic arsenic. Besides inorganic arsenic iAs, dimethylated arsenate (DMA) is often found in rice, which however because of much reduced toxicity is exempt from food safety considerations. In general, arsenic toxicity is depending on the species being present, with inorganic arsenic considered to be the most toxic.

Anyhow during the last 15 years, some organic arsenic species have been recognized to show toxicity that should not be ignored. Dimethylated monothioarsenate (DMMTA) is such a highly toxic species that has been found in pore water from paddy fields at considerable concentrations.  

Figure: Thio-arsenic species found in pore water from paddy soils

The new study:
An international group of researchers now have investigated the occurrence of DMMTA in rice grains from different locations worldwide.  In order to clarify whether DMMTA is present in rice grains globally or from some regions, the researchers analyzed more than 100 rice samples from the main rice-producing regions of China as well as 140 rice samples from 16 countries across six continents.  

Total arsenic in rice was determined by inductively coupled plasma-mass spectrometry (ICP-MS) after microwave-assisted digestion.  For speciation analysis, the researchers compared two different sample pretreatment methods using either 1 % HNO3 extraction or enzymatic extraction.  For the chromatographic separation of arsenic species two different methods were compared, using either an anion exchange column (AEC) or a reversed-phase column (RPC). Arsenic species were identified by retention time match with standard compounds and quantified by ICP-MS via external calibration, leading to limits of detection between 0.2 and 0.4 µg/kg for the different species.

The comparison of the two separation methods revealed that only the RPC showed the presence of DMMTA in rice. The authors also excluded that DMMTA was produced from other species during enzymatic extraction. By further investigating the reasons for such differences, it was established that DMMTA is converted to DMA during nitric acid extraction and therefore cannot be found by this often applied method. Even if DMMTA had survived the extraction procedure, DMMTA was strongly retained in the AEC column and therefore cannot be detected under the near-neutral pH elution conditions.

The analysis of more than 100 rice samples from China showed the presence of DMMTA in all samples, with concentrations between 2.10 and 34.8 µg/kg representing 2-19% of the total As. When looking at the relation between DMMTA concentrations and geographical origin of the rice, a clear pattern was observed with rice produced in temperate regions showing higher concentrations than rice from tropical regions. Such trend was confirmed for rice samples from other countries, showing higher DMMTA concentration in rice samples from Europe and North America than from Asia.  

By statistical analysis of all results, the authors concluded that the DMMTA concentration is about 30% of the DMA concentration in the enzymatic extraction. This relationship allows to estimate the concentration of DMMTA for samples analyzed by acid extraction, during which DMMTA was degraded.

The authors concluded, that because of the high toxicity of DMMTA, its widespread presence in rice from different countries and its stability during cooking, DMMTA likely poses a serious threat to human health that should be considered in food safety regulations.

The original study

Jun Dai, Zhu Tang, A-Xiang Gao, Britta Planer-Friedrich, Peter M. Kopittke, Fang-Jie Zhao, Peng Wang, Widespread Occurrence of the Highly Toxic Dimethylated Monothioarsenate (DMMTA) in Rice Globally, Environ. Sci. Technol., 56 (2022) 3575−3586. DOI: 10.1021/acs.est.1c08394

Related studies (newest first)

J. Dai, C. Chen, A.X. Gao, Z. Tang, P. Wang, Dynamics of dimethylated monothioarsenate (DMMTA) in paddy soils and its accumulation in rice grains. Environ. Sci. Technol., 55 (2021) 8665−8674. DOI: 10.1021/acs.est.1c00133

A.E.C. Blanco, C.F. Kerl, B. Planer-Friedrich, Detection of thioarsenates in rice grains and rice products. J. Agric. Food Chem.. 69 (2021) 2287−2294. DOI: 10.1021/acs.jafc.0c06853

C. Chen, B. Yang, Y. Shen, J. Dai, Z. Tang, P. Wang, F.-J. Zhao, Sulfate addition and rising temperature promote arsenic methylation and the formation of methylated thioarsenates in paddy soils. Soil Biol. Biochem., 154 (2021) No. 108129. DOI: 10.1016/j.soilbio.2021.108129

J.J. Wang, D. Halder, L. Wegner, L. Bruggenwirth, J. Schaller, M. Martin, D. Said-Pullicino, M. Romani, B. Planer-Friedrich, Redox dependence of thioarsenate occurrence in paddy soils and the rice rhizosphere. Environ. Sci. Technol., 54 (2020) 3940−3950. DOI: 10.1021/acs.est.9b05639

J.J. Wang, C.F. Kerl, P.J. Hu, M. Martin, T.T. Mu, L. Bruggenwirth, G.M. Wu, D. Said-Pullicino, M. Romani, L. Wu, B. Planer-Friedrich, Thiolated arsenic species observed in ricepaddy pore waters. Nat. Geosci., 13 (2020) 282−287. DOI: 10.1038/s41561-020-0533-1

C.F. Kerl, R.A. Schindele, L. Bruggenwirth, A.E.C. Blanco, C. Rafferty, S. Clemens, B. Planer-Friedrich, Methylated thioarsenates and monothioarsenate differ in uptake, transformation, and contribution to total arsenic translocation in rice plants. Environ. Sci. Technol., 53 (2019) 5787−5796. DOI: 10.1021/acs.est.9b00592

B. Moe, H. Peng, X. Lu, B. Chen, L.W.L. Chen, S. Gabos, X.F. Li, X.C. Le, Comparative cytotoxicity of fourteen trivalent and pentavalent arsenic species determined using real-time cell sensing. J. Environ. Sci., 49 (2016) 113−124. DOI: 10.1016/j.jes.2016.10.004

W.R. Cullen, Q.Q. Liu, X.F. Lu, A. McKnight-Whitford, H.Y. Peng, A. Popowich, X.W. Yan, Q. Zhang, M. Fricke, H.S. Sun, X.C. Le, Methylated and thiolated arsenic species for environmental and health research - A review on synthesis and characterization. J. Environ. Sci., 49 (2016) 7−27. DOI: 10.1016/j.jes.2016.11.004

S. Hinrichsen, F. Geist, B. Planer-Friedrich, Inorganic and methylated thioarsenates pass the gastrointestinal barrier. Chem. Res. Toxicol., 28 (2015) 1678−1680. DOI: 10.1021/acs.chemrestox.5b00268

F. Ebert, L. Leffers, T. Weber, S. Berndt, A. Mangerich, S. Beneke, A. Bürkle, T. Schwerdtle, Toxicological properties of the thiolated inorganic arsenic and arsenosugar metabolite thiodimethylarsinic acid in human bladder cells. J. Trace Elem. Med. Biol., 28 (2014) 138−146. DOI: 10.1016/j.jtemb.2013.06.004

L. Leffers, M. Unterberg, M. Bartel, C. Hoppe, I. Pieper, J. Stertmann, F. Ebert, H.U. Humpf, T. Schwerdtle, In vitro toxicological characterisation of the S-containing arsenic metabolites thio-dimethylarsinic acid and dimethylarsinic glutathione. Toxicology, 305 (2013) 109−119. DOI: 10.1016/j.tox.2013.01.007

H. Naranmandura, M.W. Carew, S. Xu, J. Lee, E.M. Leslie, M. Weinfeld, X.C. Le, Comparative toxicity of arsenic metabolites in human bladder cancer EJ-1 cells. Chem. Res. Toxicol. 24 (2011) 1586− 1596. DOI: 10.1021/tx200291p

M. Bartel, F. Ebert, L. Leffers, U. Karst, T. Schwerdtle, Toxicological characterization of the inorganic and organic arsenic metabolite thio-DMAV in cultured human lung cells. J. Toxicol., 2011 (2011) 373141. DOI: 10.1155/2011/373141.

H. Naranmandura, Y. Ogra, K. Iwata, J. Lee, K.T. Suzuki, M. Weinfeld, X.C. Le, Evidence for toxicity differences between inorganic arsenite and thioarsenicals in human bladder cancer cells. Toxicol. Appl. Pharmacol., 238 (2009) 133−140. DOI: 10.1016/j.taap.2009.05.006

R. Raml, A. Rumpler, W. Goessler, M. Vahter, L. Li, T. Ochi, K.A. Francesconi, Thio-dimethylarsinate is a common metabolite in urine samples from arsenic-exposed women in Bangladesh. Toxicol. Appl. Pharmacol., 222 (2007) 374−80. DOI: 10.1016/j.taap.2006.12.014

R. Raml, W. Goessler, P. Traar, T. Ochi, K.A. Francesconi, Novel thioarsenic metabolites in human urine after ingestion of an arsenosugar, 2’,3’-dihydroxypropyl 5-deoxy-5-dimethylarsinoyl-beta-D-riboside. Chem. Res. Toxicol., 18 (2005) 1444−1450. DOI: 10.1021/tx50111h

H.R. Hansen, A. Raab, M. Jaspars, B.F. Milne, J. Feldmann, Sulfur-containing arsenical mistaken for dimethylarsinous acid [DMA(III)] and identified as a natural metabolite in urine: major implications for studies on arsenic metabolism and toxicity. Chem. Res. Toxicol., 17 (2004) 1086−1091. DOI: 10.1021/tx049978q

Related EVISA News (Newest first)

November 14, 2013: Arsenic Speciation in Rice Cereals for Infants
May 15, 2013: Arsenic species in rice: Origin, uptake and geographical variation
February 15, 2013: JRC-IRMM has released ERM-BC211 certified rice reference material for arsenic speciation analysis
September 21, 2012: Arsenic in Rice : First results from the U.S. Food and Drug Administration
January 4, 2011: Arsenic species in rice: Call for analytical laboratories
May 19, 2010: China: Inorganic Arsenic in Rice - An Underestimated Health Threat ?

last time modified: May 12, 2022

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