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      Differences of Urinary Arsenic Metabolites and Methylation Capacity between Individuals with and without Skin Lesions in Inner Mongolia, Northern China

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          Abstract

          Incomplete arsenic (As) methylation has been considered a risk factor of As-related diseases. This study aimed to examine the difference of urinary As metabolites and the methylation capacity between subjects with and without skin lesions. Urinary inorganic arsenic (iAs), monomethylarsonic acid (MMA), and dimethylarsinic acid (DMA) were analyzed. The percentage of each As species (iAs%, MMA%, and DMA%), the primary methylation index (PMI) and secondary methylation index (SMI) were calculated. The results showed that subjects with skin lesions have higher levels of urinary iAs (99.08 vs. 70.63 μg/g Cr, p = 0.006) and MMA (69.34 vs. 42.85 μg/g Cr, p = 0.016) than subjects without skin lesions after adjustment for several confounders. Significant differences of urianry MMA% (15.49 vs. 12.11, p = 0.036) and SMI (0.74 vs. 0.81, p = 0.025) were found between the two groups. The findings of the present study suggest that subjects with skin lesions may have a lower As methylation capacity than subjects without skin lesions.

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          Mechanisms of arsenic biotransformation.

          Marie Vahter (2002)
          Inorganic arsenic, a documented human carcinogen, is methylated in the body by alternating reduction of pentavalent arsenic to trivalent and addition of a methyl group from S-adenosylmethionine. Glutathione, and possibly other thiols, serve as reducing agents. The liver is the most important site of arsenic methylation, but most organs show arsenic methylating activity. The end metabolites are methylarsonic acid (MMA) and dimethylarsinic acid (DMA). These are less reactive with tissue constituents than inorganic arsenic and readily excreted in the urine. However, reactive intermediates may be formed. Absorbed arsenate (As(V)) is fairly rapidly reduced in blood to As(III), which implies increased toxicity. Also, intermediate reduced forms of the methylated metabolites, MMA(III) and DMA(III), have been detected in human urine. In particular MMA(III) is highly toxic. To what extent MMA(III) and DMA(III) contribute to the observed toxicity following exposure to inorganic arsenic remains to be elucidated. There are marked differences in the metabolism of arsenic between mammalian species, population groups and individuals. There are indications that subjects with low MMA in urine have faster elimination of ingested arsenic, compared to those with more MMA in urine.
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            Monomethylarsonous acid (MMA(III)) is more toxic than arsenite in Chang human hepatocytes.

            J Petrick, Bruce D. Carter, F Fierro … (2000)
            Methylation has been considered to be the primary detoxication pathway of inorganic arsenic. Inorganic arsenic is methylated by many, but not all animal species, to monomethylarsonic acid (MMA(V)), monomethylarsonous acid (MMA(III)), and dimethylarsinic acid (DMA(V)). The As(V) derivatives have been assumed to produce low toxicity, but the relative toxicity of MMA(III) remains unknown. In vitro toxicities of arsenate, arsenite, MMA(V), MMA(III), and DMA(V) were determined in Chang human hepatocytes. Leakage of lactate dehydrogenase (LDH) and intracellular potassium (K(+)) and mitochondrial metabolism of the tetrazolium salt XTT were used to assess cytotoxicity due to arsenic exposure. The mean LC50 based on LDH assays in phosphate media was 6 microM for MMA(III) and 68 microM for arsenite. Using the assay for K(+) leakage in phosphate media, the mean LC50 was 6.3 microM for MMA(III) and 19.8 microM for arsenite. The mean LC50 based on the XTT assay in phosphate media was 13.6 microM for MMA(III) and 164 microM for arsenite. The results of the three cytotoxicity assays (LDH, K(+), and XTT) reveal the following order of toxicity in Chang human hepatocytes: MMA(III) > arsenite > arsenate > MMA(V) = DMA(V). Data demonstrate that MMA(III), an intermediate in inorganic arsenic methylation, is highly toxic and again raises the question as to whether methylation of inorganic arsenic is a detoxication process. Copyright 2000 Academic Press.
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              A new metabolic pathway of arsenite: arsenic-glutathione complexes are substrates for human arsenic methyltransferase Cyt19.

              Toru Hayakawa, Seishiro Hirano, Yayoi Kobayashi … (2005)
              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 (iAsIII) and arsenate (iAsV) are monomethylarsonic acid (MMA(V)) and dimethylarsinic acid (DMA(V)). On the other hand, in rat bile, the major metabolites of iAsIII have been reported to be arsenic-glutathione (As-GSH) complexes. In the present study we investigate whether these As-GSH 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 iAsIII when GSH was present at concentrations 2 mM or higher. Human recombinant Cyt19 catalyzed transfer of a methyl group from S-adenosyl-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 iAsIII to methylated arsenicals by Cyt19 was via ATG and MADG rather than by oxidative methylation of iAsIII and MMA(III).
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                Author and article information

                Journal
                Journal ID (nlm-ta): Int J Environ Res Public Health
                Journal ID (iso-abbrev): Int J Environ Res Public Health
                Journal ID (publisher-id): ijerph
                Title: International Journal of Environmental Research and Public Health
                Publisher: MDPI
                ISSN (Print): 1661-7827
                ISSN (Electronic): 1660-4601
                Publication date (Electronic): 18 July 2014
                Publication date (Print): July 2014
                Volume: 11
                Issue: 7
                Pages: 7319-7332
                Affiliations
                [1 ]Department of Occupational and Environmental Health, School of Public Health, Tianjin Medical University, No. 22, Qi Xiang Tai Road, Heping District, Tianjin 300070, China; E-Mail: qiangzhang@ 123456tijmu.edu.cn
                [2 ]Environment and Non-Communicable Disease Research Center, School of Public Health, China Medical University, No. 92, Bei Er Road, Heping District, Shenyang 110001, China; E-Mails: liyongfang_17@ 123456163.com (Y.L.); 472594283@ 123456163.com (D.W.); qmzheng@ 123456mail.cmu.edu.cn (Q.Z.)
                [3 ]Library of Tianjin Medical University, Tianjin Medical University, No. 22, Qi Xiang Tai Road, Heping District, Tianjin 300070, China; E-Mail: libliujuan@ 123456tijmu.edu.cn
                Author notes
                [* ]Author to whom correspondence should be addressed; E-Mail: sungf@ 123456mail.cmu.edu.cn ; Tel.: +86-24-326-1744; Fax: +86-24-2326-1744.
                Article
                Publisher ID: ijerph-11-07319
                DOI: 10.3390/ijerph110707319
                PMC ID: 4113878
                PubMed ID: 25046631
                SO-VID: e0d1845f-4c35-43e9-bf5a-0c6440f8d6dd
                Copyright © © 2014 by the authors; licensee MDPI, Basel, Switzerland.
                License:

                This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution license ( http://creativecommons.org/licenses/by/3.0/).

                History
                Date received : 28 April 2014
                Date revision received : 25 June 2014
                Date accepted : 09 July 2014
                Categories
                Subject: Article

                ScienceOpen disciplines: Public health
                Keywords: arsenic,groundwater,skin lesion,methylation capacity,metabolism
                Data availability:
                ScienceOpen disciplines: Public health
                Keywords: arsenic, groundwater, skin lesion, methylation capacity, metabolism

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