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Urinary Arsenic Metabolites

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648VOLUME 115 | NUMBER 4 | April 2007•Environmental Health PerspectivesResearch|Mini-MonographArsenic (As), a ubiquitous element in nature(Cullen and Reimer 1989), has been identifiedas a human carcinogen by the InternationalAgency for Research on Cancer (IARC 1987).Chronic exposure to drinking water containinghigh levels of inorganic arsenic (iAs) is associ-ated with manifestations of various skin dis-eases (Tondel et al. 1999), diabetes (Bates et al.1992; Tseng et al. 2000), cardiovascular dis-ease (Engel et al. 1994), and cancers of severalorgans (Bates et al. 1992; Chen et al. 2005). Recent development of analytical methodsfor As speciation has resulted in a large numberof publications on As metabolism in humans.Until now, two important theories concerningAs biotransformation attracted much attentionof researchers in this field. One theory is thatthe classic metabolic pathway of As in mam-mals consists mainly of two reduction stepsand two methylation steps as follows: iAsV→iAsIII→ monomethylarsonic acid (MMAV) →MMAIII→ dimethylarsinic acid (DMAV)(Aposhian et al. 2004). The other theory isthat the new As pathway is based on the forma-tion of As–glutathione complexes (Hayakawaet al. 2005). These complexes cannot bedetected in human urine because they aredegraded by γ-glutamyltransferase (GGT) inthe microvilli of the proximal tubules in thekidney. The products of the degradation maybe transformed into iAs, MMA, or DMA(Kala et al. 2004). However, both theories sug-gest the two methylation steps are involved inthe metabolism of As. If these theories arecorrect, then methylation is key to understand-ing the biotransformation of As. In addition,trivalent forms of As are more toxic than thepentavalent forms of As, with MMAIIIbeingthe most toxic As metabolite (Bing et al. 2001;Nesnow et al. 2002; Styblo et al. 2000).Cytotoxicity assays revealed the followingorder of toxicity of the arsenicals: MMAIII>iAsIII> iAsV> MMAV= DMAV(Petrick et al.2000). Consequently, the traditional beliefthat biomethylation was a detoxification path-way of iAs has been questioned (Kurttio et al.1998; Styblo et al. 2002). The new under-standing of As metabolism creates interestingchallenges in evaluating health effects of inpopulations exposed to iAs because of largevariations in the factors that affect metabolism,such as nutrition (Gamble et al. 2005), genet-ics (Chung et al. 2002), sex (Shraim et al.2003; Tseng et al. 2005), and age (Chowdhury2003; Tseng et al. 2005). Exposure to As in drinking water at con-centrations as low as 50 µg/L has been shownto adversely affect children’s intellectual func-tion in Bangladesh (Wasserman et al. 2004),but little is known about the potential differ-ences, such as concentrations or distributionof urinary As metabolites in children andadults. Here, we report the results of a cross-sectional study we conducted in InnerMongolia, China, on the concentrations andthe distribution of urinary As metabolites(iAs, MMA, and DMA) in children andadults exposed to 20, 90, or 160 µg/L As ingroundwater used for drinking. In addition,comparing urinary As concentrations and thedistribution of As species, we also apply twoindices, the primary methylation index [PMI =(MMA + DMA)/total As (TAs)] and the sec-ondary methylation index [SMI = DMA/(MMA + DMA)], to evaluate As methylationability. PMI calculated as MMA/iAs and SMIcalculated as DMA/MMA were previouslydeveloped for As methylation ability (Chenet al. 2003a, 2003b; Tseng et al. 2005). Ournew methods for calculating PMI and SMI aremore logical. According to the pathway of Asmetabolism, secondary methylation can onlyproceed on the basis of primary methylation;consequently, parts of the As products result-ing from primary methylation are furthermethylated. Therefore, to evaluate the primarymethylation ability, not only the primary butalso the secondary methylation productsshould be considered. PMI and SMI show dif-ferences in As metabolism between childrenand adults but not between males and females.Finally, we report results of urinary As metabo-lites in 11 pairs of children and their mothersto illustrate the possible role of genetics inAs metabolism. MethodsStudy population. We recruited 233 subjectsfrom three villages—Tianjiaying, Koukenban,and Naimoban near Hohhot, Inner Mongolia,China. The control group with 36 individuals(mean age, 22.6 years) resided in Tianjiayingvillage on the outskirts of Hohhot, where thedeep tube-well water with an iAs concentra-tion of 20 µg/L was provided by centralizedwaterworks. Currently, the drinking waterstandard in China for iAs is 50 µg/LThis article is part of the mini-monograph “Occurrenceand Health Effects of Arsenic in China.”Address correspondence to G. Sun, Department ofEnvironmental and Occupational Health, College ofPublic Health, China Medical University, No. 92 BeiEr Rd., Heping District, Shenyang, 110001, People’sRepublic of China. Telephone: 86 24 2326 1744. Fax:86 24 2326 1744. E-mail: [email protected] thank the people of Inner Mongolia for partici-pating in this study. We also thank Y. Zheng for hercomments during the manuscript preparation. This work was supported by National NaturalScience Foundation of China (NSFC) research grant30530640. The authors declare they have no competingfinancial interests.Received 17 April 2006; accepted 20 September2006.Urinary Arsenic Metabolites in Children and Adults Exposed to Arsenic in Drinking Water in Inner Mongolia, ChinaGuifan Sun, Yuanyuan Xu, Xin Li, Yaping Jin, Bing Li, and Xiance SunDepartment of Environmental and Occupational Health, College of Public Health, China Medical University, Shenyang, Liaoning, People’s Republic of ChinaBACKGROUND: We report the concentrations and distributions of urinary arsenic (As) metabolites in233 residents exposed to 20, 90, or 160 µg/L inorganic arsenic (iAs) in drinking water from three vil-lages in Hohhot, Inner Mongolia, China, that formed one control and two exposed groups. METHODS: We used hydride generation-atomic absorption spectrometry (HGAAS) to determineiAs, monomethylarsonic acid (MMA), and dimethylarsinic acid (DMA). RESULTS: The concentrations of each urinary As species in the two exposed groups were significantlyhigher than in the control group for both children and adults. Both children and adults in exposedgroups had higher percent iAs and MMA and lower percent DMA, and low primary and secondarymethylation indices (PMI and SMI, respectively) than those


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