Supplemental Material can be found at: http://jn.nutrition.org/content/suppl/2014/04/10/jn.113.18878 9.DCSupplemental.html
The Journal of Nutrition Nutritional Epidemiology
Folate and Cobalamin Modify Associations between S-adenosylmethionine and Methylated Arsenic Metabolites in Arsenic-Exposed Bangladeshi Adults1–3 Caitlin G. Howe,4 Megan M. Niedzwiecki,4 Megan N. Hall,5 Xinhua Liu,6 Vesna Ilievski,4 Vesna Slavkovich,4 Shafiul Alam,7 Abu B. Siddique,7 Joseph H. Graziano,4 and Mary V. Gamble4* Departments of 4Environmental Health Sciences, 5Epidemiology, and 6Biostatistics, Mailman School of Public Health, Columbia University, New York, NY; and 7Columbia University Arsenic Project in Bangladesh, Dhaka, Bangladesh
Chronic exposure to inorganic arsenic (InAs) through drinking water is a major problem worldwide. InAs undergoes hepatic methylation to form mono- and dimethyl arsenical species (MMA and DMA, respectively), facilitating arsenic elimination. Both reactions are catalyzed by arsenic (13 oxidation state) methyltransferase (AS3MT) using S-adenosylmethionine (SAM) as the methyl donor, yielding the methylated product and S-adenosylhomocysteine (SAH), a potent product-inhibitor of AS3MT. SAM biosynthesis depends on folate- and cobalamin-dependent one-carbon metabolism. With the use of samples from 353 participants in the Folate and Oxidative Stress Study, our objective was to test the hypotheses that blood SAM and SAH concentrations are associated with arsenic methylation and that these associations differ by folate and cobalamin nutritional status. Blood SAM and SAH were measured by HPLC. Arsenic metabolites in blood and urine were measured by HPLC coupled to dynamic reaction cell inductively coupled plasma MS. In linear regression analyses, SAH was not associated with any of the arsenic metabolites. However, log(SAM) was negatively associated with log(% urinary InAs) (b: 20.11; 95% CI: 20.19, 20.02; P = 0.01), and folate and cobalamin nutritional status significantly modified associations between SAM and percentage of blood MMA (%bMMA) and percentage of blood DMA (%bDMA) (P = 0.02 and P = 0.01, respectively). In folate- and cobalamin-deficient individuals, log(SAM) was positively associated with %bMMA (b: 6.96; 95% CI: 1.86, 12.05; P < 0.01) and negatively associated with %bDMA (b: 26.19; 95% CI: 212.71, 0.32; P = 0.06). These findings suggest that when exposure to InAs is high, and methyl groups are limiting, SAM is used primarily for MMA synthesis rather than for DMA synthesis, contributing additional evidence that nutritional status may explain some of the interindividual differences in arsenic metabolism and, consequently, susceptibility to arsenic toxicity. J. Nutr. 144: 690–697, 2014.
Introduction Worldwide, ;140 million people are exposed to arsenic at concentrations that exceed the safe drinking water guideline set by the WHO (10 mg/L) (1–3), and >57 million of those exposed live in Bangladesh (4). Exposure to arsenic is associated with cancers of the skin, lung, bladder, liver, and kidney (5–8), in addition to noncancer outcomes including peripheral vascular
disease (9), atherosclerosis (10), hypertension (11), peripheral neuropathy (12), and decreased intellectual function in children (13). However, individuals vary in their susceptibility to arsenicinduced health outcomes, and some of this interindividual variation may be explained by differences in arsenic metabolism (14). In contaminated drinking water, arsenic is present as inorganic arsenic (InAs)8. Ingested InAs can be methylated to
1
Supported by grants RO1 CA133595, RO1ES017875, P42 ES10349, P30 ES09089, and 5-T32-CA 09529-25 from the NIH. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institute of Environmental Health Sciences, the National Cancer Institute, or the NIH. 2 Author disclosures: C. G. Howe, M. M. Niedzwiecki, M. N. Hall, X. Liu, V. Ilievski, V. Slavkovich, S. Alam, A. B. Siddique, J. H. Graziano, and M. V. Gamble, no conflicts of interest. 3 Supplemental Tables 1–4 and Supplemental Figure 1 are available from the "Online Supporting Material" link in the online posting of the article and from the same link in the online table of contents at http://jn.nutrition.org. * To whom correspondence should be addressed. E-mail: [email protected].
690
8 Abbreviations used: AsIII, arsenite; AsV, arsenate; AS3MT, arsenic (13 oxidation state) methyltransferase; bAs, blood arsenic; bDMA, dimethyl arsenical species in blood; bInAs, inorganic arsenical species in blood; bMMA, monomethyl arsenical species in blood; DMA, dimethyl arsenical species; DMAIII, dimethylarsinous acid; DMAV, dimethylarsinic acid; FOX, Folate and Oxidative Stress; InAs, inorganic arsenical species; MMA, monomethyl arsenical species; MMAIII, monomethylarsonous acid; MMAV, monomethylarsonic acid; SAH, S-adenosylhomocysteine; SAM, S-adenosylmethionine; uAs, urinary arsenic; uCr, urinary creatinine; uDMA, dimethyl arsenical species in urine; uInAs, inorganic arsenical species in urine; uMMA, monomethyl arsenical species in urine; wAs, water arsenic; 5-methylTHF, 5-methyltetrahydrofolate.
ã 2014 American Society for Nutrition. Manuscript received November 22, 2013. Initial review completed December 24, 2013. Revision accepted February 12, 2014. First published online March 5, 2014; doi:10.3945/jn.113.188789.
Downloaded from jn.nutrition.org at GLASGOW UNIVERSITY on June 30, 2015
Abstract
SAM, like plasma folate, is associated with increased arsenic methylation, whereas blood SAH is associated with decreased arsenic methylation. We hypothesized that SAM would be negatively associated with the percentage of InAs (%InAs) for all participants. However, we predicted that the relations between SAM and the methylated metabolites (%MMA and %DMA) would differ between individuals who were sufficient or deficient for folate and cobalamin, because the relation between SAM and the methylated arsenic metabolites may depend on whether or not SAM is limiting.
Participants and Methods Study region. Our study site, which is the site of the Health Effects of Arsenic Longitudinal Study (HEALS) cohort (24), is currently a 35-km2 area within Araihazar, Bangladesh, which is situated ;30 km east of Dhaka. Participants. The Folate and Oxidative Stress (FOX) Study is a crosssectional study in 378 participants selected from 5 water arsenic (wAs)– exposure categories [300 mg/L (n = 45) (25)] who were recruited between February and July of 2008. This study had 2 major aims: 1) to study the dose-response relation between wAs exposure and oxidative stress (25,26) and 2) to study the hypotheses outlined herein. Participants between the ages of 30 and 65 y were eligible. The following individuals were excluded: 1) women who were pregnant, 2) participants taking nutritional supplements, and 3) participants with known diabetes, cardiovascular or renal disease, or other diseases known to be associated with oxidative stress. Bangladeshi field staff physicians obtained informed consent after reading an approved consent form to study participants. This study was approved by both the Bangladesh Medical Research Council and the Institutional Review Board of Columbia University Medical Center. General characteristics of study participants. General characteristics of the study participants (Table 1) were obtained by questionnaire. BMI was calculated by using the measured height and weight of each participant. Dietary intakes of folate and cobalamin were determined by FFQ (27). Sample collection and handling. During each participantÕs visit to our field clinic, a physician collected a venous blood sample. Spot urine samples were collected in 50-mL acid-washed polypropylene tubes and frozen at –20°C. After blood samples underwent initial processing in the field clinic, aliquots of blood and plasma were immediately frozen at 280°C. Samples were then transported on dry ice to Dhaka by car where they were again stored in 280°C (blood and plasma) or 220°C (urine) freezers. In Dhaka, samples were packed on dry ice and flown to Columbia University.
FIGURE 1 AsV is reduced to AsIII in a reaction thought to be dependent on GSH or other endogenous reductants. AsIII then undergoes oxidative methylation, catalyzed by AS3MT, and, with SAM as the methyl donor, forms MMAV and SAH. MMAV is reduced to MMAIII and can be methylated in a second oxidative methylation step, which is also catalyzed by AS3MT and requires SAM as the methyl donor to produce DMAV and SAH. AsIII, arsenite; AsV, arsenate; AS3MT, arsenic (13 oxidation state) methyltransferase; DMAV, dimethylarsinic acid; GSH, glutathione; GSSG, oxidized glutathione; MMAIII, monomethylarsonous acid; MMAV, monomethylarsonic acid; SAH, S-adenosylhomocysteine; SAM, S-adenosylmethionine; TRX, thioredoxin.
wAs. Field sample collection and laboratory analysis procedures are described elsewhere in detail (28,29). Water samples were analyzed by high-resolution inductively coupled plasma MS after 1:10 dilution and addition of Ge to correct fluctuations in instrument sensitivity. The detection limit of the method is typically
The Journal of Nutrition Nutritional Epidemiology
Folate and Cobalamin Modify Associations between S-adenosylmethionine and Methylated Arsenic Metabolites in Arsenic-Exposed Bangladeshi Adults1–3 Caitlin G. Howe,4 Megan M. Niedzwiecki,4 Megan N. Hall,5 Xinhua Liu,6 Vesna Ilievski,4 Vesna Slavkovich,4 Shafiul Alam,7 Abu B. Siddique,7 Joseph H. Graziano,4 and Mary V. Gamble4* Departments of 4Environmental Health Sciences, 5Epidemiology, and 6Biostatistics, Mailman School of Public Health, Columbia University, New York, NY; and 7Columbia University Arsenic Project in Bangladesh, Dhaka, Bangladesh
Chronic exposure to inorganic arsenic (InAs) through drinking water is a major problem worldwide. InAs undergoes hepatic methylation to form mono- and dimethyl arsenical species (MMA and DMA, respectively), facilitating arsenic elimination. Both reactions are catalyzed by arsenic (13 oxidation state) methyltransferase (AS3MT) using S-adenosylmethionine (SAM) as the methyl donor, yielding the methylated product and S-adenosylhomocysteine (SAH), a potent product-inhibitor of AS3MT. SAM biosynthesis depends on folate- and cobalamin-dependent one-carbon metabolism. With the use of samples from 353 participants in the Folate and Oxidative Stress Study, our objective was to test the hypotheses that blood SAM and SAH concentrations are associated with arsenic methylation and that these associations differ by folate and cobalamin nutritional status. Blood SAM and SAH were measured by HPLC. Arsenic metabolites in blood and urine were measured by HPLC coupled to dynamic reaction cell inductively coupled plasma MS. In linear regression analyses, SAH was not associated with any of the arsenic metabolites. However, log(SAM) was negatively associated with log(% urinary InAs) (b: 20.11; 95% CI: 20.19, 20.02; P = 0.01), and folate and cobalamin nutritional status significantly modified associations between SAM and percentage of blood MMA (%bMMA) and percentage of blood DMA (%bDMA) (P = 0.02 and P = 0.01, respectively). In folate- and cobalamin-deficient individuals, log(SAM) was positively associated with %bMMA (b: 6.96; 95% CI: 1.86, 12.05; P < 0.01) and negatively associated with %bDMA (b: 26.19; 95% CI: 212.71, 0.32; P = 0.06). These findings suggest that when exposure to InAs is high, and methyl groups are limiting, SAM is used primarily for MMA synthesis rather than for DMA synthesis, contributing additional evidence that nutritional status may explain some of the interindividual differences in arsenic metabolism and, consequently, susceptibility to arsenic toxicity. J. Nutr. 144: 690–697, 2014.
Introduction Worldwide, ;140 million people are exposed to arsenic at concentrations that exceed the safe drinking water guideline set by the WHO (10 mg/L) (1–3), and >57 million of those exposed live in Bangladesh (4). Exposure to arsenic is associated with cancers of the skin, lung, bladder, liver, and kidney (5–8), in addition to noncancer outcomes including peripheral vascular
disease (9), atherosclerosis (10), hypertension (11), peripheral neuropathy (12), and decreased intellectual function in children (13). However, individuals vary in their susceptibility to arsenicinduced health outcomes, and some of this interindividual variation may be explained by differences in arsenic metabolism (14). In contaminated drinking water, arsenic is present as inorganic arsenic (InAs)8. Ingested InAs can be methylated to
1
Supported by grants RO1 CA133595, RO1ES017875, P42 ES10349, P30 ES09089, and 5-T32-CA 09529-25 from the NIH. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institute of Environmental Health Sciences, the National Cancer Institute, or the NIH. 2 Author disclosures: C. G. Howe, M. M. Niedzwiecki, M. N. Hall, X. Liu, V. Ilievski, V. Slavkovich, S. Alam, A. B. Siddique, J. H. Graziano, and M. V. Gamble, no conflicts of interest. 3 Supplemental Tables 1–4 and Supplemental Figure 1 are available from the "Online Supporting Material" link in the online posting of the article and from the same link in the online table of contents at http://jn.nutrition.org. * To whom correspondence should be addressed. E-mail: [email protected].
690
8 Abbreviations used: AsIII, arsenite; AsV, arsenate; AS3MT, arsenic (13 oxidation state) methyltransferase; bAs, blood arsenic; bDMA, dimethyl arsenical species in blood; bInAs, inorganic arsenical species in blood; bMMA, monomethyl arsenical species in blood; DMA, dimethyl arsenical species; DMAIII, dimethylarsinous acid; DMAV, dimethylarsinic acid; FOX, Folate and Oxidative Stress; InAs, inorganic arsenical species; MMA, monomethyl arsenical species; MMAIII, monomethylarsonous acid; MMAV, monomethylarsonic acid; SAH, S-adenosylhomocysteine; SAM, S-adenosylmethionine; uAs, urinary arsenic; uCr, urinary creatinine; uDMA, dimethyl arsenical species in urine; uInAs, inorganic arsenical species in urine; uMMA, monomethyl arsenical species in urine; wAs, water arsenic; 5-methylTHF, 5-methyltetrahydrofolate.
ã 2014 American Society for Nutrition. Manuscript received November 22, 2013. Initial review completed December 24, 2013. Revision accepted February 12, 2014. First published online March 5, 2014; doi:10.3945/jn.113.188789.
Downloaded from jn.nutrition.org at GLASGOW UNIVERSITY on June 30, 2015
Abstract
SAM, like plasma folate, is associated with increased arsenic methylation, whereas blood SAH is associated with decreased arsenic methylation. We hypothesized that SAM would be negatively associated with the percentage of InAs (%InAs) for all participants. However, we predicted that the relations between SAM and the methylated metabolites (%MMA and %DMA) would differ between individuals who were sufficient or deficient for folate and cobalamin, because the relation between SAM and the methylated arsenic metabolites may depend on whether or not SAM is limiting.
Participants and Methods Study region. Our study site, which is the site of the Health Effects of Arsenic Longitudinal Study (HEALS) cohort (24), is currently a 35-km2 area within Araihazar, Bangladesh, which is situated ;30 km east of Dhaka. Participants. The Folate and Oxidative Stress (FOX) Study is a crosssectional study in 378 participants selected from 5 water arsenic (wAs)– exposure categories [300 mg/L (n = 45) (25)] who were recruited between February and July of 2008. This study had 2 major aims: 1) to study the dose-response relation between wAs exposure and oxidative stress (25,26) and 2) to study the hypotheses outlined herein. Participants between the ages of 30 and 65 y were eligible. The following individuals were excluded: 1) women who were pregnant, 2) participants taking nutritional supplements, and 3) participants with known diabetes, cardiovascular or renal disease, or other diseases known to be associated with oxidative stress. Bangladeshi field staff physicians obtained informed consent after reading an approved consent form to study participants. This study was approved by both the Bangladesh Medical Research Council and the Institutional Review Board of Columbia University Medical Center. General characteristics of study participants. General characteristics of the study participants (Table 1) were obtained by questionnaire. BMI was calculated by using the measured height and weight of each participant. Dietary intakes of folate and cobalamin were determined by FFQ (27). Sample collection and handling. During each participantÕs visit to our field clinic, a physician collected a venous blood sample. Spot urine samples were collected in 50-mL acid-washed polypropylene tubes and frozen at –20°C. After blood samples underwent initial processing in the field clinic, aliquots of blood and plasma were immediately frozen at 280°C. Samples were then transported on dry ice to Dhaka by car where they were again stored in 280°C (blood and plasma) or 220°C (urine) freezers. In Dhaka, samples were packed on dry ice and flown to Columbia University.
FIGURE 1 AsV is reduced to AsIII in a reaction thought to be dependent on GSH or other endogenous reductants. AsIII then undergoes oxidative methylation, catalyzed by AS3MT, and, with SAM as the methyl donor, forms MMAV and SAH. MMAV is reduced to MMAIII and can be methylated in a second oxidative methylation step, which is also catalyzed by AS3MT and requires SAM as the methyl donor to produce DMAV and SAH. AsIII, arsenite; AsV, arsenate; AS3MT, arsenic (13 oxidation state) methyltransferase; DMAV, dimethylarsinic acid; GSH, glutathione; GSSG, oxidized glutathione; MMAIII, monomethylarsonous acid; MMAV, monomethylarsonic acid; SAH, S-adenosylhomocysteine; SAM, S-adenosylmethionine; TRX, thioredoxin.
wAs. Field sample collection and laboratory analysis procedures are described elsewhere in detail (28,29). Water samples were analyzed by high-resolution inductively coupled plasma MS after 1:10 dilution and addition of Ge to correct fluctuations in instrument sensitivity. The detection limit of the method is typically
