Folate and arsenic metabolism: a double-blind, placebo-controlled folic acid-supplementation trial in Bangladesh

Abstract

Background: Populations in South and East Asia and many other regions of the world are chronically exposed to arsenic-contaminated drinking water. To various degrees, ingested inorganic arsenic (InAs) is methylated to monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) via folate-dependent one-carbon metabolism; impaired methylation is associated with adverse health outcomes. Consequently, folate nutritional status may influence arsenic methylation and toxicity.

Objective: The objective of this study was to test the hypothesis that folic acid supplementation of arsenic-exposed adults would increase arsenic methylation.

Design: Two hundred adults in a rural region of Bangladesh, previously found to have low plasma concentrations of folate (</=9 nmol/L) were enrolled in a randomized, double-blind, placebo-controlled folic acid-supplementation trial. Plasma concentrations of folate and homocysteine and urinary concentrations of arsenic metabolites were analyzed at baseline and after 12 wk of supplementation with folic acid at a dose of 400 microg/d or placebo.

Results: The increase in the proportion of total urinary arsenic excreted as DMA in the folic acid group (72% before and 79% after supplementation) was significantly (P < 0.0001) greater than that in the placebo group, as was the reduction in the proportions of total urinary arsenic excreted as MMA (13% and 10%, respectively; P < 0.0001) and as InAs (15% and 11%, respectively; P < 0.001).

Conclusions: These data indicate that folic acid supplementation to participants with low plasma folate enhances arsenic methylation. Because persons whose urine contains low proportions of DMA and high proportions of MMA and InAs have been reported to be at greater risk of skin and bladder cancers and peripheral vascular disease, these results suggest that folic acid supplementation may reduce the risk of arsenic-related health outcomes.

FIGURE 1

The arsenic metabolic pathway. Arsenate is reduced to arsenite in a reaction thought to be dependent on glutathione (GSH) or other endogenous reductants. Arsenite then undergoes an oxidative methylation, with S-adenosylmethionine (SAM) as the methyl donor, forming monomethylarsonic acid (MMA^V) and S-adenosylhomocysteine (SAH). MMA^V is reduced to MMA^III before a subsequent oxidative methylation step yielding dimethylarsinic acid (DMA^V) and SAH. Little is known about the in vivo reduction of DMA^V to DMA^III. Enzymes capable of catalyzing the illustrated reactions include arsenic-3-methyltransferase (AS3MT, formerly known as Cyt19) (12), arsenite methyltransferase and methylarsonite methyltransferase (13), and MMA^V reductase (also known as GST omega) (14). GSSG, glutathione disulfide; GST, glutathione-S-transferase; TR, thioredoxin; Trx, thioredoxin reductase.

FIGURE 2

Comparison of the effects of supplementation with folic acid (—) and with placebo (- - -) on arsenic metabolites in urine by using one-sample t tests for within-person changes from baseline to week 1 or to week 12 in each treatment group. The proportions of total urinary arsenic excreted as inorganic arsenic (InAs), dimethylarsinic acid (DMA), and monomethylarsonic acid (MMA) are indicated as %InAs, %DMA, and %MMA. *,**Significant within-person changes: *P < 0.01, **P < 0.0001. In the placebo group, small but significant (P ≤ 0.05) changes were found between baseline and week 12 for each variable.

FIGURE 3

Frequency distribution of the proportion of total urinary arsenic excreted as dimethylarsinic acid (%DMA) after 12 wk of intervention with folic acid ( ) or placebo ( ).

FIGURE 4

Overview of one-carbon metabolism. Numbers inside circles identify the different reactions. Reaction 1: Dietary folates are reduced to dihydrofolate (DHF) and tetrahydrofolate (THF) by dihydrofolate reductase. Reaction 2: The β-carbon of serine is transferred to THF by serine hydroxy-methyltransferase, forming 5,10-methylene-THF and glycine. Reaction 3: At a major branch point between transmethylation reactions and nucleotide biosynthesis, 5,10-methylene-THF can be reduced to 5-methyl-THF by 5,10-methylene-THF reductase. Reaction 4: In a reaction catalyzed by the vitamin B-12–containing enzyme methionine synthetase, the methyl group of 5-methyl-THF is transferred to homocysteine, generating methionine and regenerating THF. Reaction 5: Methionine adenosyltransferase activates methionine to form S-adenosylmethionine (SAM). Reaction 6: SAM serves as a universal methyl donor for numerous acceptors, predominantly guanidinoacetate (GAA) but also DNA, arsenic, and others, in reactions that involve numerous methyltransferases. Reaction 7: The byproduct of these methylation reactions, S-adenosylhomocysteine (SAH), is hydrolyzed to generate homocysteine. SAH is a potent inhibitor of most SAM-dependent methylations. Reaction 8: Homocysteine either is used to regenerate methionine or is directed to the transsulfuration pathway through which it is ultimately catabolized. Reaction 9: The transsulfuration pathway is also responsible for glutathione (GSH) biosynthesis. As^V, arsenate; As^III, arsenite; MMA^V, monomethylarsonic acid; MMA^III, monomethylarsonous acid; DMA^V, dimethylarsinic acid.