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BACKGROUND: It has been proposed that interactions between selenium and arsenic in the body may affect their kinetics and toxicity. However, it is unknown how the elements influence each other in humans.

OBJECTIVES: We aimed to investigate potential interactions in the methylation of selenium and arsenic.

METHODS: Urinary selenium (U-Se) and arsenic (U-As) were measured using inductively coupled plasma mass spectrometry (ICPMS) in samples collected from pregnant women (n = 226) in rural Bangladesh at gestational weeks (GW) 8, 14, 19, and 30. Urinary concentrations of trimethyl selenonium ion (TMSe) were measured by HPLC-vapor generation-ICPMS, as were inorganic arsenic (iAs), methylarsonic acid (MMA), and dimethylarsinic acid (DMA). IMethylation efficiency was assessed based on relative amounts (%) of arsenic and selenium metabolites in urine. Genotyping for the main arsenite and selenium methyffiansferases, AS3MT and INMT, was performed using TaqMan probes or Sequenom.

RESULTS: Multivariable-adjusted linear regression analyses indicated that %TMSe (at GW8) was positively associated with %MMA (beta = 1.3, 95% CI: 0.56, 2.0) and U-As, and inversely associated with %DMA and U-Se in producers of TMSe (INMT rs6970396 AG + AA, n = 7 4), who had a wide range of urinary TMSe, (12-42%). Also, %TMSe decreased in parallel to %MMA during pregnancy, especially in the first trimester (-0.58 %TMSe per gestational week). We found a gene gene interaction for %MMA (p-interaction = 0.076 for haplotype 1). In analysis stratified by INMT genotype, the association between %MMA and both AS3MT haplotypes 1 and 3 was stronger in women with the INMT (TMSe nonproducers, 5th-95th percentile,: 0.2-2%TMSe) vs. AG + AA genotype.

CONCLUSIONS: Our findings for Bangladeshi women suggest a positive association between urinary %MMA and %TMSe. Genes involved in the methylation of selenium and arsenic may interact on associations with urinary %MMA.