Distinct demethylation pathways of rice- and fish-derived methylmercury

Overview

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tems (2,3), and is subsequently biomagnified along food chains, leading reported that each part per million (ppm) increase in maternal hair Hg
(2‰–3‰) due to variations in MeHg/THg ratios (MeHg%) in rice (6,32, 5 mL HNO3 at 90–95 ◦ C for 3 h. Approximately 0.05–0.10 g of rice, hair,
maternal biomarkers from Hg-contaminated inland regions, offering digested with 25% HNO3, whereas rice (1–2 g) and blood (0.2–0.5 mL)
2.1. Sample collection and preparation (30%) was added to stabilize Hg. For maternal and cord blood, about
county, Guizhou province, China, between November 2019 and January BrCl (30%). All digests were diluted with Milli-Q water to a final THg
1. Biological samples including maternal hair, maternal blood, cord concentration of 1 μg/L for isotope analysis. Hg isotope compositions
weight, and dietary habits (Table S1). Eligibility criteria required par- Extraction Method (SEM) described by Masbou et al. (44). Purity and
ticipants to have lived in the study area for at least one year prior to recovery were assessed for each extract (Table S4). MeHg purity (%) was
cardiovascular, hepatic, or renal disease) or infectious conditions (e.g., was used to evaluate SEM extraction efficiency. After dilution with 10%
AIDS, hepatitis B, syphilis) were excluded to avoid potential bias asso- HNO3 to a final THg concentration of 1 μg/L, purified MeHg extracts
MeHg isotope values using the following equations: standard deviations. With the given sampling size (n = 23), the achieved
sample, calculated as (MeHg/THg) × 100%.
(MeHg%) for all samples, are presented in Table 1. The mean THg
Previous studies have demonstrated that there is no significant MIF (16.1 ± 8.75 μg/kg) and MeHg (8.57 ± 4.84 μg/kg) concentrations in
human biological samples can be used to quantitatively estimate the Li et al. (32) (THg: 83.0 ± 14.0 μg/kg; MeHg: 38.1 ± 9.30 μg/kg) and Du
differentiation of Hg from different sources. In this study, a binary et al. (6) (THg: 79.0 ± 78.0 μg/kg; MeHg: 17.0 ± 16.0 μg/kg) in the
data, a Monte Carlo simulation (n = 100,000) was performed utilizing implementation of the Soil Pollution Prevention and Control Action Plan
maternal blood were 3.85 ± 1.33 μg/L and 2.27 ± 1.42 μg/L, respectively,
with no values exceeding the acceptable Hg limit in blood (5.80 μg/L) set
Where, the Δ Hgi represents Δ HgTHg or Δ HgMeHg values; (THg: 12.2 ± 15.0 μg/L; MeHg: 5.78 ± 5.06 μg/L) (7), but higher than
Δ199Hgibiomarkers represents Δ199Hgi values in different biomarkers; levels observed in coastal areas (THg: 2.36 ± 0.94 μg/L; MeHg:
Δ199HgiRice and Δ199HgiFish are the mean Δ199Hgi values of locally 1.11 ± 0.47 μg/L) (17), which indicates that, despite the closure of Hg
of Hg originating from rice and fish sources, respectively. Δ199HgiRice higher than in coastal areas. The THg (geomean: 0.690 μg/g) and MeHg
and Δ199HgiFish values were used as the dietary sources of Hg for preg- (geomean: 0.276 μg/g) concentrations in maternal hair were significantly
2.5. Quality control and data analysis 0.576 μg/g; MeHg geomean: 0.420 μg/g) (17). In this study, the geometric
ensure the accuracy and reliability of the results. This included the of 250:1 (μg/kg dry wt : μg/L) (53). Similar MeHg% in hair and maternal
concentrations in CRMs are presented in Table S2. The relative per- attributed to fetal partitioning of maternal Hg, leading to a reduced
10% for both THg and MeHg, as shown in Table S4. Additionally, the pretation is supported by maternal blood and hair Hg concentrations
recoveries for the SEM are presented in Table S6, with mean recoveries population in the same region (THg hair-to-blood ratio: 268 ± 112; MeHg
the method to efficiently extract MeHg from biological samples. The (8.07 ± 2.45 μg/L) exceeded the acceptable limit of 5.8 μg/L (53).
NIST-3133 standard was used as a bracketing standard, while the NIST- Compared to background areas, such as Baoding (0.91 μg/L), Kunming
3177 standard was analyzed and used as a secondary standard. The (1.55 μg/L), and Xi’an (1.05 μg/L), the elevated THg concentrations
overall mean (±2SD) δ202Hg and Δ199Hg values for all NIST-3177 indicated potential health risks for infants due to chronic Hg exposure
(n = 15), respectively, which are consistent with previously published 4.58 ± 2.25 μg/L, which was also much higher than coastal areas
data (22,45). The BCR-482 and NIES-13 standards were used to validate (2.11 ± 0.91 μg/L) (17). The mean cord-to-maternal blood ratios for
compositions of these CRMs are presented in Table S3. higher than those reported previously, respectively. The enhanced
compared using Fisher’s least significant difference (LSD) test at a 5% the placenta via amino acid transporters, resulting in higher cord blood
effect size (Cohen’s d) was calculated from the observed means and (Fig. 1). The THg (51.6 ± 14.2 μg/kg) and MeHg (35.9 ± 11.1 μg/kg) in
Maternal blood (μg/L) 23 3.85 1.33 3.66 2.27 1.42 1.88 56.4 22.4 51.4
Maternal hair (μg/kg) 23 997 1068 690 467 568 276 48.4 25.0 39.9
(THg: 28.0 ± 10.9 μg/kg; MeHg: 13.3 ± 5.70 μg/kg) (17). The mean derstand Hg metabolism and transport in pregnant women. Relying
values for each sample provided in Table S6. The average δ202HgTHg

Methods (brief)

shan Hg mining area, China. Even after remediation, THg in all cord blood samples still exceeded the U.S. EPA
limited insight into in vivo metabolism and maternal–fetal transfer. In Maternal and cord blood samples were collected at delivery using
biomarkers with stable Hg isotopes, and whether MeHg derived from of placenta was collected immediately after birth, wrapped in clean tin
mass-independent fractionation (MIF) (19). MDF arises from biogeo- samples were untreated (not dyed or permed) (6). Except for hair, all
chemical processes, including redox reactions, demethylation, absorp- samples were frozen immediately and transported to the laboratory.
tion, and tissue partitioning (20,21). In contrast, MIF is primarily driven Rice samples were collected directly from the households of participants
tissues has been attributed to microbial demethylation of MeHg in the were randomly selected from the study cohort, and all samples were
Compound-specific stable isotope analysis further differentiates MeHg In the laboratory, hair and placenta samples underwent additional
in rice (32). For example, hair Δ199Hg values near 0 indicate rice-based pretreatment. Hair samples were processed following previously estab-
exposure, while elevated values indicate fish consumption (6,33). Bi- lished protocols (40). Placenta samples were rinsed with non-ionic
the isotopic offset between rice and rice consumers’ hair is likely greater All rice, maternal hair, placenta, and blood samples were digested in
35,36). and placenta, and 0.2–0.5 mL of blood were used for digestion. THg
Integrating MDF and MIF analysis can provide robust constraints on concentrations in the digested samples were measured by cold vapor
Hg source attribution and internal transformations, including mater- atomic fluorescence spectroscopy (CVAFS; Brooks Rand Model III,
biomarkers (e.g., hair, urine), leaving a critical knowledge gap in un- centrations in rice were additionally analyzed using a DMA-80 analyzer
novel insights into Hg metabolism and transplacental transfer in preg- were processed using KOH–methanol digestion followed by solvent
using isotope fractionation. These findings contribute to a better un- CVAFS (GC-CVAFS; Brooks Rand Model III, Brooks Rand Lab, U.S.) ac-
digested in 5 mL HNO3 at 95 ◦ C for 3 h. After digestion, 0.5 mL of BrCl

Implications

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Update history

The five most recent substantive edits to this page, classified major (evidence or structure moved), correction (a published value or statement was wrong and has been fixed), or minor (narrative rewritten without changing the underlying evidence). Each description is derived from what the edit did to this page; the linked commit is the authoritative record, routine regeneration passes are excluded, and the full version history lives in git. When DOI minting comes online (see schema docs), each entry below will also link to a version-pinned DataCite DOI.

Commit	Date	Change	Description
b01ec52c	2026-08-04	major	2 sections added
d49e450f	2026-08-03	major	5 sections added; narrative text revised