Hemoglobin as a major binding protein for methylmercury in white-sided dolphin liver

Emerging evidence on selenoneine and its public health relevance in coastal populations: a review and case study of dietary Se among Inuit populations in the Canadian Arctic

Selenium is an essential mineral yet both deficiency and excess are associated with adverse health effects. Dietary intake of Se in humans varies greatly between populations due to food availability, dietary preferences, and local geological and ecosystem processes impacting Se accumulation into agricultural products and animal populations. We argue there is a need to evaluate and reconsider the relevance of public health recommendations on Se given recent evidence, including the metabolic pathways and health implications of Se. This argument is particularly pertinent for Inuit populations in Northern Canada, who often exceed dietary tolerable upper intake levels and exhibit very high whole blood Se concentrations due to their dependence on local country foods high in the newly discovered Se compound, selenoneine. Since selenoneine appears to have lower toxicity compared to other Se species and does not contribute to the circulating pools of Se for selenoprotein synthesis, we argue that total dietary Se or total Se in plasma or whole blood are poor indicators of Se adequacy for human health in these populations. Overall, this review provides an overview of the current evidence of Se speciation, deficiency, adequacy, and excess and implications for human health and dietary recommendations, with particular reference to Inuit populations in the Canadian Arctic and other coastal populations consuming marine foods.

Exploring mercury detoxification in fish: The role of selenium from tuna byproduct diets for sustainable aquaculture

Exposure to mercury (Hg) through fish consumption poses significant environmental and public health risks, given its status as one of the top ten hazardous chemicals. Aquaculture is expanding, driving a surge in demand for sustainable aquafeeds. Tuna byproducts, which are rich in protein, offer potential for aquafeed production, yet their use is challenged by the high content of heavy metals, particularly Hg. However, these byproducts also contain elevated levels of selenium (Se), which may counteract Hg adverse effects. This study examines the fate of dietary Hg and Se in an aquaculture model fish. Biomolecular speciation analyses through hyphenated analytical approaches were conducted on the water-soluble protein fraction of key organs of juvenile rainbow trout (Oncorhynchus mykiss) exposed to various combinations of Hg and Se species, including diets containing tuna byproducts, over a six-month period. The findings shed light on the dynamics of Hg and Se compounds in fish revealing potential Hg detoxification mechanisms through complexation with Hg-biomolecules, such as cysteine, glutathione, and metallothionein. Furthermore, the trophic transfer of selenoneine is demonstrated, revealing novel opportunities for sustainable aquafeed production. Understanding the interactions between Hg and Se in aquaculture systems is crucial for optimizing feed formulations and mitigating environmental risks. This research contributes to the broader goal of advancing sustainable practices in aquaculture while addressing food security challenges.

Assessment of dietary Selenium and its role in Mercury fate in cultured fish rainbow trout with two sustainable aquafeeds

This study enhances the current limited understanding of the interaction between mercury (Hg) and selenium (Se) species in fish. Rainbow trout (Oncorhynchus mykiss), a model aquaculture fish, was exposed to Hg and Se species through controlled dietary conditions. Over a 6-month feeding trial, the impact of dietary Se on Hg bioaccumulation in fish, including flesh, brain, and liver, was tracked. Twelve dietary conditions were tested, including plant-based diets (0.25 µgSe g-1) and tuna byproduct diets (0.25 µgHg g-1, 8.0 µgSe g-1) enriched with methylmercury and/or Se as selenite or selenomethionine. The tuna byproduct diet resulted in lower Hg levels than the plant-based diets, with muscle Hg content below the European Commission's safe threshold. This study highlights the significant impact of specific Se compounds in the diet, particularly from tuna-based aquafeed, on Hg bioaccumulation. These promising results provide a strong recommendation for future use of fisheries byproducts in sustainable aquafeeds.

First-Time Isotopic Characterization of Seleno-Compounds in Biota: A Pilot Study of Selenium Isotopic Composition in Top Predator Seabirds

This study pioneers the reporting of Se isotopes in marine top predators and represents the most extensive Se isotopic characterization in animals to date. A methodology based on hydride generation─multicollector inductively coupled plasma mass spectrometry─was established for such samples. The study was conducted on various internal organs of giant petrels (Macronectes spp.), encompassing bulk tissues (δ82/78Sebulk), distinct Se-specific fractions such as selenoneine (δ82/78SeSEN), and HgSe nanoparticles (δ82/78SeNPs). The δ82/78Sebulk results (2.0-5.6‰) offer preliminary insights into the fate of Se in key internal organs of seabirds, including the liver, the kidneys, the muscle, and the brain. Notably, the liver of all individuals was enriched in heavier Se isotopes compared to other examined tissues. In nanoparticle fraction, δ82/78Se varies significantly across individuals (δ82/78SeNPs from 0.6 to 5.7‰, n = 8), whereas it exhibits remarkable consistency among tissues and individuals for selenoneine (δ82/78SeSEN, 1.7 ± 0.3‰, n = 8). Significantly, there was a positive correlation between the shift from δ82/78Sebulk to δ82/78SeSEN and the proportion of Se present as selenoneine in the internal organs. This pilot study proves that Se species-specific isotopic composition is a promising tool for a better understanding of Se species fate, sources, and dynamics in animals.

Sea turtle egg yolk and albumen as biomonitoring matrices for maternal burdens of organic pollutants

To establish the use of eggs as biomonitoring tools for maternal body burdens, we investigated the mother-to-egg ratio of 56 PCB, 12 OCP and 34 PBDE unique compounds from maternal plasma into replicate egg yolk and albumen samples in the loggerhead turtle (Caretta caretta) as a case study of a threatened migratory marine species. We applied robust Regression on Order Statistics to fully account for the information in both censored and uncensored data. Our results added new insights into the use of yolk as a suitable biomonitoring matrix; the difference between yolk and albumen which were previously analysed as a homogeneous mixture; and the value of accounting for censored data. Overall, compound-specific mother-to-egg ratios need to be considered when translating yolk levels back to maternal pollution burdens, and when assessing the risk to the subsequent generations of turtle embryos.

Selenium in the liver facilitates the biodilution of mercury in the muscle of Planiliza haematocheilus in the Jiaozhou Bay, China

There are increasing evidences that the biodilution effect can significantly reduce the biomagnification of mercury (Hg) in fish. The significant antagonism of selenium (Se) -Hg may have a potential diluting effect on Hg in fish; however, there is still lack of knowledge on such effect. To reveal the Se-Hg interaction and its role in controlling the biodilution effect of Hg, we investigated levels of Hg and Se in the muscle and liver of redlip mullet from Jiaozhou Bay, China, an urbanized semi-enclosed bay highly impacted by human activities. In general, Hg levels in fish muscle were significantly negatively correlated to the levels of Se in the liver and fish size for fish with a size of < 200 mm, indicating that the antagonistic effect of Se on Hg increased with fish growth. This relationship was not significant for fish with a size of > 200 mm, possibly because the normal metabolism of Hg in muscle was hindered by homeostatic regulation or physiological activities such as gonadal development in vivo. Furthermore, the molar ratio of Se in the liver/Hg in the muscle was significantly increasing with Se/Hg in the liver, suggesting that the liver may be the key organ involved in Se-Hg antagonism. Moreover, both ratios continued to decrease with increasing fish size, implying that the antagonistic effect weakens with fish growth. These results indicate that Hg sequestration by liver may be a key mechanism of Se-Hg antagonism in fish and function as a driver for the biodilution effect of Hg, especially at a size of < 200 mm. These findings are further supported by the established linear model of Se-Hg antagonism at different developmental stages.

The ATTAC guiding principles to openly and collaboratively share wildlife ecotoxicology data

The inability to quantitatively integrate scattered data regarding potential threats posed by the increasing total amount and diversity of chemical substances in our environment limits our ability to understand whether existing regulations and management actions sufficiently protect wildlife. Systematic literature reviews and meta-analyses are great scientific tools to build upon the current push for accessibility under the Open Science and FAIR movements. Despite the potential of such integrative analyses, the emergence of innovative findings in wildlife ecology and ecotoxicology is still too rare relative to the potential that is hidden within the entirety of the available scattered data. To promote the reuse of wildlife ecotoxicology data, we propose the ATTAC workflow which comprises five key steps (Access, Transparency, Transferability, Add-ons, and Conservation sensitivity) along the chain of collecting, homogenizing, and integrating data for subsequent meta-analyses. The ATTAC workflow brings together guidelines supporting both the data prime movers and re-users. As such, the ATTAC workflow could promote an open and collaborative wildlife ecotoxicology able to reach a major objective in this applied field, namely, providing strong scientific support for regulations and management actions to protect and preserve wildlife species.

Internal Dynamics and Metabolism of Mercury in Biota: A Review of Insights from Mercury Stable Isotopes

Monitoring mercury (Hg) levels in biota is considered an important objective for the effectiveness evaluation of the Minamata Convention. While many studies have characterized Hg levels in organisms at multiple spatiotemporal scales, concentration analyses alone often cannot provide sufficient information on the Hg exposure sources and internal processes occurring within biota. Here, we review the decadal scientific progress of using Hg isotopes to understand internal processes that modify the speciation, transport, and fate of Hg within biota. Mercury stable isotopes have emerged as a powerful tool for assessing Hg sources and biogeochemical processes in natural environments. A better understanding of the tissue location and internal mechanisms leading to Hg isotope change is key to assessing its use for biomonitoring. We synthesize the current understanding and uncertainties of internal processes leading to Hg isotope fractionation in a variety of biota, in a sequence of better to less studied organisms (i.e., birds, marine mammals, humans, fish, plankton, and invertebrates). This review discusses the opportunities and challenges of using certain forms of biota for Hg source monitoring and the need to further elucidate the physiological mechanisms that control the accumulation, distribution, and toxicity of Hg in biota by coupling new techniques with Hg stable isotopes.

Mercury concentrations in tuna blood and muscle mirror seawater methylmercury in the Western and Central Pacific Ocean

Understanding the relationship between mercury in seafood and the distribution of oceanic methylmercury is key to understand human mercury exposure. Here, we determined mercury concentrations in muscle and blood of bigeye and yellowfin tunas from the Western and Central Pacific. Results showed similar latitudinal patterns in tuna blood and muscle, indicating that both tissues are good candidates for mercury monitoring. Complementary tuna species analyses indicated species- and tissue- specific mercury patterns, highlighting differences in physiologic processes of mercury uptake and accumulation associated with tuna vertical habitat. Tuna mercury content was correlated to ambient seawater methylmercury concentrations, with blood being enriched at a higher rate than muscle with increasing habitat depth. The consideration of a significant uptake of dissolved methylmercury from seawater in tuna, in addition to assimilation from food, might be interesting to test in models to represent the spatiotemporal evolutions of mercury in tuna under different mercury emission scenarios.

Reply to the comment on "New insights into the biomineralization of mercury selenide nanoparticles through stable isotope analysis in giant petrel tissues" by A. Manceau, J. Hazard. Mater. 425 (2021) 127922. doi: 10.1016/j.jhazmat.2021.127922

In the comments reported by A. Manceau [1], relating to our recent paper on mercury (Hg) species-specific isotopic characterization in giant petrel tissues [2] two critical questions were raised. Firstly, according to A. Manceau, our method of extraction and isolation of nanoparticles was not able to efficiently isolate mercury selenide nanoparticles (HgSe NPs) and therefore the δ²⁰²Hg values measured are not species-specific, but rather δ²⁰²Hg of mixtures of complexes such as MeHgCys, Hg(Sec)₄, and HgSe. Secondly, he suggests that our main findings showing that no isotopic fractionation is induced during the HgSe NPs biomineralization step from the precursor-demethylated species is erroneous because it contradicts the conclusion of two recent articles by A. Manceau and co-workers [3,4]. In this reply we defend our scientific findings and respectively respond to the questions and comments raised by A. Manceau.

New insights into the biomineralization of mercury selenide nanoparticles through stable isotope analysis in giant petrel tissues

Tiemannite (HgSe) is considered the end-product of methylmercury (MeHg) demethylation in vertebrates. The biomineralization of HgSe nanoparticles (NPs) is understood to be an efficient MeHg detoxification mechanism; however, the process has not yet been fully elucidated. In order to contribute to the understanding of complex Hg metabolism and HgSe NPs formation, the Hg isotopic signatures of 40 samples of 11 giant petrels were measured. This seabird species is one of the largest avian scavengers in the Southern Ocean, highly exposed to MeHg through their diet, reaching Hg concentrations in the liver up to more than 900 µg g^-1. This work constitutes the first species-specific isotopic measurement (δ²⁰²Hg, Δ¹⁹⁹Hg) of HgSe NPs in seabirds and the largest characterization of this compound in biota. Similar δ²⁰²Hg values specifically associated to HgSe (δ²⁰²Hg_HgSe) and tissues (δ²⁰²Hg_bulk) dominated by inorganic Hg species were found, suggesting that no isotopic fractionation is induced during the biomineralization step from the precursor (demethylated) species. In contrast, the largest variations between δ²⁰²Hg_bulk and δ²⁰²Hg_HgSe were observed in muscle and brain tissues. This could be attributed to the higher fraction of Hg present as MeHg in these tissues. Hg-biomolecules screening highlights the importance of the isotopic characterization of these (unknown) complexes.

First Time Identification of Selenoneine in Seabirds and Its Potential Role in Mercury Detoxification

Birds are principally exposed to selenium (Se) through their diet. In long-lived and top predator seabirds, such as the giant petrel, extremely high concentrations of Se are found. Selenium speciation in biota has aroused great interest in recent years; however, there is a lack of information about the chemical form of Se in (sea)birds. The majority of publications focus on the growth performance and antioxidant status in broilers in relation to Se dietary supplementation. The present work combines elemental and molecular mass spectrometry for the characterization of Se species in wild (sea)birds. A set of eight giant petrels (Macronectes sp.) with a broad age range from the Southern Ocean were studied. Selenoneine, a Se-analogue of ergothioneine, was identified for the first time in wild avian species. This novel Se-compound, previously reported in fish, constitutes the major Se species in the water-soluble fraction of all of the internal tissues and blood samples analyzed. The levels of selenoneine found in giant petrels are the highest reported in animal tissues until now, supporting the trophic transfer in the marine food web. The characterization of selenoneine in the brain, representing between 78 and 88% of the total Se, suggests a crucial role in the nervous system. The dramatic decrease of selenoneine (from 68 to 3%) with an increase of Hg concentrations in the liver strongly supports the hypothesis of its key role in Hg detoxification.

Determination of the Intracellular Complexation of Inorganic and Methylmercury in Cyanobacterium Synechocystis sp. PCC 6803

Understanding of mercury (Hg) complexation with low molecular weight (LMW) bioligands will help elucidate its speciation. In natural waters, the rate of this complexation is governed by physicochemical, geochemical, and biochemical parameters. However, the role of bioligands involved in Hg intracellular handling by aquatic microorganisms is not well documented. Here, we combine the use of isotopically labeled Hg species (inorganic and monomethylmercury, iHg and MeHg) with gas or liquid chromatography coupling to elemental and molecular mass spectrometry to explore the role of intracellular biogenic ligands involved in iHg and MeHg speciation in cyanobacterium Synechocystis sp. PCC 6803, a representative phytoplankton species. This approach allowed to track resulting metabolic and newly found intracellular Hg biocomplexes (e.g., organic thiols) in Synechocystis sp. PCC 6803 finding different intracellular Hg species binding affinities with both high and low molecular weight (HMW and LMW) bioligands in the exponential and stationary phase. Furthermore, the parallel detection with both elemental and molecular ionization sources allowed the sensitive detection and molecular identification of glutathione (GSH) as the main low molecular weight binding ligand to iHg ((GS)₂-Hg) and MeHg (GS-MeHg) in the cytosolic fraction. Such a novel experimental approach expands our knowledge on the role of biogenic ligands involved in iHg and MeHg intracellular handling in cyanobacteria.

Mercury interactions with selenium and sulfur and the relevance of the Se:Hg molar ratio to fish consumption advice

Eating fish is often recommended as part of a healthful diet. However, fish, particularly large predatory fish, can contain significant levels of the highly toxic methylmercury (MeHg). Ocean fish in general also contain high levels of selenium (Se), which is reported to confer protection against toxicity of various metals including mercury (Hg). Se and Hg have a high mutual binding affinity, and each can reduce the toxicity of the other. This is an evolving area of extensive research and controversy with variable results in the animal and epidemiologic literature. MeHg is toxic to many organ systems through high affinity for -SH (thiol) ligands on enzymes and microtubules. Hg toxicity also causes oxidative damage particularly to neurons in the brain. Hg is a potent and apparently irreversible inhibitor of the selenoenzymes, glutathione peroxidases (GPX), and thioredoxin reductases (TXNRD) that are important antioxidants, each with a selenocysteine (SeCys) at the active site. Hg binding to the SeCys inhibits these enzymes, accounting in part for the oxidative damage that is an important manifestation of Hg toxicity, particularly if there is not a pool of excess Se to synthesize new enzymes. A molar excess of Se reflected in an Se:Hg molar ratio > 1 is often invoked as evidence that the Hg content can be discounted. Some recent papers now suggest that if the Se:Hg molar ratio exceeds 1:1, the fish is safe and the mercury concentration can be ignored. Such papers suggested that the molar ratio rather than the Hg concentration should be emphasized in fish advisories. This paper examines some of the limitations of current understanding of the Se:Hg molar ratio in guiding fish consumption advice; Se is certainly an important part of the Hg toxicity story, but it is not the whole story. We examine how Hg toxicity relates also to thiol binding. We suggest that a 1:1 molar ratio cannot be relied on because not all of the Se in fish or in the fish eater is available to interact with Hg. Moreover, in some fish, Se levels are sufficiently high to warrant concern about Se toxicity.

Implications for an imidazol-2-yl carbene intermediate in the rhodanase-catalyzed C-S bond formation reaction of anaerobic ergothioneine biosynthesis

In the anaerobic ergothioneine biosynthetic pathway, a rhodanese domain containing enzyme (EanB) activates tne hercynine's sp² ε-C-H Dona ana replaces it with a C-S bond to produce ergothioneine. The key intermediate for this trans-sulfuration reaction is the Cys412 persulfide. Substitution of the EanB-Cys412 persulfide with a Cys412 perselenide does not yield the selenium analog of ergothioneine, selenoneine. However, in deuterated buffer, the perselenide-modified EanB catalyzes the deuterium exchange between hercynine's sp² ε-C-H bond and D₂O. Results from QM/MM calculations suggest that the reaction involves a carbene intermediate and that Tyr353 plays a key role. We hypothesize that modulating the pK_a of Tyr353 will affect the deuterium-exchange rate. Indeed, the 3,5-difluoro tyrosine containing EanB catalyzes the deuterium exchange reaction with k _ex of ~10-fold greater than the wild-type EanB (EanB_WT). With regards to potential mechanisms, these results support the involvement of a carbene intermediate in EanB-catalysis, rendering EanB as one of the few carbene-intermediate involving enzymatic systems.

Cellular and Molecular Mechanisms Mediating Methylmercury Neurotoxicity and Neuroinflammation

Methylmercury (MeHg) toxicity is a major environmental concern. In the aquatic reservoir, MeHg bioaccumulates along the food chain until it is consumed by riverine populations. There has been much interest in the neurotoxicity of MeHg due to recent environmental disasters. Studies have also addressed the implications of long-term MeHg exposure for humans. The central nervous system is particularly susceptible to the deleterious effects of MeHg, as evidenced by clinical symptoms and histopathological changes in poisoned humans. In vitro and in vivo studies have been crucial in deciphering the molecular mechanisms underlying MeHg-induced neurotoxicity. A collection of cellular and molecular alterations including cytokine release, oxidative stress, mitochondrial dysfunction, Ca²⁺ and glutamate dyshomeostasis, and cell death mechanisms are important consequences of brain cells exposure to MeHg. The purpose of this review is to organize an overview of the mercury cycle and MeHg poisoning events and to summarize data from cellular, animal, and human studies focusing on MeHg effects in neurons and glial cells. This review proposes an up-to-date compendium that will serve as a starting point for further studies and a consultation reference of published studies.

Capabilities of selenoneine to cross the in vitro blood-brain barrier model

The naturally occurring selenoneine (SeN), the selenium analogue of the sulfur-containing antioxidant ergothioneine, can be found in high abundance in several marine fish species. However, data on biological properties of SeN and its relevance for human health are still scarce. This study aims to investigate the transfer and presystemic metabolism of SeN in a well-established in vitro model of the blood-brain barrier (BBB). Therefore, SeN and the reference Se species selenite and Se-methylselenocysteine (MeSeCys) were applied to primary porcine brain capillary endothelial cells (PBCECs). Se content of culture media and cell lysates was measured via ICP-MS/MS. Speciation analysis was conducted by HPLC-ICP-MS. Barrier integrity was shown to be unaffected during transfer experiments. SeN demonstrated the lowest transfer rates and permeability coefficient (6.7 × 10-7 cm s-1) in comparison to selenite and MeSeCys. No side-directed accumulation was observed after both-sided application of SeN. However, concentration-dependent transfer of SeN indicated possible presence of transporters on both sides of the barrier. Speciation analysis demonstrated no methylation of SeN by the PBCECs. Several derivatives of SeN detected in the media of the BBB model were also found in cell-free media containing SeN and hence not considered to be true metabolites of the PBCECs. In concluding, SeN is likely to have a slow transfer rate to the brain and not being metabolized by the brain endothelial cells. Since this study demonstrates that SeN may reach the brain tissue, further studies are needed to investigate possible health-promoting effects of SeN in humans.

Characterization of the mercury-binding proteins in tuna and salmon sashimi: Implications for health risk of mercury in food

Fish consumption is one of the major ways through which humans receive exposure to mercury (Hg). The existing forms of Hg in food, particularly Hg bound to proteins, may affect the absorption of Hg by humans and subsequently its potentially toxic effects. However, the knowledge regarding Hg-binding proteins in edible fish muscle is scarce. In the present study, salmon and tuna fish muscles, collected from seven different regions and countries, were analyzed using metallomics- and proteomics-based techniques. The concentration of Hg in sashimi samples ranged from 4.4 to 317.4 ng/g. Size exclusion chromatography (SEC) coupled with inductively coupled plasma mass spectrometer (ICP-MS) showed that beta-actin was a novel Hg-binding protein from the fish muscles, and this protein could also bind bismuth (Bi), silver (Ag), and copper (Cu). Hg bound to beta-actin accounted for approximately 30.2-37.6% of the total Hg in the tuna muscles and was significantly correlated to total Hg in the fish muscles (r = 0.98, p < 0.01) and in the fraction of soluble proteins (r = 0.94, p < 0.01). These findings suggest that proteins act as the main Hg accumulation sites in edible fish; thus, increasing human exposure to Hg following gastrointestinal digestion.

Metalloproteomic approach of mercury-binding proteins in liver and kidney tissues of Plagioscion squamosissimus (corvina) and Colossoma macropomum (tambaqui) from Amazon region: Possible identification of mercury contamination biomarkers

Fish is an important source of protein, vitamins, and minerals. However, this food is also a major source of human exposure to toxic contaminants such as mercury. Thus, this paper aimed to evaluate mercury-binding proteins for possible application as biomarkers of mercury contamination in hepatic and renal tissues of Plagioscion squamosissimus (carnivorous fish) and Colossoma macropomum (omnivorous fish) from the Amazon region using metalloproteomic approach. The proteome of hepatic and renal tissues of fish species was separated by two-dimensional polyacrylamide gel electrophoresis (2D-PAGE), and the mercury concentrations in protein spots were determined by graphite furnace atomic absorption spectrometry (GFAAS). Finally, the protein spots associated to mercury were characterized by electrospray ionization mass spectrometry (ESI-MS/MS). The activity of antioxidant enzymes (SOD, CAT, GPx, and GST) and lipid peroxidation (LPO) were also determined. The results showed that the highest concentrations of mercury were found in the carnivorous species (P. squamosissimus) and that the accumulation pattern of this metal was higher in hepatic tissues than in renal tissues for both species. A tendency was observed for greater enzymatic activity in the hepatic and renal tissues of P. squamosissimus, the species with the highest concentration of mercury. Only GPx activity in the kidney and GST in the liver were lower for the P. squamosissimus species, and this finding can be explained by the interaction of mercury with these enzymes. The data obtained by ESI-MS/MS allowed for the characterization of the protein spots associated to mercury, revealing proteins involved in energy metabolism, biomolecules transport, protein synthesis and degradation, cell differentiation, gene regulation, and the antioxidant system. The results obtained in the present study can contribute to understanding the physiological processes underlying mercury toxicity and have provided new perspectives on possible candidates for mercury contamination biomarkers in fish.

Selenium and stable mercury isotopes provide new insights into mercury toxicokinetics in pilot whales

High exposures of mammalian species to inorganic mercury (Hg^II) and methylmercury (MeHg) have been associated with adverse effects on behavior and reproduction. Different mammalian species exhibit varying responses to similar external exposure levels, reflecting potential differences in Hg toxicokinetics. Here, we use Hg stable isotopes, total Hg, MeHg and selenium (Se) concentrations measured in multiple tissues of North Atlantic pilot whales (Globicephala melas) to investigate processes affecting the distribution and accumulation of Hg^II and MeHg. We find that simple mixing of two distinct isotopic end-members: MeHg (1.4‰) and Hg^II (-1.6‰) can explain the observed variability of δ²⁰²Hg in brain tissue. A similar isotopic composition for the MeHg end-member in the brain, muscle, heart, and kidney suggests efficient exchange of MeHg in blood throughout the body. By contrast, the Hg isotopic composition of the liver of adult whales is different from younger whales and other tissues that follow the two-end member mixing model. Measured Se:Hg ratios are lowest in adult whales with the highest levels of MeHg exposure. In these individuals, Se availability is likely reduced by complexation with demethylated Hg^II. We speculate that this results in a higher fraction of labile Hg^II eliminated from the liver of adult whales compared to young whales and subsequent redistribution to other tissues, potentially affecting toxicity.

Toxicity of thimerosal in biological systems: Conformational changes in human hemoglobin, decrease of oxygen binding capacity, increase of protein glycation and amyloid's formation

Thimerosal (TH), an organomercurial compound, is used as a preservative in vaccines and cosmetics. Its interaction with human hemoglobin (Hb) was investigated under physiological conditions using biophysical and biological assays, aiming to evaluate hazardous effects. TH interacts spontaneously with Hb (stoichiometry 2:1, ligand-protein), preferably by electrostatic forces, with a binding constant of 1.41 × 10⁶ M^-1. Spectroscopic data allows to proposing that TH induces structural changes in Hg, through ethylmercury transfer to human Hb-Cys93 residues, forming thiosalicylic acid, which, in turn, interacts with the positive side of the amino acid in the Hb-HgEt adduct chain. As a consequence, inhibition of Hb-O₂ binding capacity up to 72% (human Hb), and 50% (human erythrocytes), was verified. Dose-dependent induction of TH forming advanced glycation end products (AGE) and protein aggregates (amyloids) was additionally observed. Finally, these results highlight the toxic potential of the use of TH in biological systems, with a consequent risk to human health.

Deiminated proteins and extracellular vesicles as novel biomarkers in pinnipeds: Grey seal (Halichoerus gryptus) and harbour seal (Phoca vitulina)

Peptidylarginine deiminases (PADs) are phylogenetically conserved calcium-dependent enzymes which post-translationally convert arginine into citrulline in target proteins in an irreversible manner, leading to functional and structural changes in target proteins. Protein deimination can cause the generation of neo-epitopes, affect gene regulation and also allow for protein moonlighting and therefore facilitate multifaceted functions of the same protein. PADs are furthermore a key regulator of cellular release of extracellular vesicle (EVs), which are found in most body fluids and participate in cellular communication via transfer of cargo proteins and genetic material. In this study, post-translationally deiminated proteins and EVs were assessed in sera of two seal species, grey seal and harbour seal. We report a poly-dispersed population of serum-EVs, which were positive for phylogenetically conserved EV-specific markers and characterised by transmission electron microscopy. A number of deiminated proteins critical for immune and metabolic functions were identified in the seal sera and varied somewhat between the two species under study, while some targets were in common. EV profiles of the seal sera further revealed that key microRNAs for inflammation, immunity and hypoxia also vary between the two species. Protein deimination and EVs profiles may be useful biomarkers for assessing health status of sea mammals, which face environmental challenges, including opportunistic infection, pollution and shifting habitat due to global warming.

Deiminated proteins and extracellular vesicles - Novel serum biomarkers in whales and orca

Peptidylarginine deiminases (PADs) are a family of phylogenetically conserved calcium-dependent enzymes which cause post-translational protein deimination. This can result in neoepitope generation, affect gene regulation and allow for protein moonlighting via functional and structural changes in target proteins. Extracellular vesicles (EVs) carry cargo proteins and genetic material and are released from cells as part of cellular communication. EVs are found in most body fluids where they can be useful biomarkers for assessment of health status. Here, serum-derived EVs were profiled, and post-translationally deiminated proteins and EV-related microRNAs are described in 5 ceataceans: minke whale, fin whale, humpback whale, Cuvier's beaked whale and orca. EV-serum profiles were assessed by transmission electron microscopy and nanoparticle tracking analysis. EV profiles varied between the 5 species and were identified to contain deiminated proteins and selected key inflammatory and metabolic microRNAs. A range of proteins, critical for immune responses and metabolism were identified to be deiminated in cetacean sera, with some shared KEGG pathways of deiminated proteins relating to immunity and physiology, while some KEGG pathways were species-specific. This is the first study to characterise and profile EVs and to report deiminated proteins and putative effects of protein-protein interaction networks via such post-translationald deimination in cetaceans, revealing key immune and metabolic factors to undergo this post-translational modification. Deiminated proteins and EVs profiles may possibly be developed as new biomarkers for assessing health status of sea mammals.

Selenoneine ameliorates peroxide-induced oxidative stress in C. elegans

SCOPE

Selenoneine (2-selenyl-N_α, N_α, N_α-trimethyl-L-histidine), the selenium (Se) analogue of the ubiquitous thiol compound and putative antioxidant ergothioneine, is the major organic selenium species in several marine fish species. Although its antioxidant efficacy has been proposed, selenoneine has been poorly characterized, preventing conclusions on its possible beneficial health effects.

METHODS AND RESULTS

Treatment of Caenorhabditis elegans (C. elegans) with selenoneine for 18 h attenuated the induction of reactive oxygen and nitrogen species (RONS). However, the effect was not immediate, occurring 48 h post-treatment. Total Se and Se speciation analysis revealed that selenoneine was efficiently taken up and present in its original form directly after treatment, with no metabolic transformations observed. 48 h post-treatment, total Se in worms was slightly higher compared to controls and no selenoneine could be detected.

CONCLUSION

The protective effect of selenoneine may not be attributed to the presence of the compound itself, but rather to the activation of molecular mechanisms with consequences at more protracted time points.

Organ-specific differences in mercury speciation and accumulation across ringed seal (Phoca hispida) life stages

Methylmercury (MeHg) is a central nervous system toxicant and exposures can adversely affect the health of marine mammals. Mercuric selenide (HgSe) in marine mammal tissues is hypothesized to result from a protective detoxification mechanism, but toxicokinetic processes contributing to its formation are poorly understood. Here, new data is reported on speciated Hg concentrations in multiple organs of n = 56 ringed seals (Phoca hispida) from Labrador, Canada, and compare concentrations to previously published data from Greenland seals. A higher proportion of Hg is found to accumulate in the kidney of young-of-the-year (YOY) ringed seals compared to adults. A toxicokinetic model for Hg species is developed and evaluated to better understand factors affecting variability in Hg concentrations among organs and across life stages. Prior work postulated that HgSe formation only occurs in the liver of mature seals, but model results suggest HgSe formation occurs across all life stages. Higher proportions of HgSe in mature seal livers compared to YOY seals likely results from the slow accumulation and elimination of HgSe (total body half-life = 500 days) compared to other Hg species. HgSe formation in the liver reduces modeled blood concentrations of MeHg by only 6%. Thus, HgSe formation may not substantially reduce MeHg transport across the blood-brain barrier of ringed seals, leaving them susceptible to the neurotoxic effects of MeHg exposure.

Mercury Exposure: Protein Biomarkers of Mercury Exposure in Jaraqui Fish from the Amazon Region

This study presents data on the extraction and characterization of proteins associated with mercury in the muscle and liver tissues of jaraqui (Semaprochilodus spp.) from the Madeira River in the Brazilian Amazon. Protein fractionation was carried out by two-dimensional electrophoresis (2D-PAGE). Mercury determination in tissues, pellets, and protein spots was performed by graphite furnace atomic absorption spectrometry (GFAAS). Proteins in the spots that showed mercury were characterized by electrospray ionization tandem mass spectrometry (ESI-MS/MS). The highest mercury concentrations were found in liver tissues and pellets (426 ± 6 and 277 ± 4 μg kg^-1), followed by muscle tissues and pellets (132 ± 4 and 86 ± 1 μg kg^-1, respectively). Mercury quantification in the protein spots allowed us to propose stoichiometric ratios in the range of 1-4 mercury atoms per molecule of protein in the protein spots. The proteins characterized in the analysis by ESI-MS/MS were keratin, type II cytoskeletal 8, parvalbumin beta, parvalbumin-2, ubiquitin-40S ribosomal S27a, 39S ribosomal protein L36 mitochondrial, hemoglobin subunit beta, and hemoglobin subunit beta-A/B. The results suggest that proteins such as ubiquitin-40S ribosomal protein S27a, which have specific domains, possibly zinc finger, can be used as biomarkers of mercury, whereas mercury and zinc present characteristics of soft acids.

Biosynthesis and isolation of selenoneine from genetically modified fission yeast

Selenoneine was successfully purified from genetically modifiedSchizosaccharomyces pombegrown in selenate containing culture medium.

Characterization of mercury-binding proteins in human neuroblastoma SK-N-SH cells with immobilized metal affinity chromatography

Metal-binding proteins play important roles in biological functions of metals. However, only very limited mercury-binding proteins with high abundance were characterized in cells or organisms. Characterization of mercury-binding proteins in proteome-wide is important for elucidating mechanisms of mercury toxicity comprehensively. In this study, a method based on immobilized mercury ion affinity chromatography was developed for identification of putative mercury-binding proteins. The method was then successfully applied to profile mercury-binding proteins in human neuroblastoma SK-N-SH cells. In total, 38 proteins were identified as mercury-binding proteins, in which most of them were uncharacterized to associate with mercury in cells. The identified mercury-binding proteins did not show obvious relevance to protein abundance and were mainly involved in protein processing in endoplasmic reticulum, protein folding, and cytoskeleton organization. The newly built metalloproteomic approach provided valuable information on the possible molecular mechanisms and protein candidates for mercury transport and toxicity.

Remarkable differences in the biochemical fate of Cd2+, Hg2+, CH3Hg+ and thimerosal in red blood cell lysate

The application of a metallomics method revealed that all investigated Hg species bound to hemoglobin and that these interactions are of toxicological significance.

Natural Hg isotopic composition of different Hg compounds in mammal tissues as a proxy for in vivo breakdown of toxic methylmercury

In the last decade, specific attention has been paid to total mercury (HgT) stable isotopic composition, especially in natural samples such as aquatic organisms, due to its potential to track the cycle of this toxic element in the environment. Here, we investigated Hg Compound Specific stable Isotopic Composition (CSIC) of natural inorganic Hg (iHg) and methylmercury (MMHg) in various tissues of aquatic mammals (Beluga whale from the Arctic marine environment and seals from the freshwater lake Baikal, Russia). In seals' organs the variation in mass dependent fractionation (MDF, δ(202)Hg) for total Hg was significantly correlated to the respective fraction of iHg and MMHg compounds, with MMHg being enriched by ∼ 3‰ in heavier isotopes relative to iHg. On the other hand, we observe insignificant variation in Hg mass independent isotope fractionation (MIF, Δ(199)Hg) among iHg and MMHg in all organs for the same mammal species and MMHg in prey items. MIF signatures suggest that both MMHg and iHg in aquatic mammals have the same origin (i.e., MMHg from food), and are representative of Hg photochemistry in the water column of the mammal ecosystem. MDF signatures of Hg compounds indicate that MMHg is demethylated in vivo before being stored in the muscle, and the iHg formed is stored in the liver, and to a lesser extent in the kidney, before excretion. Thus, Hg CSIC analysis in mammals can be a powerful tool for tracing the metabolic response to Hg exposure.

Investigating the adduct formation of organic mercury species with carbonic anhydrase and hemoglobin from human red blood cell hemolysate by means of LC/ESI-TOF-MS and LC/ICP-MS

The interaction of mercury species with human erythrocytes is studied to investigate possible high molecular binding partners for mercury species. Human blood hemolysate was spiked with methylmercury and investigated by means of liquid chromatography (LC) coupled to electrospray ionization time of flight mass spectrometry (ESI-ToF-MS) and inductively coupled plasma mass spectrometry (ICP-MS). Beside adduct formation of mercury species with hemoglobin, the main compound of the erythrocytes, mercury binding to the enzyme carbonic anhydrase was revealed. Due to an enzymatic digest of the protein-mercury adduct, the binding site at the free thiol group of the protein was identified. These results indicate that carbonic anhydrase might play a role in mercury toxicity.

[Recent Epidemiological Studies on Methylmercury, Mercury and Selenium]

More than sixty years has passed since the outbreak of Minamata disease, and high-level methylmercury contaminations now seem nonexistent in Japan. However, mercury has been continuously discharged from natural sources and industrial activities, and the health effects on children susceptible to methylmercury exposure at low levels, in addition to mercury contamination from mercury or gold mining areas in developing countries, become a worldwide concern. In this article, we provide a recent overview of epidemiological studies regarding methylmercury and mercury. The following findings were obtained. (1) Many papers on exposure assessment of methylmercury/mercury have been published since the Minamata Convention on Mercury was adopted in 2013. (2) The most crucial problem is child developmental neurotoxicity resulting from prenatal exposure to methylmercury, but its precise assessment seems to be difficult because most of such effects are neither severe nor specific. (3) Several problems raised in birth cohort studies (e.g., whether IQ deficits due to prenatal methylmercury exposure remain when the children become adults, or whether the postnatal exposure at low levels also causes such adverse effects in children) remain unsolved. (4) Concurrent exposure models of methylmercury, lead, polychlorinated biphenyls, aresenic, and organochlorine pesticides, as well as possible antagonists such as polyunsaturated fatty acids and selenium, should be considered in the study design because the exposure levels of methylmercury are extremely low in developed countries. (5) Further animal experiments and molecular biological studies, in addition to human studies, are required to clarify the mechanism of methylmercury toxicity.

Synthesis, characterization, DFT calculations, and electrochemical comparison of novel iron(ii) complexes with thione and selone ligands

Characterization of Fe(ii)–thone and –selone complexes revealed that these ligands oxidize before Fe(ii), suggesting an iron-binding mechanism for similar antioxidants.

Specific Effects of Dietary Methylmercury and Inorganic Mercury in Zebrafish (Danio rerio) Determined by Genetic, Histological, and Metallothionein Responses

A multidisciplinary approach is proposed here to compare toxicity mechanisms of methylmercury (MeHg) and inorganic mercury (iHg) in muscle, liver, and brain from zebrafish (Danio rerio). Animals were dietary exposed to (1) 50 ng Hg g(-1), 80% as MeHg; (2) diet enriched in MeHg 10000 ng Hg g(-1), 95% as MeHg; (3) diet enriched in iHg 10000 ng Hg g(-1), 99% as iHg, for two months. Hg species specific bioaccumulation pathways were highlighted, with a preferential bioaccumulation of MeHg in brain and iHg in liver. In the same way, differences in genetic pattern were observed for both Hg species, (an early genetic response (7 days) for both species in the three organs and a late genetic response (62 days) for iHg) and revealed a dissimilar metabolization of both Hg species. Among the 18 studied genes involved in key metabolic pathways of the cell, major genetic responses were observed in muscle. Electron microscopy revealed damage mainly because of MeHg in muscle and also in liver tissue. In brain, high MeHg and iHg concentrations induced metallothionein production. Finally, the importance of the fish origin in ecotoxicological studies, here the seventh descent of a zebrafish line, is discussed.

Specific Pathways of Dietary Methylmercury and Inorganic Mercury Determined by Mercury Speciation and Isotopic Composition in Zebrafish (Danio rerio)

An original approach is proposed to investigate inorganic (iHg) and methylmercury (MeHg) trophic transfer and fate in a model fish, Danio rerio, by combining natural isotopic fractionation and speciation. Animals were exposed to three different dietary conditions: (1) 50 ng Hg g(-1), 80% as MeHg; (2) diet enriched in MeHg 10,000 ng Hg g(-1), 95% as MeHg, and (3) diet enriched in iHg 10,000 ng Hg g(-1), 99% as iHg. Harvesting was carried out after 0, 7, 25, and 62 days. Time-dependent Hg species distribution and isotopic fractionation in fish organs (muscle, brain, liver) and feces, exhibited different patterns, as a consequence of their dissimilar metabolization. The rapid isotopic re-equilibration to the new MeHg-food source reflects its high bioaccumulation rate. Relevant aspects related to Hg excretion are also described. This study confirms Hg isotopic fractionation as a powerful tool to investigate biological processes, although its deconvolution and fully understanding is still a challenge.

Ultrasensitive, rapid, and selective detection of mercury using graphene assisted laser desorption/ionization mass spectrometry

We report an extremely sensitive and specific detection of mercuric ions (Hg²⁺) based on graphene assisted laser desorption/ionization mass spectrometry (GALDI-MS). Combining the highly selective coordination interactions between thymine (T) and Hg²⁺, we present a simple, effective, and novel approach, based on π-π interactions of the T-Hg²⁺-T complex and G that can serve as a platform and matrix for GALDI-MS. The present sensor not only exhibits high selectivity and sensitivity (picomolar) to Hg²⁺ in aqueous solution, but also can elucidate the chemical structures of the metal complexes. The significant advantage in the current approach is that there is no need for a sophisticated instrument, and no sample pretreatment is required to detect the Hg²⁺ ions.