IS OCEAN FISH CONSUMPTION ASSOCIATED WITH CAUSING MERCURY TOXICITY OR PREVENTING IT?
Selenium (Se), is an essential trace nutrient that is required for synthesis of selenocysteine, the 21st genetically encoded amino acid. Selenocysteine is required for the functions of ~25 genetically and functionally unique enzymes (selenoenzymes) that are expressed in human tissues; many with activities vital for brain function, health, and development. Mercury (Hg) is the only environmental insult known to cross placental and blood brain barriers and irreversibly inhibit selenoenzyme activities. Maternal exposures to methyl-Hg, (MeHg) can be especially harmful because MeHg can sequester Se in maternal tissues, thus limiting Se transport to the fetus. Studies indicate that the toxic effects of high MeHg exposures can be prevented by increasing dietary Se intakes, including those from eating Se-rich ocean fish. MeHg exposure from fish consumption is proportional to risk only in cases where MeHg concentration occurs in molar excess of selenium (Se). In this study, weanling male Long Evans rats were fed 0.2 µmol Se/kg diets for 5 weeks to deplete their Se body stores, making them highly dependent on dietary Se to support growth and brain selenoenzyme synthesis. Following the depletion period, rats were weighed and pseudo-randomly assigned to diets that contained either 0.2, 1.0, or 10 µmol Se/kg or similar amounts of Se from delipidated fish protein and were supplemented with either 0 or 40 µmol MeHg/kg and fed ab libitum for 5 weeks. During the course of the study, neurotoxicity was only noted in rats fed diets containing 0.2 µmol Se with 40 µmol MeHg. At the end of the study, brain Hg, Se, and selenoenzyme activities of thioredoxin reductase (TRx) glutathione peroxidase (GPx) were assessed along with F2-isoprostane (an indicator of oxidative damage). In animals with low dietary Se intakes, brain TRx and GPx activities were severely impaired by MeHg exposures, but increasing dietary Se from delipidated ocean fish protein increasingly counteracted the inhibitory effects of high MeHg exposures. Oxidative damage as indicated by F2-isoprostane concentrations in the brains of MeHg exposed rats was inversely related to brain GPx and TRx, strongly indicating that the oxidative damage known to be associated with high MeHg exposures is due to loss of selenoenzyme activities. This study found that dietary HBV is a superior index of risks due to high MeHg exposures. Because the HBV provides a direct reflection of the bimolecular reaction mechanisms associated with MeHg toxicity, it provides a more reliable approach to risk assessment than seafood safety criteria based on MeHg exposures alone.
METALLOMIC QUANTIFYING METALS IN METALLOPROTEINS: THE LEVELS OF HG AND SE BINDING TO SERUM SELENOPROTEINS FROM METHYLMERCURY-POISONED RATS AFTER SE TREATMENT
Metalloproteins - proteins containing metal atoms or clusters -, accounting for nearly half of all proteins in nature are involved in a wide range of important biological processes, like photosynthesis, respiration, water oxidation, molecular oxygen reduction, nitrogen fixation and metal homeostasis. Proteomic approach has been widely used in the characterization of metalloproteins, however, the in situ quantification of metals in metalloproteins remains a challenge. Metallomics is an emerging field addressing the role, uptake, transport and storage of trace metals in biological systems, which provides complementary tools for metalloproteomics.
It has been known for decades that selenium (Se) could antagonize the toxicity of mercury (Hg), both in inorganic (Hg2+) and organic (i.e. methylmercury, MeHg) forms in different organisms when co-exposed, but the mechanism underlying this antagonism is still not clear .It was proposed that selenite could react with Hg2+ in blood to form a Hg-Se-selenoprotein P (Sel P) complex, however, it is not known if such kind of complex will also form when selenite meet methylmercury (MeHg) in blood stream. Besides, it is not known how much Hg and Se in serum were bound to selenoproteins.
In this study, selenite was supplemented for 30 days in methylmercury-poisoned rats. Blood samples were collected and the contents of Se and Hg in serum were measured using inductively coupled plasma mass spectrometry (ICP-MS). Affinity chromatography (AF) coupled to ICP-MS was used to find out the Hg, Se binding selenoproteins in serum samples. Post-column isotope dilution analysis (IDA) using both enriched 78Se and 199Hg as spike was applied to quantify the levels of Hg and Se binding to selenoproteins. It was found that both Hg and Se were mainly bound to Sel P in serum in MeHg-poisoned rats after selenite treatment. Se promoted Hg to bind to Sel P in MeHg-poisoned rats, suggesting that Sel P plays an important role in fighting against the toxicity of MeHg.
SELENIUM PROTECTS AGAINST TOXIC EFFECTS OF MERCURY ON CARDIOVASCULAR HEALTH AMONG INUIT IN CANADA
Background: Selenium (Se) has been reported to protect against the neurotoxicity of mercury (Hg). However, the effect of Se against Hg on cardiovascular diseases remains unclear. Inuit living in the Arctic have high exposure to both Se and Hg through their marine mammal and fish rich traditional diet.
Objective: To characterize the co-exposure of Hg and Se among Inuit in Canada and to assess the interaction between Hg and Se exposure and its effect on cardiovascular health outcomes, such as stroke and hypertension.
Methods: Data was collected from the International Polar Year Inuit Health Survey (IHS) conducted in 2007 and 2008. Blood Se and Hg were measured, and self-report cardiovascular health outcomes were collected through a questionnaire interview from 2169 adults aged 18 and above.
Results: The mean age was 42.4 years, and 38.7% of the participants were male. The geometric means (GM) of blood Se and total Hg were 319.5 µg/L and 7.0 µg/L, respectively. The crude prevalence of heart attack, stroke and hypertension were 3.55%, 2.36%, and 24.47% respectively. Participants were categorized into 4 exposure groups according to blood Hg (high: ≥ 7.8 µg/L; low: < 7.8 µg/L), and Se (high: ≥ 280 µg/L; low: < 280 µg/L). The odds ratio (OR) of cardiovascular outcomes were estimated using general linearized models. Results showed the low Se and high Hg group had a higher prevalence of cardiovascular disease (OR=1.76 for hypertension, 1.57 for stroke, and 1.26 for myocardial infarction (MI)). However, the prevalence was decreased in both the high Se and low Hg group (OR=0.57 for hypertension, 0.44 for stroke, and 0.27 for MI) and the high Se and high Hg group (OR=1.14 for hypertension, 0.31 for stroke, and 0.80 for MI).
Conclusions: The high Se and low Hg group had the lowest prevalence of cardiovascular outcomes, except for stroke. These results suggest that Se exhibits a protective effect on cardiovascular disease through interaction with Hg.
SELENIUM HEALTH BENEFIT VALUES ARE A MORE RELIABLE INDEX OF MERCURY ASSOCIATED RISKS
Seafood safety evaluations regarding methylmercury (MeHg) are currently based on fish MeHg contents. Although this provides an accurate index of MeHg exposures, it fails to provide an accurate indication of risk. Studies of maternal MeHg exposures from eating certain types of highly predatory sharks or whales find subtle dose-dependent adverse effects on their children. However, studies involving consumption of typical varieties of ocean fish do not support the premise that maternal MeHg exposures result in adverse child outcomes. Instead, ocean fish consumption is directly associated with improved neurodevelopment. Exposures to MeHg has been found to be proportional to risk only in cases where MeHg concentration occurs in molar excess of selenium (Se) in the seafood consumed i.e., meats of highly predatory sharks or whales. Since MeHg is an irreversible inhibitor of Se-dependent enzymes (selenoenzymes), studies indicate that the toxic effects of high MeHg exposures can be prevented by increasing dietary Se, including Se-rich ocean fish. Selenoenzymes are normally present in all vertebrate cells, but are most important in brain and endocrine tissues where, among other functions, they prevent and reverse oxidative damage. However, these vital activities are lost when consumption of foods with disproportionately high MeHg contents transiently result in MeHg-dependent inhibition of selenoenzymes. The Se-Health Benefit Value (HBV) criterion is a predictive indicator of dose-effect relationships between MeHg and Se concentrations in the fish or seafood. Negative HBV's predict relative risks (e.g., pilot whale; -83) while consumption of increasing amounts of ocean fish with positive values (e.g., yellowfin tuna; 15.7 ±3.4) predict benefits. Epidemiological studies which report adverse outcomes in children have involved MeHg exposures from eating seafood’s with negative HBV's. Although increasing ocean fish consumption increases maternal and fetal MeHg exposures, health benefits have been consistently observed when the seafoods have positive HBVs. Therefore, the HBV is a seafood safety indicator that reflects the beneficial nutrients provided by fish while allowing consideration of MeHg risks when present. Studies performed in-vitro, in cell culture and laboratory animal models, as well as epidemiological studies coincide in confirming that HBV-dependent predictions are superior to predictions based on MeHg exposures alone. Fish and background diets consumed by human populations can vary widely in Se content, and thus influence health outcomes observed in association with MeHg exposures. Epidemiological studies are likely to obtain more consistently reliable results if they consider dietary Se intakes as well as MeHg exposures in relation to health outcomes.
CHRONIC EXPOSURE TO LOW-DOSE METHYLMERCURY (MEHG) INDUCES DIFFERENTIAL ACCUMULATION AND ASSOCIATED PROTEOME CHANGES IN DIFFERENT REGIONS OF MAMMALIAN BRAINS
Human exposes to MeHg mainly through consumption of contaminated seafood. Paresthesia and numbness of the skin are some of the earliest syndromes of acute MeHg poisoning. As the MeHg load keeps increasing, other syndromes like dystaxia, dysarthria and visual impairment will follow. The sequential occurrence of pathological syndromes indicates that different regions of the brain respond differently to MeHg. In our previous study, we have observed that different amounts of Hg was loaded into different regions of the rat brain after chronic exposure to low dose of MeHg (40 µg/ kg bodyweight/ day) for 12 weeks. Comparative proteomic analysis revealed that there was a dramatic proteome changes in the somatosensory cortex upon chronic MeHg exposure. Proteins related to glycolysis, ATP production, neurotransmission, and protein synthesis were down-regulated in the somatosensory cortex, resulting in a metabolic deficit without observable abnormality phenotypically. In contrary to results obtained from the somatosensory cortex, we found different sets of differentially expressed proteins in the other regions. In the motor cortex, visual cortex and the cerebellum, some proteins involved in synaptic transmission and glycolysis (like synapsin and aldolase) were up-regulated. Beside rats, Hg loads and associated proteome changes in brain tissues from other top marine consumers including ringed seals and polar bears were also examined. In general, proteins related to glycolysis, ATP production, and neurotransmission showed differential expressions upon chronic MeHg exposure. Interestingly, only a small fraction of the proteomes was affected in the frontal and occipital lobe of polar bear while relatively more proteins showed differential expressions in polar bear cerebellum. In ringed seals, differences in proteome changes across frontal lobe, occipital lobe and cerebellum were also observed. Based on the current findings, neurotransmission, glycolysis and other metabolism involved in energy production in mammalian brains were primarily affected by chronic MeHg exposure. From our results, it is also obvious that different regions of the brain showed different proteome changes and this may account for the sequential occurrence of different pathological syndromes in human suffered from acute and increasing MeHg loads.
MERCURY AND THE IMMUNE SYSTEM: WHAT IS THE CONTRIBUTION OF IMMUNOTOXIC MECHANISMS TO MERCURY TOXICITY
Mercury compounds, including elemental (Hg°), inorganic (iHg), and organic forms (including alkyl, methyl, and ethyl Hg), have effects on the immune system as demonstrated in experimental, cellular, ecotoxicological and epidemiological studies. However, the role of immunotoxicity in the major health outcomes associated with exposures to these mercury compounds is at present unclear. Inconsistencies among studies may have reduced attention to this question. However, the lack of concordance may be due to the importance of interactions between mercury compounds and other risk factors in eliciting these adverse outcomes. For example, suppression of primary immune responses may be manifest in populations exposed to an infectious pathogen or vaccination; induction of autoimmune responses may be manifest in populations exposed to an unrelated triggering event such as a virus; neurotoxicity may involve viral infection or other pyrogens. Under these conditions, serious clinical outcomes may be observed, including increased morbidity and recurrence of malaria; failure of HBV vaccinations; greater severity of autoimmune myocarditis; and structural disruption of neurodevelopmental programming.
This model suggests that mercury may be a necessary but not sufficient risk factor for outcomes involving immunotoxicity and other stressors This hypothesis has implications for both mechanistic models and the design of epidemiological and ecotoxicological studies since the observation of adverse effects may depend in most but not all cases on mercury exposure and concurrent infections or immunosuppressive medications, the host immunogenome, sexual dimorphism of immune function, all of which are likely to be important modifiers of risk and outcome.
FACTORS AFFECTING MERCURY EXPOSURE FROM FISH
Studies of mercury in blood and hair provide a direct assessment of mercury exposure and are often linked with estimates of fish consumption frequency. These studies provide a basis to compare estimates of mercury intake based on fish consumption rates and mercury concentrations; however, estimates of fish consumption rates are difficult to measure and are highly variable. Estimates of mercury exposure also should consider ratios of total mercury (THg) to methylmercury (meHg), bioavailability of fish mercury, and impacts of cooking on fish concentrations. Studies conducted around the world in the past ten years provide a basis for assessing fish consumption-based risk estimates. In some cases biomarker data indicated exposures much less than predicted, suggesting lower than assumed fish consumption, fish mercury concentrations, mercury bioavailability, or a combination of factors. MeHg concentrations typically are predicted based on THg concentrations. United States Environmental Protection Agency (USEPA), Health Canada and the European Food Safety Authority apply a factor of 1.0-0.95 (unitless) to convert total mercury levels in fish tissue samples into meHg levels, but our compilation of recent studies indicates a range of 61-85% may be more representative. Mercury concentrations are expected to increase during cooking because mercury is concentrated in muscle rather than in water and fat that may be lost during cooking. Recently, USEPA applied an adjustment factor of 1.5 to mercury concentrations in raw fish when estimating mercury doses from cooked fish portions. Recent studies suggest that while high heat cooking methods such at grilling result in cooking factors of 1.4-1.5, lower heat methods such as steaming, roasting or frying fish result in little or no increase in mercury concentrations. Recent studies also indicate that mercury absorption from fish meals may be much lower than the 95% assumed by North American and European regulatory authorities, especially when testing cooked fish. The practical impact of these factors is that larger amounts of fish may be safely consumed than currently is assumed. For example, for raw fish with 0.3 ug/g THg, the default assumptions (THg:meHg = 0.95, cooking factor = 1.5, bioavailability = 95%) lead to a conclusion that 2.5 meal/month may be safely consumed, whereas reasonable alternate assumptions (THg:meHg = 0.8, cooking factor = 1.3, bioavailability = 80%) yield an estimate that it is safe to consume 4 meals/month. Considering the nutritional benefits of fish consumption, understanding the reliability of such estimates is critical to providing appropriate guidance to fish consumers.
MITIGATING THE HUMAN HEALTH IMPACTS OF HYDROELECTRIC DEVELOPMENT IN CANADA
Developing hydroelectric plants in circumpolar regions is a part of the energy strategy of many northern countries; in Canada alone there are multiple existing plants and 22 facilities are currently under construction or are planned to be built. Flooding, including the flooding associated with hydroelectric developments, has been shown to increase microbial production of methylmercury (MeHg), a known neurotoxin which bioaccumulates in country foods (e.g. fish) and the humans and wildlife (e.g. marine mammals) who consume them. Most of these existing, new or planned hydrolectric projects are located in the vicinity of indigenous communities, who are frequent consumers of country foods, including fish. Health Canada has established its tolerable daily intakes (TDI) for methylmercury at 0.2 ug/kg bw per day for children and women of childbearing age and at 0.47 ug/kg bw per day for the general population. Analysis of human samples (usually hair or blood) can provide information on baseline MeHg levels in indigenous communities, which in some cases are already elevated. Consumption surveys allow for estimation of daily intakes and development of consumption advisories, but due to the inherent uncertainty in these surveys, they may not be the most effective tool in limiting exposure to MeHg. In order to provide meaningful advice on the safety of consuming country foods (fish and marine mammals) from these reservoirs and adjacent water bodies further study in the area of MeHg production and transfer in newly flooded areas is needed, in addition to more information on the short-term toxicokinetics of MeHg. This presentation studies the ways MeHg intake is currently being mitigated, explores the new research being done in the area of MeHg mitigation, and examines the risks and benefits of country foods consumption by indigenous communities in Canada and elsewhere.