Biomarkers of adverse response to mercury: histopathology versus thioredoxin reductase activity

Mercury-induced toxicity: Mechanisms, molecular pathways, and gene regulation

Mercury is a well-known neurotoxicant for humans and wildlife. The epidemic of mercury poisoning in Japan has clearly demonstrated that chronic exposure to methylmercury (MeHg) results in serious neurological damage to the cerebral and cerebellar cortex, leading to the dysfunction of the central nervous system (CNS), especially in infants exposed to MeHg in utero. The occurrences of poisoning have caused a wide public concern regarding the health risk emanating from MeHg exposure; particularly those eating large amounts of fish may experience the low-level and long-term exposure. There is growing evidence that MeHg at environmentally relevant concentrations can affect the health of biota in the ecosystem. Although extensive in vivo and in vitro studies have demonstrated that the disruption of redox homeostasis and microtube assembly is mainly responsible for mercurial toxicity leading to adverse health outcomes, it is still unclear whether we could quantitively determine the occurrence of interaction between mercurial and thiols and/or selenols groups of proteins linked directly to outcomes, especially at very low levels of exposure. Furthermore, intracellular calcium homeostasis, cytoskeleton, mitochondrial function, oxidative stress, neurotransmitter release, and DNA methylation may be the targets of mercury compounds; however, the primary targets associated with the adverse outcomes remain to be elucidated. Considering these knowledge gaps, in this article, we conducted a comprehensive review of mercurial toxicity, focusing mainly on the mechanism, and genes/proteins expression. We speculated that comprehensive analyses of transcriptomics, proteomics, and metabolomics could enhance interpretation of "omics" profiles, which may reveal specific biomarkers obviously correlated with specific pathways that mediate selective neurotoxicity.

Lipid-extracted muscle and liver tissues: Can they reveal mercury exposure of pelagic sharks?

Pelagic sharks are apex predators in oceanic ecosystems and tend to accumulate high amounts of mercury (Hg). The conventional method for assessing Hg exposure in sharks involves analyzing tissue samples without any chemical treatment. However, a substantial number of chemically treated tissue samples are still being preserved in laboratories or museums. It is critical to maximize the utilization of existing samples to reduce the need for additional sampling of pelagic sharks, especially endangered species. Lipid extraction is a widely employed pretreatment process for carbon isotope analysis in shark trophic ecology, while its impact on Hg quantification remains uncertain. Here, we evaluated the feasibility of using lipid-free muscle and liver tissues for investigation of Hg exposure in four endangered pelagic sharks inhabiting the eastern Pacific, including bigeye thresher (Alopias superciliosus), pelagic thresher (A. pelagicus), blue shark (Prionace glauca) and silky shark (Carcharhinus falciformis). Results showed that total Hg concentrations (THg) differed between untreated (THgbulk) and lipid-free (THglipid-free) samples for each tissue type of each species. In addition, dichloromethane-methanol extractions significantly altered the amount of Hg. This may result from the removal of lipoprotein compounds that vary between tissues and species. The THgbulk can be calculated by THglipid-free using the following formulas, THgbulk = 1.14 × THglipid-free + 0.30 and THgbulk = 0.33 × THglipid-free + 0.18, for muscle and liver tissues, respectively. These findings emphasize the applications of lipid-free tissues in THg analysis. This study may have important implications for improving evaluation of Hg exposure in endangered pelagic sharks.

Selenium and Redox Enzyme Activity in Pregnant Women Exposed to Methylmercury

Selenium (Se) is a micronutrient with essential physiological functions achieved through the production of selenoproteins. Adequate Se intake has health benefits and reduces mercury (Hg) toxicity, which is important due to its neurotoxicity. This study determined the Se status and redox enzyme, including selenoproteins', activity in pregnant women highly exposed to Hg (between 1 to 54 µg Hg/L blood) via fish consumption. A cross-sectional study enrolling 513 women between the first and third trimester of pregnancy from Madeira, Portugal was conducted, encompassing collection of blood and plasma samples. Samples were analyzed for total Se and Hg levels in whole blood and plasma, and plasma activity of redox-active proteins, such as glutathione peroxidase (GPx), thioredoxin reductase (TrxR) and thioredoxin (Trx). Enzyme activities were related to Se and Hg levels in blood. Se levels in whole blood (65.0 ± 13.1 µg/L) indicated this population had a sub-optimal Se status, which translated to low plasma GPx activity (69.7 ± 28.4 U/L). The activity of TrxR (12.3 ± 5.60 ng/mL) was not affected by the low Se levels. On the other hand, the decrease in Trx activity with an increase in Hg might be a good indicator to prevent fetal susceptibility.

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.

Inorganic mercury effects on biomarker gene expressions of a freshwater amphipod at two temperatures

Mercury (Hg) is a global contaminant resulting of both natural processes and human activities. In aquatic environments, studies conducted on vertebrates highlighted changes of gene expression or activity of antitoxic and oxidative enzymes. However, although Hg is a highly toxic compound in aquatic environments, only a few studies have evaluated the lethal and sublethal effects of inorganic Hg on Gammarus sp. Therefore, this study aimed at evaluating the effects of inorganic Hg (HgCl₂) on the expression of 17 genes involved in crucial biological functions or mechanisms for organisms, namely respiration, osmoregulation, apoptosis, immune and endocrine system, and antioxidative and antitoxic defence systems. The study was performed in males of the freshwater amphipod Gammarus pulex exposed to two environmentally relevant concentrations (50 and 500 ng/L) at two temperature regime fluctuations (16 °C and 20 °C +/-2 °C) for 7 and 21 days. Results showed that G. pulex mortality was dependent on Hg concentration and temperature; the higher the concentration and temperature, the higher the mortality rate. In addition, the Integrated Biomarker Response emphasized that HgCl₂ toxicity was dependent on the concentration, time and temperature of exposure. Overall, antioxidant and antitoxic defences, as well as the endocrine and immune systems, were the biological functions most impacted by Hg exposure (based on the concentration, duration, and temperature tested). Conversely, osmoregulation was the least affected biological function. The results also demonstrated a possible adaptation of G. pulex after 21 days at 500 ng/L, regardless of the exposure temperature. This study allowed us to show that Hg deregulates many crucial biological functions after a short exposure, but that during a long exposure, an adaptation phenomenon could occur, regardless of temperature.

Bioremediation by Cupriavidus metallidurans Strain MSR33 of Mercury-Polluted Agricultural Soil in a Rotary Drum Bioreactor and Its Effects on Nitrogen Cycle Microorganisms

Nitrogen cycle microorganisms are essential in agricultural soils and may be affected by mercury pollution. The aims of this study are to evaluate the bioremediation of mercury-polluted agricultural soil using Cupriavidus metallidurans MSR33 in a rotary drum bioreactor (RDB) and to characterize the effects of mercury pollution and bioremediation on nitrogen cycle microorganisms. An agricultural soil was contaminated with mercury (II) (20-30 ppm) and subjected to bioremediation using strain MSR33 in a custom-made RDB. The effects of mercury and bioremediation on nitrogen cycle microorganisms were studied by qPCR. Bioremediation in the RDB removed 82% mercury. MSR33 cell concentrations, thioglycolate, and mercury concentrations influence mercury removal. Mercury pollution strongly decreased nitrogen-fixing and nitrifying bacterial communities in agricultural soils. Notably, after soil bioremediation process nitrogen-fixing and nitrifying bacteria significantly increased. Diverse mercury-tolerant strains were isolated from the bioremediated soil. The isolates Glutamicibacter sp. SB1a, Brevundimonas sp. SB3b, and Ochrobactrum sp. SB4b possessed the merG gene associated with the plasmid pTP6, suggesting the horizontal transfer of this plasmid to native gram-positive and gram-negative bacteria. Bioremediation by strain MSR33 in an RDB is an attractive and innovative technology for the clean-up of mercury-polluted agricultural soils and the recovery of nitrogen cycle microbial communities.

Oxidative Stress Biomarkers in Erythrocytes of Captive Pre-Juvenile Loggerhead Turtles Following Acute Exposure to Methylmercury

This study describes the use of erythrocytes (RBCs) of loggerhead turtles as in vitro models for evaluating their toxicity to methylmercury. Blood samples of loggerhead turtles that were born in the Colombian Caribbean were used. The LC50 of RBCs to methylmercury was determined at 96 h using methylmercury concentrations of 0.5–100 mg L−1. Next, the viability of the RBCs and the activity of the enzymes superoxide dismutase (SOD), glutathione S-transferase (GST), and lipid peroxidation by malondialdehyde (MDA) at 6 and 12 h of exposure to acute concentrations of 0, 1, and 5 mg L−1 were evaluated. The LC50 for loggerhead turtle RBCs was 8.32 mg L−1. The cell viability bioassay of RBCs exposed for 12 h only showed 100% cell viability. Increasing in vitro MeHg concentrations caused a corresponding increase in MDA concentration as well as decreases in the activities of SOD and GST. The RBCs represent an excellent model for ecotoxicological studies and SOD, GST, and MDA are biomarkers of environmental pollution and oxidative stress in loggerhead turtles. This was the first study conducted on loggerhead turtle where the response of RBCs to MeHg-induced oxidative stress is evaluated.

Thermal stress accelerates mercury chloride toxicity in Oreochromis niloticus via up-regulation of mercury bioaccumulation and HSP70 mRNA expression

Mercury (Hg) is an environmental pollutant that threatens aquatic life. Many environmental factors, including water temperature, are reported to influence the toxicity of dissolved chemicals in the aquatic ecosystem. Therefore, we investigated the impact of thermal stress on Hg-induced subchronic toxicity in Nile tilapia (Oreochromis niloticus). Fish were randomly allocated into five groups. Group I served as the control and kept at 25 °C. Groups II, III, IV, and V were reared at 25, 28, 31, and 34 °C, respectively, and co-exposed to HgCl₂ (1/10 LC₅₀) for 42 days. Blood and tissue samples were collected after 21 and 42 days. All HgCl₂-exposed groups exhibited significant elevations in serum levels of alanine aminotransferase (ALT), aspartate aminotransferase (AST), urea, and creatinine, along with decreases in the serum total protein and albumin. In addition, marked reductions in antioxidant enzymes, including superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GSPx), were observed. Remarkable increases in Hg tissue concentrations were detected along with increases in heat shock protein (HSP) 70 mRNA expression. Interestingly, the patterns data that were recorded were more coincident with the water temperature than the period of exposure. In conclusion, water temperature and exposure period are two crucial factors modulating HgCl₂-induced toxicity and bioaccumulation in Nile tilapia. Our findings provide new insights concerning the impact of thermal stress as an environmental factor on Hg toxicity and bioaccumulation in Nile tilapia and, in turn, on fish and fish consumer health.

Deleterious effects of chronic mercury exposure on in vitro LTP, memory process, and oxidative stress

Heavy metal contamination in aquatic environments plays an important role in the exposure of humans to these toxicants. Among these pollutants, mercury (Hg) is one main concern due to its high neurotoxicity and environmental persistence. Even in low concentrations, Hg bioaccumulation is a major threat to human health, with higher impact on populations whose diet has fish as chief consumption. Mercury compounds have high affinity for neuronal receptors and proteins, which gives Hg its cumulative feature and have the ability to cross cell membranes and blood-brain barrier to show their neurotoxicity. Intoxication with Hg increases levels of reactive oxygen species (ROS), thus depleting faster the resource of antioxidant proteins. To evaluate Hg-induced hippocampal ROS production, synaptic plasticity, anxiety, and memory, a total of 11 male Wistar rats were exposed to HgCl₂ (Hg₃₀ group) to produce a residual concentration of 8 ng/mL at the end of 30 days. Behavioral tests (plus-maze discriminative avoidance task), in vitro electrophysiology, and ROS assays were performed. Western blot assay showed decreased levels of antioxidant proteins GPx and SOD in Hg₃₀ group. Increased ROS production was observed in the CA1 and CA3 regions in the Hg-exposed group. Plus-maze task detected long-term memory impairment in Hg₃₀ group, linked to poorer in vitro long-term potentiation as compared to control group. Hg intoxication also promoted higher anxiety-like behavior in the exposed animals. In conclusion, our data suggests that low doses of HgCl₂ resulted in impaired long-term memory and unbalance between decreased antioxidant protein expression and increased ROS production in the hippocampus.

The thioredoxin system as a target for mercury compounds

BACKGROUND

Mercury interaction with selenium in vivo has been recognized for >50 years. Several researchers attempted to use selenium to mitigate the detrimental effects of mercurial compounds but the results were controversial. Selenium pools in living organisms are quite low and the high affinity of mercury to bind selenols pointed out selenoproteins as possible targets of toxicity. Such was the case of the selenoenzyme thioredoxin reductase (TrxR) which is an integrant part of the thioredoxin system. Given the important role of this redox system for cellular functioning and the high affinity of mercury for TrxR's active site, this interaction can be key to understand the mechanism by which Hg causes cell death.

SCOPE OF THE REVIEW

This review discusses the current state of knowledge concerning the interaction between mercury compounds and the thioredoxin system, its implications for the development of toxicity and the effects of selenium co-exposure.

MAJOR CONCLUSIONS

The mechanism of toxicity of mercurials is a complex chain of events starting with inhibition of the selenoenzyme, TrxR. Selenium supplementation protects TrxR from the toxicity of inorganic forms of mercury (i.e., Hg(II)) to a certain extent, but not from methylmercury. When TrxR is inhibited, thioredoxin is reduced by alternative mechanisms involving glutathione and glutaredoxin and only when this pathway is hampered does cell death occur.

GENERAL SIGNIFICANCE

Understanding the molecular mechanism of mercury toxicity and the mechanisms of enzymatic compensation allows the design of mitigation strategies and, since TxrR and Trx exist in the plasma, puts forward the possibility for future use of changes in activity/expression of these enzymes as biomarkers of mercury toxicity, thus refining the risk assessment process.

Toxicodynamics of Lead, Cadmium, Mercury and Arsenic- induced kidney toxicity and treatment strategy: A mini review

Environmental pollution has become a concerning matter to human beings. Flint water crisis in the USA pointed out that pollution by heavy metal is getting worse day by day, predominantly by Lead, Cadmium, Mercury and Arsenic. Despite of not having any biological role in flora and fauna, they exhibit detrimental effect following exposure (acute or chronic). Even at low dose, they affect brain, kidney and heart. Oxidative stress has been termed as cause and effect in heavy metal-induced kidney toxicity. In treatment strategy, different chelating agent, vitamins and minerals are included, though chelating agents has been showed different fatal drawbacks. Interestingly, plants and plants derived compounds had shown possible effectiveness against heavy metals induced kidney toxicity. This review will provide detail information on toxicodynamics of Pb, Cd, Hg and As, treatment strategy along with the possible beneficiary role of plant derived compound to protect kidney.

Neurodevelopmental Effects of Mercury

The toxicology of mercury (Hg) is of concern since this metal is ubiquitously distributed in the environment, and living organisms are routinely exposed to Hg at low to high levels. The toxic effects of Hg are well studied and it is known that they may differ depending on the Hg chemical species. In this chapter, we emphasize the neurotoxic effects of Hg during brain development. The immature brain is more susceptible to Hg exposure, since all the Hg chemical forms, not only the organic ones, can harm it. The possible consequences of Hg exposure during the early stages of development, the additive effects with other co-occurring neurotoxicants, and the known mechanisms of action and targets will be addressed in this chapter.

The role of the thioredoxin/thioredoxin reductase system in the metabolic syndrome: towards a possible prognostic marker?

Mammalian thioredoxin reductase (TrxR) is a selenoprotein with three existing isoenzymes (TrxR1, TrxR2, and TrxR3), which is found primarily intracellularly but also in extracellular fluids. The main substrate thioredoxin (Trx) is similarly found (as Trx1 and Trx2) in various intracellular compartments, in blood plasma, and is the cell's major disulfide reductase. Thioredoxin reductase is necessary as a NADPH-dependent reducing agent in biochemical reactions involving Trx. Genetic and environmental factors like selenium status influence the activity of TrxR. Research shows that the Trx/TrxR system plays a significant role in the physiology of the adipose tissue, in carbohydrate metabolism, insulin production and sensitivity, blood pressure regulation, inflammation, chemotactic activity of macrophages, and atherogenesis. Based on recent research, it has been reported that the modulation of the Trx/TrxR system may be considered as a new target in the management of the metabolic syndrome, insulin resistance, and type 2 diabetes, as well as in the treatment of hypertension and atherosclerosis. In this review evidence about a possible role of this system as a marker of the metabolic syndrome is reported.

Possible Mechanisms of Mercury Toxicity and Cancer Promotion: Involvement of Gap Junction Intercellular Communications and Inflammatory Cytokines

A number of observations indicate that heavy metals are able to alter cellular metabolic pathways through induction of a prooxidative state. Nevertheless, the outcome of heavy metal-mediated effects in the development of human diseases is debated and needs further insights. Cancer is a well-established DNA mutation-linked disease; however, epigenetic events are perhaps more important and harmful than genetic alterations. Unfortunately, we do not have reliable screening methods to assess/validate the epigenetic (promoter) effects of a physical or a chemical agent. We propose a mechanism of action whereby mercury acts as a possible promoter carcinogen. In the present contribution, we resume our previous studies on mercury tested at concentrations comparable with its occurrence as environmental pollutant. It is shown that Hg(II) elicits a prooxidative state in keratinocytes linked to inhibition of gap junction-mediated intercellular communication and proinflammatory cytokine production. These combined effects may on one hand isolate cells from tissue-specific homeostasis promoting their proliferation and on the other hand tamper the immune system defense/surveillance checkmating the whole organism. Since Hg(II) is not a mutagenic/genotoxic compound directly affecting gene expression, in a broader sense, mercury might be an example of an epigenetic tumor promoter or, further expanding this concept, a “metagenetic” effector.

Effect of thimerosal on thyroid hormones metabolism in rats

Mercury seems to exert an inhibitory effect on deiodinases, but there are few studies using Thimerosal (TM) as the mercury source. We aimed to elucidate the effect of TM on thyroid hormones peripheral metabolism. Adult Wistar female rats received 0.25 µg or 250 µg TM/100 g BW, IM, twice a week, for a month. We evaluated serum total T₃ and T₄, D1 activity using ¹²⁵I-rT₃ as tracer, and D2 activity using ¹²⁵I-T₄ NADPH oxidase activity was measured by Amplex-red/HRP method and mRNA levels by real time PCR. Serum T₄ was increased and T₃ decreased by the greatest dose of TM. Even though D1 activity in pituitary and kidney was reduced by the highest dose of TM, hepatic D1 activity and D1 mRNA levels remained unchanged. D2 activity was also significantly decreased by the highest dose of TM in all CNS samples tested, except cerebellum, but D2 mRNA was unaltered. mRNA levels of the tested NADPH oxidases were not affected by TM and NADPH oxidase activity was either unaltered or decreased. Our results indicate that TM might directly interact with deiodinases, inhibiting their activity probably by binding to their selenium catalytic site, without changes in enzyme expression.

Biomarkers of mercury toxicity: Past, present, and future trends

Mercury (Hg) toxicity continues to represent a global health concern. Given that human populations are mostly exposed to low chronic levels of mercurial compounds (methylmercury through fish, mercury vapor from dental amalgams, and ethylmercury from vaccines), the need for more sensitive and refined tools to assess the effects and/or susceptibility to adverse metal-mediated health risks remains. Traditional biomarkers, such as hair or blood Hg levels, are practical and provide a reliable measure of exposure, but given intra-population variability, it is difficult to establish accurate cause-effect relationships. It is therefore important to identify and validate biomarkers that are predictive of early adverse effects prior to adverse health outcomes becoming irreversible. This review describes the predominant biomarkers used by toxicologists and epidemiologists to evaluate exposure, effect and susceptibility to Hg compounds, weighing on their advantages and disadvantages. Most importantly, and in light of recent findings on the molecular mechanisms underlying Hg-mediated toxicity, potential novel biomarkers that might be predictive of toxic effect are presented, and the applicability of these parameters in risk assessment is examined.

Sex- and structure-specific differences in antioxidant responses to methylmercury during early development

Methylmercury (MeHg) is a ubiquitous environmental contaminant and neurotoxin, particularly hazardous to developing and young individuals. MeHg neurotoxicity during early development has been shown to be sex-dependent via disturbances in redox homeostasis, a key event mediating MeHg neurotoxicity. Therefore, we investigated if MeHg-induced changes in key systems of antioxidant defense are sex-dependent. C57BL/6J mice were exposed to MeHg during the gestational and lactational periods, modeling human prenatal and neonatal exposure routes. Dams were exposed to 5ppm MeHg via drinking water from early gestational period until postnatal day 21 (PND21). On PND21 a pair of siblings (a female and a male) from multiple (5-6) litters were euthanized and tissue samples were taken for analysis. Cytoplasmic and nuclear extracts were isolated from fresh cerebrum and cerebellum and used to determine thioredoxin (Trx) and glutathione (GSH) levels, as well as thioredoxin reductase (TrxR) and glutathione peroxidase (GPx) activities. The remaining tissue was used for mRNA analysis. MeHg-induced antioxidant response was not uniform for all the analyzed antioxidant molecules, and sexual dimorphism in response to MeHg treatment was evident for TrxR, Trx and GPx. The pattern of response, namely a decrease in males and an increase in females, may impart differential and sex-specific susceptibility to MeHg. GSH levels were unchanged in MeHg treated animals and irrespective of sex. Trx was reduced only in nuclear extracts from male cerebella, exemplifying a structure-specific response. Results from the gene expression analysis suggest posttranscriptional mechanism of sex-specific regulation of the antioxidant response upon MeHg treatment. The study demonstrates for the first time sex-and structure-specific changes in the response of the thioredoxin system to MeHg neurotoxicity and suggests that these differences in antioxidant responses might impart differential susceptibility to developmental MeHg exposure.

Impairment of mitochondrial energy metabolism of two marine fish by in vitro mercuric chloride exposure

The goal of this work was to understand the extent of mercury toxic effects in liver metabolism under an episode of acute contamination. Hence, the effects of in vitro mercuric chloride in liver mitochondria were assessed in two commercial marine fish: Senegalese sole (Solea senegalensis) and gilthead seabream (Sparus aurata). Liver mitochondria were exposed to 0.2mgL(-1) of mercury, the average concentration found in fish inhabiting contaminated areas. Mercuric chloride depressed mitochondrial respiration state 3 and the maximal oxygen consumption in the presence of FCCP indicating inhibitory effects on the oxidative phosphorylation and on the electron transport chain, respectively. The inhibition of F1Fo-ATPase and succinate-dehydrogenase activities also corroborated the ability of mercury to inhibit ADP phosphorylation and the electron transport chain. This study brings new understanding on the mercury levels able to impair fish mitochondrial function, reinforcing the need for further assessing bioenergetics as a proxy for fish health status.

Multiple classes of environmental chemicals are associated with liver disease: NHANES 2003-2004

Biomonitoring studies show that humans carry a body burden of multiple classes of contaminants which are not often studied together. Many of these chemicals may be hepatotoxic. We used the 2003-2004 National Health and Nutrition Examination Survey to evaluate the relationship between alanine aminotransferase (ALT) and 37 environmental contaminants, comprising heavy metals, non-dioxin-like polychlorinated biphenyls (PCBs), and dioxin-like compounds, using a novel method. Linear regression models were constructed for each chemical separately, then as a class, using quartiles to represent exposure and adjusting for age, sex, race, income, and BMI. We then used an optimization approach to compile a weighted sum of the quartile scores, both within and across chemical classes. Using the optimization approach to construct weighted quartile scores, the dioxin like PCB, the non-dioxin like PCB and metal class-level scores were significantly associated with elevated ALT. A significant interaction was detected between the class-level score for metals, and the score for non-dioxin-like PCBs. When including all chemicals in one model, 3 chemicals accounted for 78% of the weight (mercury, PCB 180, 3,3',4,4',5-PNCB) with the remaining 22% associated with 4 chemicals (a dioxin and 3 PCBs). Validation with a holdout dataset indicated that the weighted quartile sum estimator efficiently identifies reproducible significant associations.