MeHg Demethylation Research Articles

Effects of Reactive Oxygen Modulators on in vivo Demethylation of Methylmercury(PROCEEDINGS OF 24TH SYMPOSIUM ON TOXICOLOGY AND ENVIRONMENTAL HEALTH)

To elucidate the involvement of reactive oxygen in in vivo demethylation of methylmercury(MeHg), the effects of paraquat (PQ) and other reactive oxygen modulators on inorganic-Hg (I-Hg) production in MeHg-administered rats were examined. Rats were intravenously (i.v.) injected with MeHgCl (2mg/kg). After MeHg administration, I-Hg levels time-dependently increased in the liver up to 9h, whereas the renal levels did not change during the first 3h, and then increased up to 24h in a time-dependent manner. PQ stimulated I-Hg production in the liver but not in the kidney, whereas it increased 2-thiobarbituric acid-reactive substance (TBA-RS) levels in both tissues. PQ-induced stimulation of I-Hg production was not further accelerated by an OH· enhancer, such as FeSO4 or Fe(III)EDTA. Hepatic I-Hg production in MeHg-administered rats (without PQ) was suppressed by NaCN (a potent inhibitor of mitochondrial cytochorome oxidase) but not by desferal or Fe(III) EDTA (an OH· modulalator). These results suggest that hepatic mitochondria may play an important role in in vivo demethylation of MeHg, and that reactive oxygen species other than OH· may participate in it.

Comparison of Elements in Bottlenose Dolphins Stranded on the Beaches of Texas and Florida in the Gulf of Mexico over a One-Year Period

We analyzed tissue samples from bottlenose dolphins (Tursiops truncatus) that had stranded on beaches in Texas and Florida over a 1-year period starting in September 1991. The concentrations of 10 elements plus methyl mercury (MeHg) were determined in brain, kidney, and liver, and we examined these results for differences based upon age, site, sex, and tissue type. A strong inverse relationship between total mercury (Hg) and the percentage that was MeHg was found in liver, kidney, and brain tissue, presumably due to demethylation of MeHg. A threshold concentration was found for total Hg in brain tissue, indicating that most Hg was present as MeHg up to about 8 years of age. Increases in total Hg after this age were accompanied by an increase in the ratio of total Hg to MeHg, indicating demethylation. Strong relationships were found between total Hg in liver and age and between total Hg and selenium in liver, which have been observed before in many fish- and squid-eating marine mammals. The only difference based on sex of the animals was observed for MeHg, which was higher in females and contrary to the pattern often observed for organic contaminants. Several elements (copper, Hg, lead, zinc) exhibited intersite differences, which were not consistent. Bottlenose dolphin from Florida exhibited the highest levels of MeHg and total Hg, while animals from Texas exhibited the highest levels of lead, copper, and zinc. The essential elements copper and zinc were expected to be the same for the Texas and Florida animals; however, observed differences may indicate population differences in basic physiological levels, dietary intake, or health status.

A Preliminary Assessment of Wet Deposition and Eposodic Transport of Total and Methyl Metcury From Low Order Blue Ridge Watersheds, S.E. U.S.A.

Results from a preliminary sampling program designed to investigate total (THg) and methyl Hg (MeHg) deposition, cycling and transport at the Coweeta Hydrologic Laboratory western North Carolina are presented. Wet deposition samples were collected in June and July 1994 and throughfall, seep and streamwaters were intensively collected during and after a rainfall event in June 1994. All water samples were collected using ultra clean trace sampling protocol. Low elevation Watershed 18 streamwater THg concentrations peaked with discharge, increasing 6 fold to 9 ng L-1. High elevation Watershed 27 which received less than one half the precipitation Watershed 18 received during the event, exhibited THg concentrations only 1.3 times over base flow conditions. Methyl Hg concentrations remained near detection limits (≤ 0.025 ng L-1) in both streams. Dissolved MeHg concentrations were higher in shallow seep (0.097 ng L-1), throughfall (0.135 ng L-1) and precipitation (0.16 – 0.035 ng L-1) than streamwaters. Initial estimates of annual THg and MeHg deposition and transport indicate >90% retention of Thg and a >80% retention or demethylation of wet deposition MeHg is occurring in these low order watersheds.

Demethylation of Methyl Mercury in Different Brain Sites of Macaca-fascicularis Monkeys during Long-Term Subclinical Methyl Mercury Exposure

Total (T-Hg) and inorganic (I-Hg) mercury concentrations were determined in specific brain sites (cerebellum, occipital pole, pens, motor strip, frontal pole, temporal pole, thalamus, and pituitary) of female Macaca fascicularis monkeys exposed to daily peroral doses (50 μg Hg/kg body weight) of methyl mercury (MeHg) for 6, 12, or 18 months, or to continuous iv infusion of HgCl 2 (200 μg Hg/kg body wt). In normal weight monkeys (2.4-4.1 kg body wt), the average concentration of MeHg (calculated as T-Hg minus I-Hg) was about the same in all brain sites, except the pituitary-3.0 μg Hg/g at 6 months, 4.2 μg/g at 12 months, and 4.3 μg Hg/g at 18 months. MeHg concentrations in the pituitary were about 50% of those in the other brain sites. In a group of monkeys that were kept unexposed for 6 months following 12 months of MeHg exposure, T 1 2 for MeHg was about 37 days in all brain sites, with the exception of the pituitary, where it was shorter. The concentration of I-Hg increased in all brain sites, but especially in the thalamus and pituitary, with the time of MeHg exposure. In most brain sites, I-Hg constituted about 9% of T-Hg at 6 and 12 months, and 12% of T-Hg at 18 months. In the pituitary, I-Hg increased from 20% of T-Hg at 6 months to 46% at 18 months. Elimination T 1 2 for I-Hg was extremely long, 230-540 days in most brain sites and considerably longer in the thalamus and pituitary. The concentration of I-Hg in the thalamus did not decrease during the clearance period (6 months), while I-Hg in the pituitary continued to increase in spite of no additional exposure. The MeHg exposed monkeys had several times higher I-Hg concentrations in the brain than monkeys exposed to HgCl 2, indicating that I-Hg was formed by demethylation of MeHg in the brain, and not by brain uptake of I-Hg formed by demethylation elsewhere in the body. There were large variations in the relative concentration of I-Hg between individual monkeys, but not between brain sites (except thalamus and pituitary). Obese monkeys (5.0-6.1 kg body wt) exposed to MeHg had higher concentrations of both MeHg and I-Hg than normal weight monkeys in all brain sites, except in the pituitary.

Transplacental and Lactational Exposure to Mercury in Hamster Pups after Maternal Administration of Methyl Mercury in Late Gestation

Pregnant Syrian golden hamsters were given a single oral dose of 203Hg-labelled methyl mercury (MeHg), 1.6 mumol/kg body weight, on day 12 of gestation. The uptake, retention and tissue distribution of 203Hg in the dams and pups was studied by gamma-counting during the following three weeks. The average transplacental transfer of 203Hg was 1.1% of the administered dose per pup, corresponding to 11% of the administered dose to a whole litter. This was considerably more than in our previous studies when the dams were treated on gestational day 2 (1.3%) or 9 (4.6%). The amount of 203Hg transferred to each pup in utero was independent of the litter size. The average additional transfer of 203Hg to a litter via milk was 1.7% of the administered dose. In the pups, the content of 203Hg in the liver and brain decreased, while the content in the kidneys and pelt increased during the second and third week. The highest amount of 203Hg was generally found in the pelt, which indicated that unweaned hamster pups primarily excrete MeHg by binding to hair. The chemical form of mercury in the liver and kidneys of the pups was determined by ion-exchange separation of inorganic Hg and MeHg followed by gamma-counting. The amount of inorganic Hg in the liver of the pups remained constant throughout the experiment, while it increased in the kidneys after one week due to the demethylation of MeHg. The inorganic Hg in the liver of newborn pups was probably due to maternal demethylation of MeHg and transplacental transfer of inorganic Hg.(ABSTRACT TRUNCATED AT 250 WORDS)

Autometallographic determination of inorganic mercury distribution in the cortex of the calcarine sulcus of the monkey Macaca fascicularis following long-term subclinical exposure to methylmercury and mercuric chloride.

The distribution of accumulated inorganic mercury deposits in the cortex of the calcarine sulcus of adult female Macaca fascicularis following long-term subclinical exposure to methyl-mercury (MeHg) and mercuric chloride (inorganic mercury-IHg) has been determined by autometallography. Four groups of monkeys were exposed to MeHg (50 micrograms Hg/kg body wt/day) by mouth for 6, 12, and 18 months or 12 months followed by 6 months without exposure (clearance group). A fifth group of monkeys was administered inorganic mercury (as HgCl2; 200 micrograms Hg/kg body wt/day) for 3 months by constant rate intravenous infusion via an indwelling catheter. Staining of IHg deposits in the MeHg-exposed groups increased for all cell types with increased length of exposure. The astrocytes and microglia in the MeHg exposure groups contained the largest deposits of IHg. Neurons in the 6-month MeHg exposure group were either not labeled or contained very fine deposits of IHg. The frequency of labeled neurons increased somewhat in the 12-month and clearance exposure groups. Virtually all neurons in the 18-month exposure group contained labeled deposits of IHg; however, these total deposits were considerably smaller than those present within the astrocytes and microglia. The majority of endothelial cells and pericytes did not contain notable mercury deposits, although scattered individual cells were heavily labeled. Labeled oligodendrocytes were relatively rare in all MeHg-exposed groups. Gitter cells, primarily located in a perivascular position, were common in the 12-month, 18-month, and clearance groups and many of these were found to be heavily labeled. The staining of mercury deposits in the IHg-exposed animals was low compared to the MeHg-exposed groups. The astrocytes and microglia were the primary cell types labeled. It is concluded that the astrocytes, and possibly microglia, are the primary location of the demethylation of MeHg into IHg within the cortex of the calcarine sulcus.

Methylmercury oxidative degradation potentials in contaminated and pristine sediments of the carson river, nevada

Sediments from mercury-contaminated and uncontaminated reaches of the Carson River, Nevada, were assayed for sulfate reduction, methanogenesis, denitrification, and monomethylmercury (MeHg) degradation. Demethylation of [(sup14)C]MeHg was detected at all sites as indicated by the formation of (sup14)CO(inf2) and (sup14)CH(inf4). Oxidative demethylation was indicated by the formation of (sup14)CO(inf2) and was present at significant levels in all samples. Oxidized/reduced demethylation product ratios (i.e., (sup14)CO(inf2)/(sup14)CH(inf4) ratios) generally ranged from 4.0 in surface layers to as low as 0.5 at depth. Production of (sup14)CO(inf2) was most pronounced at sediment surfaces which were zones of active denitrification and sulfate reduction but was also significant within zones of methanogenesis. In a core taken from an uncontaminated site having a high proportion of oxidized, coarse-grain sediments, sulfate reduction and methanogenic activity levels were very low and (sup14)CO(inf2) accounted for 98% of the product formed from [(sup14)C]MeHg. There was no apparent relationship between the degree of mercury contamination of the sediments and the occurrence of oxidative demethylation. However, sediments from Fort Churchill, the most contaminated site, were most active in terms of demethylation potentials. Inhibition of sulfate reduction with molybdate resulted in significantly depressed oxidized/reduced demethylation product ratios, but overall demethylation rates of inhibited and uninhibited samples were comparable. Addition of sulfate to sediment slurries stimulated production of (sup14)CO(inf2) from [(sup14)C]MeHg, while 2-bromoethanesulfonic acid blocked production of (sup14)CH(inf4). These results reveal the importance of sulfate-reducing and methanogenic bacteria in oxidative demethylation of MeHg in anoxic environments.

Demethylation and placental transfer of methyl mercury in the pregnant hamster

The demethylation and placental transfer of methylmercury (MeHg) was studied in Syrian Golden hamsters administered a single oral dose of 203Hg-labeled MeHgCl, 1.6 μmol/kg body weight, on day 2 or 9 of gestation and sacrificed 1 day before expected parturition. In order to evaluate the role of demethylation for transplacental transport of MeHg, four hamster were administered 203Hg-labeled HgCl 2 intravenously on day 9 of gestation. The mean biological halftime of 203Hg in animals administered radiolabeled MeHg was 7.7 days and the fecal route was the main excretory pathway. The fetal content of 203Hg-in hamsters administered radiolabeled MeHg on gestational day 2 or 9 corresponded to 1.3% and 4.6% of the administered dose, respectively. The distribution of 203Hg in the fetus was more even than in the dam and the concentration of 203Hg in the fetal brain, liver and kidney was similar to that of the placenta. Inorganic Hg was found in maternal liver (18% of total Hg), kidney (31%) and placenta (21%) and fetal liver (3%). The amount of inorganic 203Hg in fetal liver corresponded to about 0.015% of the dose administered to the dam as MeHg. When hamsters were administered 203HgCl 2 by intravenous injection on day 9 of gestation, the concentration of 203Hg in fetal liver corresponded to 0.03% of the administered dose. The inorganic 203Hg detected in fetal liver after maternal exposure to MeHg was probably due to demethylation of MeHg in the dam and transplacental transfer of inorganic Hg.

Biotic and abiotic mercury methylation and demethylation in sediments

Inorganic mercury (Hg(II)) methylation and methylmercury (MeHg) demethylation may occur in the water column, sediment-water interface and subsurficial sediment of aquatic ecosystems. These transformations involve mainly microbial mechanisms, although abiotic methylation may play a more important role in the water compartment. The relative importance of biotic versus abiotic mechanisms of methylation has not been determined however, and abiotic demethylation remains unknown. Little quantitative information is available on the role of bacterial activity in mercury transformations. It has been reported that at least 16 genera of aerobic and anaerobic microorganisms are able to methylate HG(II), and that a greater number are able to demethylate MeHg. Nevertheless, not all populations of these species are capable of methyl- and demethyl-transformations. The actual concentration of MeHg in the aquatic environment is regulated by the relative production and decomposition rates. This, in turn, depends on the availability of Hg(II), MeHg, and bacteria as well as on the physico-chemical properties of the sample. The objective of this study was to compare mercury methylation and demethylation rates in sediment samples with and without active bacterial populations. We therefore performed experiments to follow bacterial evolution during the course of Hg(II) methylation and MeHg demethylation in sediment slurries containingmore » both sterile and non-sterile sediments.« less

Speciation of Mercury in the Primate Blood and Brain Following Long-Term Exposure to Methyl Mercury

Total (I-Hg) and inorganic (I-Hg) mercury in blood and brain of female Macaca fascicularis monkeys, exposed to daily peroral doses of methyl mercury (MeHg; 50 μg Hg/kg body wt) for 6, 12, or 18 months, or to continuous iv infusion of HgCI2 (200 μg Hg/kg body wt) for 3 months, were determined. In normal weight monkeys (2.4-4.1 kg body wt) exposed to MeHg, steady state of T-Hg in blood (1.1 μg Hg/g) was reached in about 4 months. The elimination T 1 2 in blood was 26 days. I-Hg constituted 7% of T-Hg in blood. The average concentration of MeHg in occipital pole and thalamus was about 3 μg Hg/g at 6 months and 4.5 μg Hg/g at 12-18 months. Accumulation in brain seemed to be biphasic. Following termination of 12 months exposure, elimination T 1 2 for MeHg in brain was 35 days. I-Hg constituted about 9% of T-Hg in brain at 6-12 months, 18% at 18 months, and 74% at 6 months after termination of exposure. The I-Hg concentrations were somewhat higher in thalamus than in occipital pole. The elimination T 1 2 for I-Hg was extremely long, on the order of years. Most likely, the I-Hg was formed by demethylation of MeHg in the brain. In monkeys exposed to HgCl 2, blood levels of 0.6 μg I-Hg/g gave rise to brain I-Hg levels of about 0.1 μg/g only. In three heavy weight monkeys (5.0-6.1 kg body wt) exposed to MeHg, blood Hg increased to about 2 μg Hg/g, indicating a limited distribution of MeHg to fat. The Hg concentrations in brain (7-22 μg Hg/g) were considerably higher than those in normal weight monkeys, due to the high blood Hg levels in combination with a high brain-to-blood distribution ratio.

Effect of Fe-Overload on the Biotransformation of Methylmercury in Rat.

It is well documented that methylmercury (MeHg) in vivo slowly undergoes demethylation reaction to change to inorganic mercury (Hg-i). The cleavage of the C-Hg bond was suggested to occur via a reactive oxygen-mediated process in vitro. To study the possibility of the involvement of hydroxyl radical (·OH) in the demethylation of MeHg in vivo, the combined effect of Fe-overload and carbon tetrachloride (CCl4) treatment on the rate of biotransformation of MeHg was examined in rats. The effects of this treatment on H2O2-scavenging enzyme activities (catalase and glutathione peroxidase) were also studied. Rats were fed 3.5% Fe (II) fumalate-containing diet for 0, 7 or 21 d to load Fe. Feeding of the Fe-containing diet brought about a time-dependent increase of the hepatic Fe levels by nearly 7-fold after 3 weeks. The serum Fe levels showed a maximum on day 7, while the renal levels increased after day 7. TBA-reactive substance levels in the liver significantly increased along with the Fe feeding, and this increase was drastically accelerated by CCl4 treatment, suggesting the effective production of ·OH. The CCl4-induced stimulation of the lipid peroxidation was observed also in serum and kidney, though not as marked as in the liver. The hepatic accumulations of total and inorganic Hg at 72 h after MeHg administration were significantly increased by the Fe-feeding. The proportion of Hg-i to total Hg in this tissue increased markedly by the combined effect of Fe and CCl4 with the concomitant decrease of catalase activity. On the other hand, the renal accumulation of Hg-i drastically decreased by the Fe-load, while the MeHg levels remained unchanged. Since the renal metallothionein levels were found to be lowered after the Fe-feeding, this might account for the reduced retention of Hg-i in the whole kidney. The present results suggest that reactive oxygen species, probably ·OH, induced by Fe and CCl4 treatment may play a critical role in the biotransformation of MeHg in the liver.

Distribution of Mercury in Rabbits Subchronically Exposed to Low Levels of Radiolabeled Methyl Mercury

The metabolism of methyl mercury (MeHg) has been studied in rabbits administered 203Hg-labeled methyl mercuric chloride, 0.125 mumol/kg body weight, twice a week for 9 weeks, by intravenous injection. Twelve weeks after cessation of treatment, about 54% of the administered dose had been excreted in faeces and 5% in urine. After one week, the highest concentration of 203Hg was found in fur (8.6 nmol Hg/g). Substantially lower concentrations were found in kidney (2.5 nmol/g), liver (0.9 nmol/g), brain (0.4 nmol/g), muscle (0.3 nmol/g) and blood (0.1 nmol/g). The rate of elimination of 203Hg from brain, muscle and blood was faster (t1/2 about 12 days) than that from kidney and liver (t1/2 about 28 days). The relative amount of inorganic Hg in kidney and liver increased with time after cessation of treatment. The highest fractions were 85 and 70%, respectively. In brain, no significant demethylation of MeHg could be detected.

The effect of cadmium pretreatment on the disposition and excretion of methylmercury and trace elements (zinc, copper) in rats.

Cadmium chloride (Cd) was injected s.c. into male rats at a dose rate of 3 mg Cd/kg 3 times a week for 4 weeks. The animals were maintained for administration of methylmercury (203 Hg) chloride at a dose of 3 mg CH3Hg/kg given p.o. 3 times a week for 2 weeks, followed by 3 weeks of recovery period. Animals were sacrificed 24 h after the final dose of MeHg, or 5 weeks after cessation of Cd administration. Cd-pretreatment significantly decreased total Hg concentration in the kidney and RBC and almost completely inhibited demethylation of MeHg in the kidney (from 32% to 3% of inorganic Hg). Cd-pretreatment did not affect urinary excretion of total Hg, but significantly increased daily excretion of total Hg in feces. MeHg given alone significantly increased renal but not hepatic copper levels and decreased copper in the plasma and brain. In Cd-pretreated rats, both renal and hepatic copper concentration were in the normal ranges. Zinc levels in Cd-pretreated rats significantly increased in the kidney, liver and brain but decreased in plasma (compared to control and MeHg-alone treated animals). From these results it can be concluded that Cd-pretreatment may decrease MeHg toxicity by increasing the fecal mercury excretion and by inhibiting the formation of inorganic mercury in the kidney, which is a more potent renal toxin than MeHg.