Studies on the biotransformation of 203Hg-labeled methyl mercury chloride in rats

The effects of CCl4 on the accumulation of mercury in rat tissues after methylmercury injection

Rats were administered a dose of 10 mg of CH3HgCl/kg by single s.c. injection. They were subsequently treated with 0.5 ml of CCl4/kg by three i.p. injections at 48-h intervals. The treatment with CCl4 resulted in the rise of total mercury level in brain, although the increase of mercury level was transitory and the difference between with and without CCl4 treatment gradually diminished. In addition, the mercury levels in other tissues involving liver, kidney, and muscle were increased by the treatment with CCl4. These results suggest that a hepatic damage by CCl4 is responsible for the duration of mercury accumulation in brain and other tissues after the single injection of methylmercury.

Mutagenicity and teratogenicity of mercury compounds

Agriculture, consumption of fossil fuels and, to a lesser extent, industry, are the main sources of pollution by mercury which is discharged into the environment as metallic mercury, as inorganic mercury compounds, or as organic compounds. Once in the environment, mercury compounds are capable of a variety of transformations. Some professional or accidental mercury poisonings have been reported in human populations, but they can easily be minimized by appropriate preventive measures. Production of C-mitosis in plant material is the most noticeable genetic effect of mercury compounds. No positive report that mercury could be carcinogenic in man has appeared up to now and animal experiments have also provided negative results. Although placenta may represent a certain barrier to mercury, embryotoxicity and teratogenicity of organic mercury compounds have been observed in numerous systems such as fish, birds and mammals.

Degradation of methylmercury by selenium

When methylmercury was incubated in the presence of selenite and reduced glutathione (GSH), the mercury which was extracted into benzene under acidic condition decreased gradually with the elapse of time. This decrease was due to the cleavage of mercury-carbon bond of methylmercury. The reaction did not proceed when selenite or GSH was singly added to the reaction mixture. L-Cysteine, 2-mercaptoethanol and sodium sulfide in place of GSH also were effective for decomposition of methylmercury in combination with selenite, but oxidized glutathione (GSSG) and L-cystine were not. This suggests that reduction of selenite is needed for the degradation of methylmercury. Thus, the effect of reduced metabolites of selenite produced by GSH was investigated. Glutathione selenotrisulfide (GSSeSG) required GSH for the degradation of methylmercury, whereas H2Se possessed a strong activity even in the absence of GSH. This may indicate that H2Se is involved directly in the conversion of methylmercury to inorganic mercury. This phenomenon found in in vitro experiments is discussed in relation to the biotransformation of methylmercury.

Methyl mercury decomposition in mice treated with antibiotics

The role of intestinal flora in the decomposition and faecal excretion of methyl mercury was studied in mice treated with antibiotics. The antibiotics, neomycin sulfate and chloramphenicol, were given to mice in drinking water for six days before intraperitoneal administration of methyl mercuric chloride (MMC), and intestinal microorganisms were thereby reduced. Inorganic and organic mercury were determined separately for faeces, intestinal contents and organs. On the fourth day after the mercury administration, the percentage ratios of inorganic mercury to total mercury in the contents of the caecum and large intestine were less in the mice treated with antibiotics, at 37% and 39%, respectively, than in the control mice (66% and 65%, respectively). Administration of the antibiotics reduced the excretion of inorganic mercury in the faeces to 26% of that of control mice and also reduced the excretion of total mercury to 60%. Reduction of intestinal microorganisms by the antibiotics was assumed to have caused the reduced decomposition of methyl mercury in the caecal contents and the reduced excretion of total mercury in the faeces.

Distribution and biotransformation of methyl mercuric chloride in different tissues of mice

The distribution of 203Hg radioactivity has been studied in various organs of adult male and female mice from one hour to 21 days after treating with 203 Hg-labeled methyl mercuric chloride (MMC). The amount of methyl mercury (MeHg) and inorganic mercury (Hg) has also been determined by injecting single doses of non-radioactive MMC, and subsequently measuring total, organic and inorganic Hg content by atomic absorption technique. In addition, photoemulsion histochemical method (PEHM) was used to demonstrate localization of Hg grains in various cellular compartments of organs and tissues. The highest levels of radioactivity were attained at 7 hours post-treatment in all organs except for brain and testis. The testis showed the highest radioactivity at one day and the brain at two days post-treatment. MeHg persisted in brain over a longer period though the level was not as high. The content of MeHg and inorganic Hg was maximum in kidneys as compared to other organs. The brain and the reproductive organs contained the least amount of inorganic Hg. By PEHM, Hg grains were most prominently observed in the sinusoids, Kupfer cells, hepatic cells and bile duct epithelium of liver; in the lumen of blood vessels, convoluted and collecting tubules of kidneys; and in the gastrointestinal epithelium. The pattern of uptake and distribution of MeHg correlated well with the morphological demonstration of Hg grains in tissue sections.

Peripheral neuropathy following intraneural injection of mercury compounds

Between 0.25-25 micrograms of an aqueous solution of either mercuric chloride or methyl mercuric acetate was injected directly into the sciatic nerve of 28 adult Wistar rats. The resultant pathological changes in the nerve were examined by teasing individual fibres and by light and electron microscopy. In most respects mercuric chloride was more toxic than methyl mercuric acetate. The large doses of mercuric chloride produced a hind limb paresis within 24 h but no clinical signs followed injection of organic mercury. The predominant effect of mercuric chloride was on Schwann cells which showed cytoplasmic swelling and necrosis, associated with extensive segmental demyelination. In contrast methyl mercuric acetate caused axonal degeneration in many of the large myelinated fibres but only minor alterations were observed in Schwann cells.

Permeability of lipid bilayers to methylmercuric chloride: quantification by fluorescence quenching of a carbazole-labeled phospholipid

We investigated the permeabilities of lipid bilayers to the neurotoxin methylmercuric chloride (MMC). This mercurial is an efficient collisional quencher of the fluorescence of N-alkyl carbazole derivatives. Quenching of the fluorescence of beta-(3-(9-carbazole)-propionyl-L-alpha-phosphatidylcholine (CPA-PC) in vesicles of dimyristoyl phosphatidylcholine and of dioleoyl phosphatidylcholine reveal rapid diffusion of MMC in the alkyl side chain regions of these bilayers. By a combination of (1) the lipid concentration dependence of the apparent quenching constants, (2) the solubility of MMC in concentrated lipid dispersions and (3) the 270 MHz proton magnetic resonance of methylmercury in the presence of lipid bilayers we conclude that the lipid-water partition coefficient of this mercurial is less than or equal to two. Using the fluorescence quenching and the partitioning data we estimate the diffusion coefficient of MMC in these bilayers to range from 0.13 to 0.31 X 10(-5) cm2/sec, or 20--47% of its diffusion coefficient in ethanol. These data indicate that lipid bilayers do not pose a significant permeability barrier to the diffusional transport of MMC.

The role of biotransformation in organic mercury neurotoxicity

Although the clinical patterns of organic and inorganic mercury poisoning are very different, systemic toxicity experiments have shown that the histological changes in the kidneys and dorsal root ganglia neurones are identical with the 2 classes of compounds. It has been further suggested that the toxicity of organic mercurials is the result of biotransformation to inorganic mercury. To test this hypothesis, between 10(-7) and 10(-10) mol of mercuric chloride and methyl mercuric acetate were injected directly into the cerebrum of rats. The comparative size of lesions was estimated anatomically and by reference to blood brain barrier dysfunction. Inorganic lesions were only slightly larger than those produced by equimolar amounts of organic mercury. Consequently both organic and inorganic mercury must be regarded as neurotoxic in their own right. Conversion of organic mercury certainly occurs but is not the only mechanism by which organic mercury exerts its toxicological effect.

The uptake of methyl mercury in guinea pig cochlea in relation to its ototoxic effect

Guinea pigs were treated for 7 days by daily subcutaneous injection of methyl mercury chloride labeled with 14C, the total dose of which was 17 mg Hg/kg. In these animals the cochlear microphonics and whole-nerve action potentials were suppressed in the basal turn but there was no marked losses in the third turn of the cochlea. The endocochlear potential was not decreased in magnitude. At the end of the treatment there was no accumulation of mercury in the perilymph, endolymph and cerebrospinal fluid. Uptake and elimination of mercury in the cochlear fluids were studied in guinea pigs which were treated by a single intravenous injection of 203Hg-labeled methyl mercury, the dose of which ranged from 0.2 to 17 mg Hg/kg. The results indicated that mercury concentration ratio of the blood relative to cochlear fluids was comparable with the blood to plasma ratio reported previously. In contrast to lack of accumulation in the extra cellular environment, it is likely that tissues of the sensory end organs in the cochlea accumulated methyl mercury.

Accumulation and removal of Hg203 in different regions of the rat brain

We have studied the brain regional distribution of methyl mercury following intravenous administration of CH3 203HgCl in rat. Early peak levels were obtained in cerebellum, medulla oblongata and midbrain. The efficacy of removal of 203Hg by different chelators is also region dependent. The most efficient chelator for brain mercury proved to be mesodimercaptosuccinic acid.

Effect of methylmercury on some constituents of serum and urine

To evaluate some of the early effects of methymercury chloride (MMC) male rats were given 10, 20 or 30 mg MMC/kg intraperitoneally. Urine was analysed for vanilmandelic acid (VMA), leucine aminopeptidase (LAP), alkaline phosphatase (AP), and creatinine, blood for glucose-6-phosphatase (G-6-P) and glucose, serum for glutamate-oxalate-transaminase (GOT) and urea. Except for LAP and AP excretion there is no effect of MMC on the parameters investigated. However, the effects on these 2 renal enzymes are to variable to permit their use as a test for MMC toxicity.

Catalytic effects by thioltransferase on the transfer of methylmercury and p-mercuribenzoate from macromolecules to low molecular weight thiol compounds

Thiol agarose and glyceraldehyde-3-phosphate dehydrogenase were blocked with methylmercury or p-mercuribenzoate. The exchange of mercurials between the thiol-containing polymers and glutathione or dithioerythritol was investigated. The activity of glyceraldehyde-3-phosphate dehydrogenase was inhibited by blocking thiol-groups with the mercury compounds. Inhibition was reversible when a short period of inactivation was used. Inactivation for longer periods resulted in reduced regain of enzyme activity. The activity was in part regained when either of the 2 thiol compounds was added. Thioltransferase, known to catalyze thiol-disulfide exchange reactions, increased the regain of glyceraldehyde-3-phosphate dehydrogenase activity to nearly the original value. Here, thioltransferase is proposed to catalyze the transfer of organomercurial from one thiol complex to another. Some consequences of the observations in vivo are discussed.

Influence of protein or cystein deficiency on hepatic subcellular distribution of methyl mercury in two rat strains

The influence of protein deprivation and cystein deficiency on the distribution of methyl mercury between 4 subcellular fractions of liver was studied in 2 rat strains (Wistar, strain R and Sprague-Dawley). Kept on a standard diet, the 2 strains showed a similar distribution pattern, with the highest mercury level found in the cytosol, followed by the mitochondrial, microsomal and nuclei fractions. The protein free diet caused on increase in the total amount of bound mercury in both strains, the greatest increase, being found in livers from strain R rats. The cystein deficient diet, on the other hand, gave rise to diverging results. Whereas the level of mercury bound to the subcellular fractions was increased in livers from strain R rats, it was markedly reduced in livers from Sprague-Dawley rats.

Hemolysis and morphological changes in rat erythrocytes with mercurials

Effects of mercurials on rat erythrocytes were studied morphologically using an electron microscope. In the scanning study, the normal biconcave shape of the erythrocytes was changed to rugged surface spherocytes when mercuric chloride was added to the erythrocyte suspension. Methylmercuric chloride produced an irregularity of cell shape with spicules including the final stage of spherocytes. p-Chloromercuribenzoic acid formed crenated cells with protrusion, then spherocytes. By a carbon replica technique, it was revealed that control erythrocytes had a granular surface structure; however the surface of mercuric chloride-treated and methyl-mercuric chloride-treated erythrocytes appeared less granulated. By a negative staining technique, severe damage was observed on the erythrocytes lysed by mercurials. As a decrease in content of reduced glutathione, inhibition of glucose-6-phosphate dehydrogenase activity and formation of methemoglobin in erythrocytes treated with mercurials to induce hemolysis were not observed, it was concluded that the hemolysis induced by mercurials was not due to a disturbance in erythrocyte metabolism but rather to the direct action of mercurials on the cell membrane.

Normal and lethal mercury levels in human beings

Ethyl mercury in the form of Granosan M was used as a fungicide in dressing grains in Iraq. Disregarding warnings and precautions by the authorities, some villagers used this grain in making their bread. Tissue specimens of poisoned people were analysed for total mercury contents using the flameless atomic absorption spectroscopic technique. The analytical method used is highly sensitive (1 ppb/1% absorbance), and the precision in terms of relative standard deviation (RSD) was about 1.5%. The ranges of mercury content in ppm units in the two cases of poisoning were 8-9 for the kidneys, 6-7 for livers, 3-5 for the cerebella, and about 15 for the blood. The analyses included some other tissues as well. Control values were also present. These were obtained from human beings who died by accident and showed no signs of mercury poisoning.

Toxicity of methylmercury chloride in rats. III. Long-term toxicity study

Four groups, each of 25 male and 25 female weanling rats, were given dietary levels of 0, 0.1, 0.5 and 2.5 ppm MeHgCl for 2 years. Observations were made on behaviour, growth, food intake, haematology, serum enzymes, urinalysis, microsomal liver enzymes, organ weights and histology with special reference to the nervous system, histochemistry of the kidneys and cerebellum and on tissue Hg concentrations. Significant findings included a slight growth reduction in females at 2.5 ppm, increased relative kidney weight at 2.5 ppm and histochemical changes in kidney enzymes at 2.5 ppm. No effect was seen on the nature or incidence of pathological lesions or tumours at any level. From the results obtained in the short-term, reproduction and long-term studies, the no-toxic effect level for rats appears to be between 0.1 and 0.5 ppm MeHgCl in the diet. Exposure of the Dutch population does not appear to present a health hazard at the moment because the mean intake of total Hg is still far below the intake deemed to be safe.

The kinetics of methylmercury administered repeatedly to rats

Female rats (65-75 days old) were given orally 0.84 or 3.36 mg Hg/kg as methylmercury chloride (MeHgCl) 5 times a week for 13 and 3 weeks, respectively. The proportion of inorganic to total mercury remained as low as 6% in whole animal though it increased to above 40% in the kidneys. Differences in organ half times and the negative correlation with time for blood to liver, brain and kidney mercury ratios indicated more than one compartment for MeHg+. Brain had 26 days half time with a 32% final equilibrium concentration in relation to the body concentrations. Brain concentrations of mercury reported on rats dosed repeatedly with MeHg+ agreed with these values which justifies their use when experiments are planned to give a certain brain MeHg+ concentration. Half time for the whole body was 34 days but patholgical changes-weight loss, tubular damage, slow gastrointestinal passage-disturbed the accumulation curves in the higher dose group. Blood to kidney ratio and uptake of MeHg+ by kidneys also changed significantly.

Metabolism and toxicity of cadmium, mercury, and lead in animals: a review

Cadmium, mercury, and lead are toxic to humans and animals. Although cadmium and inorganic mercury toxicities occur in humans, they have not been observed in domestic livestock under practical conditions. In contrast, cattle, especially young calves, are extremely susceptible to lead toxicity. Apparently, cattle are more tolerant of cadmium than are other animal species. Due partially to higher absorption and longer retention times in the body, the alkyl mercuries, especially methyl mercury, are more toxic than inorganic mercury compounds. Inorganic forms of cadmium, mercury, and lead are poorly absorbed from the intestine. However, due to lack of effective homeostasis, after absorption retention time is long. Injected cadmium, mercury, and lead are metabolized differently from that naturally absorbed. Most cadmium and mercury are in kidney and liver (50 and 23% of total body in goats); but highest total load of methyl mercury is in muscle (72% in cows). With low to moderate body burden, most lead is retained in the skeleton. However, beyond a certain point, the kidney accumulates large quantities. Only minute amounts of cadmium and mercury are secreted into milk, but milk is only moderately well protected from dietary lead. Likewise, little cadmium and inorganic mercury pass the placental barrier whereas lead and methyl mercury pass more readily.