Methylmercury half-life

The model simulations were in close agreement with the observed results from the distribution and metabolism studies. Physiological processes that were highlighted by the results and the discrepancies that did occur include the probable active transport into the brain (versus passive diffusion) of a methyl-mercury-cysteine complex, the bidirectional transport of methylmercury between the gut lumen and gut tissue as a more important determinant of methylmercury fecal excretion than biliary secretion, the importance for the determination of methylmercury half-life in rats of the recycling of mercury from ingested hair, and the need for better estimates of the rate constants for the demethylation of methylmercury in order to adapt the model to other species. 

Alkyl mercury compounds in the blood stream are found mainly in the blood cehs, and only to a smah extent in the plasma. This is probably the result of the greater stabhity of the alkyl mercuric compounds, as well as their pecuflar solubiUty characteristics. Alkyl mercury compounds affect the central nervous system and accumulate in the brain (17,18). Elimination of alkyl mercury compounds from the body is somewhat slower than that of inorganic mercury compounds and the aryl and alkoxy mercurials. Methylmercury is eliminated from humans at a rate indicating a half-life of 50—60 d (19) inorganic mercurials leave the body according to a half-life pattern of 30—60 d (20). Elimination rates are dependent not only on the nature of the compound but also on the dosage, method of intake, and the rate of intake (21,22). 

In their proposed method, contamination only from the ammoniacal glutathione solution is expected. However, any inorganic mercury in this solution will be adsorbed on the glass container walls with a half-life about 2d, i.e. the blank value becomes zero if the solution is left to stand for more than a week. This method for mercury in sediments does not distinguish between the different forms of organomercury. Results are calculated as methylmercury. 

On the basis of chemical profile, Wood (38) predicted that arsenic, selenium, and tellurium will be methylated in the environment, and lead, cadmium, and zinc will not. Elemental concentration in the aquatic food chain has been reported for As (39), Hg (40), Cd (41), Pb (42), and Cu (43). The biological half-life of methylmercury in fish, for example, is one to two years (44). Pillay et al. (40) implicated heavy coal burning in the mercurial contamination of plankton and fish populations of Lake Erie. Other metals, notably cadmium, have been shown to be incorporated into the grazing grasses surrounding a coal burning source (27). Trace element contamination, therefore, can enter the food chain at various points. Disposal of solid wastes in the form of ash and slag is yet another environmental consideration (45). 

Methylmercury is a primary example of this type of model in humans. A human mercury one-compartment PK model has been developed on the basis of several exposure datasets in which blood or hair concentrations have been measured at various times after methyl mercury exposure (Stern 1997 Kershaw et al. 1980 Sherlock et al. 1984). The one-compartment model takes into account the elimination half-life of mercury from blood and methylmercury volume distribution (in blood) to back-calculate the... 

Human pharmacokinetic studies indicate that methylmercury has a half-life in blood and the whole body of about 50 days (CDC 2005). Hair grows at about 1 cm/month with a delay of around 20 days between current blood concentration and appearance of mercury in hair (Myers et al. 2003). Thus, postnatal maternal hair can be analyzed sequentially to evaluate timing of methylmercury exposure during pregnancy. However, the potential that this affords to document critical periods of prenatal methylmercury exposure has yet to be realized. 

Methylmercury is rapidly and nearly completely absorbed from the gastrointestinal tract 90-100% absorption is estimated. Methylmercury is somewhat lipophilic, allowing it to pass through lipid membranes of cells and facilitating its distribution to all tissues, and it binds readily to proteins. Methylmercury binds to amino acids in fish muscle tissue. The highest methylmercury levels in humans generally are found in the kidneys. Methylmercury in the body is considered to be relatively stable and is only slowly transformed to other forms of mercury. Methylmercury readily crosses the placental and blood/brain barriers. Its estimated half-life in the human body ranges from 44 to 80 days. Excretion of methylmercury is via the feces, urine, and breast milk. Methylmercury is also distributed to human hair and to the fur and feathers of wildlife measurement of mercury in hair and these other tissues has served as a useful biomonitor of contamination levels. 

In contrast, in the brain of 21-day-old neonatal rats that had been previously exposed to a gavage dose of 6.4 mg Hg/kg as methylmercury chloride in utero, the cerebellum had the highest mercury concentrations and the brainstem had the lowest (Braghiroli et al. 1990). By 60 days of age, concentrations in the brain reached normal values, with an estimated half-life of approximately 37 days (Braghiroli et al. 1990). Therefore, age can affect regional distribution in the brain of animals. 

The accumulation of methylmercury and inorganic mercury in the brain of female monkeys (Macaca fascicularis) was studied by Vahter et al. (1994). In this study, animals received oral doses of 50 g/kg/day for either 6, 12, or 18 months. In normal-weight monkeys (2.4-4.1 kg), a steady-state blood concentration for total mercury was attained in approximately 4 months. The elimination half-life in the blood was found to be 26 days. Accumulation in the brain appeared to be biphasic, with an elimination half-life of 35 days for brain methylmercury in those monkeys exposed for 12 months. The elimination half-life of inorganic mercury, on the other hand, was reported be on the order of years. It... 

Because methylmercury undergoes enterohepatic recirculation, nonabsorbable agents have been used to "trap" methylmercury excreted into the bile (Lund et al. 1984). A polystyrene resin containing sulfhydryl groups added to food at a concentration of 1% doubled the elimination rate of methylmercuric chloride when administered to mice. The elimination half-life decreased from 65 to 20 days (Clarkson et al. 

EPA carried out the analysis using the polynomial model and the Weibull model. The results of the two models were within 3% of each other. EPA based its analysis on the Weibull model due to goodness of fit and history of use. The Benchmark dose is an estimate of an experimental dose associated with a specified low incidence of adverse effects. According to the Integrated Risk Information System (IRIS), the following uncertainty factors were applied 3 for the variability in human population (variability in the half-life of methylmercury and in hair-to-blood ratio) and 3 for the lack of a two-generation reproductive study and data on the effect of exposure duration on sequelae of the developmental neurotoxicity effects and on adult paresthesia. 

Bioalkylation of Hg has received considerable attention owing to its suspected involvement in Minimata Disease in which human poisonings in Japan and Sweden occurred after ingestion of fish and shellfish containing methylmercury. Methylmercury is a potent neurotoxin at low concentrations with a long biological half-life, and thus can be concentrated in food chains, even if present in small amounts in waters -. ... 

Methylmercury nitrate introduced through the mouth into the stomach appears in the bloodstream only 15 minutes after absorption, and attains its highest concentration after six hours. Of the absorbed quantity, 34% was excreted in 49 days in the feces and only 3% in the urine. The biological half-life of methylmercury compounds is about 70 days. As to the site of accumulation, half of the methylmercury compounds accumulate in the liver and 10% in the brain (Alberg et al, 1969). Moreover, it has been established that the concentration of methylmercury in the red blood cells is ten times that in the blood plasma. 

Canli and Erdem 1994, Wachs 1989). In sardines, the relative methylmercury concentration was high and remained constant with age in muscle (>85%), but decreased with age in liver (from 50% to 20%), though this might indicate the existence of a slow demethylation process (Joiris et al. 1999). The biological half-life of mercury in fish is long for inorganic mercury it has been calculated as 722 309 days in catfish (Ictalurus punctatus) (Schultz et al. 1996). 

Respiratory absorption of elemental mercury in air (1.0-30.0 xg Hg/m ) by humans ranged from 74 to 100% when inhaled through the nose and exhaled through the mouth for dogs, this value was 25%. However, respiratory absorption in humans was only about 20% when inspiration and expiration was through mouth only. Two human males exposed to radiolabeled methylmercury as CH ° HgCl via inhalation had variable half-life retention times (Tbl/2) of mercury. One subject had a Tbl/2 value of 103 days for the early period of observation (days 1-44 post-administration) and 39 days thereafter the second had a... 

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