organic compound are in usual more dangerous than inorganic compounds and in case of mercury the organic compounds can accumilate in the organisms and in food web

Hello!  Although both inorganic and organic mercury compounds are in fact toxic, it is the source or pathway of exposure that is at issue.  According to the U. S. Environmental Protection Agency (U. S. EPA), most inorganic mercury compounds have been banned since 1990 in the United States, so exposure to them is limited or minimal.  The risk of exposure to organic mercury compounds (i. e., methyl and ethyl mercury), however, is much more prevalent via the pathway of food consumption.  

Organic mercury compounds are highly lipophilic, tend to bioaccumulate within fish tissues. After consumption, said compounds in turn bioaccumulate in mylein, the fatty layers that coat and protect nerve fibers, the brain and spinal cord within the central nervous system. 

The following articles may be found useful to you, as well:

Elemental Mercury and Inorganic Mercury Compounds:  Human Health Impacts (WHO, 2003)

http://www.who.int/ipcs/publications/cicad/en/cicad50.pdf

Methyl Mercury in Fish:  A Review of Residue Levels, Fish Consumption and Regulatory Action in the United States (TOLLEFSON and CORDLE, 1986)

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1474253/pdf/envhper00439-0195.pdf

Methyl Mercury Exposure and Health Effects (HONG et al., 2012)

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3514465/pdf/jpmph-45-353.pdf

Many would advise against even thinking of working with dimethyl mercury.  It is extremely toxic, and there is a well known case where just 1 or 2 drops spilled on a pair of latex gloves killed a Dartmouth Chemistry Professor.  The link below is an excellent overview of the dangers of working with organic mercury and how Karen Wetterhahn died as a result of working with it.  

https://www.youtube.com/watch?v=h049Hgfk-BI

https://www.youtube.com/watch?v=h049Hgfk-BI

Mr. Mei-Ling Hamilton is correct that the most common exposure pathway to mercury is via consumption of mercury, particularly fish. However, the mercury in fish is almost all monomethylmercury in muscle, not in fat. This seems a bit counter intuitive since there are important neutral mercury species that can pass through biological membranes (DOI: 10.1021/es950373d) which one might think would accumulate in fat. In fact, there are neutral complexes of Hg(II) and monomethyl Hg, and of course dimethyl Hg that are lipophilc and so do pass through membranes, but they are only moderately lipophilic. Their lipophilicty may explain how MeHg gets into aquatic food webs initially, but not why they bioaccumulate to such high levels. 

Another part of the story is that monomethylmercury binds to cysteine in fluids in the body, while not so lipophilic, it is transported across cell membranes by amino acid transporters since it mimics methionine (e.g., DOI: 10.1146/annurev.pa.33.040193.002553; DOI: 10.1111/j.1471-4159.2008.05683.x; doi:10.1006/taap.2000.9018). Thus, its trophic transfer is very efficient, which is why the Hg in fish is almost entirely monomethyl Hg. For the same reason, it is efficiently absorbed and can move around in bodies of animals and humans (DOI: 10.1111/j.1471-4159.2008.05683.x).

Finally, it accumulates in muscle rather than fat because it binds to the thiol moities of proteins located there (DOI: 10.1021/es0628856) since bonds of Hg(II) and MeHg with reduced sulfur compounds are extremely strong.

Other references include: doi:10.1016/j.taap.2011.01.010

I agree with Robert Hudson: you need to consider both polarity of the substance but also the fact that mercury binds strongly to sulphur

 All form of mercury are very toxic and   high accumulation with food chain but  organic mercury is absorbed by some amino acid . 

Any organic matter have the ability to react with the organic component of the cell or tissues. Mercury even organic or inorganic is very toxic. So, organic mercury like methyl mercury or methyl mercury chloride more toxic than mercury chloride.

Hg toxicity, at least at low levels, does not come from a general reaction of Hg with organic matter, so it is not a matter of organic mercury preferentially reacting with organic matter. It has to do with the ability of MeHg to be effiiciently absorbed and transported within the body. Hg2+ is also toxic, but it does not mimic methionine when bound to cysteine, so it is not absorbed so efficiently and does not travel within the body so well.

Methyl Mercury Toxicity 

Among the compounds of mercury, Organomercury ones are highly poisonous and are easily absorbed by the gastric and intestinal organs, and they are carried by blood into the brain, liver, kidney and even foetus (Guzzi 2008). MeHg is slowly metabolized to inorganic mercury mainly by microflora in the intestines at a rate of  1% of the body burden per day (WHO, 1990). Although, MeHg is the predominant form of mercury, its conversion to inorganic mercury slowly accumulates and resides for long periods in the central nervous system (WHO, 1991). Urinary excretion of MeHg is negligible and undergoes extensive enterohepatic cycling (WHO, 1990). MeHg is present in the body as water-soluble complexes mainly, if not exclusively, attached to the sulfur atom of thiol ligands. It enters the endothelial cells of the blood–brain barrier as a complex with l-cystein (WHO, 1991). 

The major toxic effects of MeHg are on the central nervous system. In adult, the action of MeHg is characterized by a latent period between exposure and onset of symptoms depending of the dose and exposure period. The mechanism underlying this long latent period is still unknown. Paresthesia is the first symptom to appear at the lowest dose (Bakir et al., 1973). This may progress to cerebellar ataxia, dysarthria, constriction of the visual fields and loss of hearing (Bakir et al., 1973). These signs and symptoms are caused by the loss of neuronal cells in specific anatomical regions of the brain (McKeown-Eyssen and Ruedy, 1983).

Kindly read more details of the toxicity due to MeHg from the following references  which i can avail to you and more upon request. Thank you.

I recommend the EFSA Scientific Opinion on the risk for public health related to the presence of mercury and methylmercury in food by EFSA Panel on Contaminants in the Food Chain (CONTAM). It is really detailed.

regards

stefania

Not only in fish and aquatic fauna!

The toxicity of organic mercury compounds in the terrestrial fauna was reported already in the 1950ies.

Carnaghan, R.B.A. and Blaxland, J.D.: The toxic Effect of Certain Seed-dressings on Wild and Game Birds. The Veterinary Record. 69, 324-325. 1957.

Borg, K. The Effect of Dressed Seed on Game Birds. (In Swedish) Inverkan av betat utsäde på viltfaunan. VIII. Nordic  Veterinary Congress. Helsinki 1958. p. 394

Borg, K., Wanntorp,H., Erne, K., Hanko, E.: Alkyl Mercury Poisoning in Terrestrial Swedish Wildlife. Viltrevy, Swedish Wildlife.

Published by Swedish Sportmen’s Association, Vol. 6. Nr.4.301-379. 1969.

I thinj this recent Opinion from European Food Safety Authority (EFSA) " Scientific Opinion on the risk for public health related to the presence of mercury and methylmercury in food "could help you.

regards

stefania

This is a further question, rather than an answer:

There is a lot of talk about toxins, such as methyl mercury from excess oily fish consumption, being sequestered in visceral fat in humans and affecting a person's ability to lose weight (as the body is primed to avoid releasing toxins into the blood stream following weight loss).  Based on Robert Hudson's response above, it would seem that this is unlikely, as he states that methyl mercury accumulates in muscle rather than fat.  Does anyone have any data or references that would indicate that a high body burden of mercury (in any form - either from fish or amalgams) might affect ability of overweight person to lose weight - either due to being sequestered in fat, or due to endocrine-disrupting effects such as lowering metabolic rate via thyroid?

many thanks

Inorg Hg is salt of Hg with Cl, S, Acetate. These also have toxic effects. Organic Hg is with C or phenyl. The most toxic is methyl- and dimethyl Hg.These are produced in soil by MO &accumulate in food , fish etc, soluble and consumed extensivly. It conjugates with Cystein, absorbs well, enters the cell where Hg easily detaches .Traverses placenta. Enters brain, oxidezes to elementary Hg which retains in brain because it cannot pass through BBB, and causes neuronal apoptosis. So, org Hg causes severe brain damage in addition damaging other organs (kidney liver, lung).