Chronic low dose mercury exposure and the disease states which result have not been well studied. The role of low dose mercury in chronic disease is controversial, especially in regard to the use of mercury dental amalgams, mercury-containing thiomersal in vaccines and the consumption of mercury containing fish. There has been speculation about the role of low dose mercury in Alzheimer’s dementia, amyotrophic lateral sclerosis, Parkinson’s disease, multiple sclerosis and autism.
Acute mercury poisoning has been well studied, and no maximum safe dosage of mercury has been determined. The aim of the present article is to review the knowledge of the toxicity of mercury at the molecular level.
Elemental mercury (HgO) comes in some thermometers, dental amalgams and other industrial products. It does not absorb well in the gastrointestinal tract, but it does absorb well on inhalation in the lungs. HgO vaporizes and is easily inhaled at room temperature and can cross the blood-brain barrier and the placenta. It is easily oxidized to inorganic mercury (Hg2+), which can stay in the body for years.
Inorganic mercury (Hg2+) is found in cosmetics, industrial products and household products. It is easily absorbed in the gastrointestinal tract and through the skin but does not cross the blood-brain barrier easily. Inorganic mercury is excreted in the urine or feces or can stay in the kidneys. It can appear in the brain as the product of metabolism of other forms of mercury and can remain there for years.
Organic mercury is mainly from methylmercury from fish or ethylmercury as a preservative used in vaccines. Organic mercury absorbs well in both the gastrointestinal and the pulmonary tracts. It easily passes the placenta and the blood-brain barrier.
Dimethylmercury is an organic form of mercury seen only in the laboratory. It is easily absorbed through the skin and a few drops can be lethal by causing degeneration of the cerebellum.
Elimination of mercury depends on the form of mercury. It can be eliminated through the urine, feces and the expired air. There is no general agreement about which form of mercury is most toxic to the nervous system.
In the blood, 99% of mercury is bound to thiol groups, which stay in the blood. The rest is bound to “diffusible thiols” that can carry mercury into cells through their membranes. Mercury is bound to thiols and carried into cells where mercury becomes part of the structure of the cells. Both N-acetyl-cysteine (NAC) and homocysteine can bind to mercury in different forms that carry mercury into cells.
Chelating agents bind tightly to heavy metals, including mercury. Good chelators have a high excretion rate which helps eliminate the metals from the body. DMPS and DMSA are good mercury dithiol chelating agents. DMSA, which has a three hour half-life, is used for treating pediatric lead toxicity. DMPS is not currently approved by the FDA.
ALA (alpha lipoic acid) and its metabolite, DHLA, form chelates with a number of heavy metals. ALA reduces the toxic effects of mercury if given before or with the mercury.
N-acetylcysteine (NAC) and glutathione are considered beneficial in mercury toxicity, but recent studies have shown that they seem to increase the passage of mercury into the nervous system. They may not be good to use for cases of mercury toxicity.
Zinc and selenium seem to be neuroprotective in cases of mercury toxicity. Patients with symptoms of “mercury amalgam illness” tend to have lower selenium levels than do asymptomatic patients with amalgam.
Wheat bran has been shown to bind mercury and other metals. Apple pectin has been to shown to reduce the period of mercury toxicity in children and increase its urinary excretion.
CONCLUSION: There is no general agreement about the best way to remove mercury from the body. DMPS and DMSA are the best drugs for chelation, however, they are not effective for chelating mercury from the brain. At the present time ALA is the only chelator that removes mercury from the nervous system. GSH and NAC are not advisable at the present time. Zinc and selenium protect from mercury toxicity and some studies show that dietary fiber helps eliminate mercury.
NOTE: A thiol is an organic compound which contains a -SH (sulphydrol) group. A ligand is a chemical that forms bonds with metallic ions.
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