According to a new CDC study, different organomercury compounds share the same fundamental mechanisms related to their toxicity. There is no harmless mercury, both methylmercury and ethylmercury are extremely toxic.
Background:
It is well known that toxicity of metal compounds depends on the species, therefore different mercury species differ both with respect to level of toxicity but also with respect to the action of toxicity and target organ. Organomercury compounds are well known for their neurotoxicity. Since one of its potent group members methylmercury is produced by microbial activity from inorganic mercury and then enters especially the aquatic food chain, consumers of seafood are regularly warned to limit the consumption of contaminated fish. Such warnings are especially addressed to pregnant women, since prenatal exposure is especially harmful for the development of the child.
Structure of thimerosal
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Another organomercury compound "Thimerosal" is used as preservative and adjuvant in some vaccines. Interestingly, this compound is declared to be safe by the producers of the vaccines as well as public health organizations such as the CDC. While the compounds releases ethylmercury to the body, this compounds is declared to be totally different from methylmercury with respect to metabolism and toxicity. Now, two CDC scientists have published research dissenting the assertion that thimerosal is safe. As they conclude in their study, there is "no good mercury" and "bad mercury" but both species are equally poisonous to the brain.
The CDC researchers J.F. Risher and P. Tucker published their study last month in the journal "Reviews of Environmental Contamination and Toxicology". The 45-pages meta-review of relevant scientific publications examines the various ways that alkylmercury compounds harms the human body. The authors conclude that published studies reveal that there are some similarities between the mechanisms of the toxic action of the mono-alkyl mercury compounds methylmercury (MeHg) and ethylmercury (EtHg).:
- Methylmercury, the highly-regulated neurotoxin found in fish, and ethylmercury (found in medical products, including influenza and tetanus vaccines, ear drops and nasal sprays) are similarly toxic to humans. Methylmercury and ethylmercury share common chemical properties, and both significantly disrupt central nervous system development and function.
- Thimerosal is extremely toxic at very low exposures, and is more damaging than methylmercury in some studies. For example, ethylmercury is even more destructive to the mitochondria in cells than methylmercury.
- The difference in manifested toxicity of MeHg and EtHg are likely the result of the differences in exposure, metabolism, and elimination from the body, rather than differences in mechanisms of action between the two.
CDC's website continues to feature now discredited safety assurances:
"Thimerosal contains ethylmercury, which is cleared from the human body more quickly than methylmercury, and is therefore less likely to cause any harm."
However, the new study makes the opposite conclusion:
"Thimerosal is quickly metabolized in vivo (in a living organism) due to its reactions with protein and non-protein thiols … so the effects of thimerosal reported in numerous articles are very likely the result of exposure to the metabolite ethylmercury."
The
CDC's webpage stubbornly insists that the "two types of mercury to which people may be exposed—methylmercury and ethylmercury—are very different." The new CDC study directly contradicts this assertion, "There are many commonalities/similarities in the mechanisms of toxic action of methylmercury and ethylmercury …"
The study meticulously details identical toxicity pathways shared by both forms of mercury:
- Both ethyl and methyl mercury cause DNA damage or impair DNA synthesis (Burke et al. 2006; Sharpe et al. 2012; Wu et al. 2008).
- Both cause oxidative stress/creation of reactive oxygen species (Dreiem and Seegal 2007; Garg and Chang 2006; Sharpe et al. 2012; Yin et al. 2007).
- Both decrease glutathione activity, thus providing less protection from the oxidative stress caused by MeHg and EtHg (Carocci et al. 2014; Ndountse and Chan (2008); Choi et al. 1996; Franco et al. 2006; Mori et al. 2007; Müller et al. 2001; Wu et al. 2008).
- Both cause effects on cell division by damaging the spindle apparatus during mitosis (Burke et al. 2006; Castoldi et al. 2000; Gribble et al. 2005; Kim et al. 2007; Ou et al. 1999; Machaty et al. 1999; Rodier et al. 1984).
- Both MeHg and EtHg bind to the amino acid cysteine (Clarkson 1995; Wu et al. 2008).
- Both MeHg and EtHg strongly inhibit the reacylation of arachidonic acid, thus inhibiting the reincorporation of this fatty acid into membrane phospholipids (Shanker et al. 2002; Verity et al. 1994; Zarini et al. 2006).
- Both cause an increase in NOS, causing an overproduction of NO (Chen et al. 2003; Chuu et al. 2001; Shinyashiki et al. 1998).
- Both disrupt glutamate homeostasis (Farina et al. 2003a, b; Manfroi et al. 2004; Mutkus et al. 2005; Yin et al. 2007).
- Both alter intracellular calcium homeostasis (Elferink 1999; Hare et al. 1993;Kang et al. 2006; Limke et al. 2004b; Machaty et al. 1999; Marty and Atchison1997; Minnema et al. 1987; Peng et al. 2002; Sayers et al. 1993; Sirois and Atchison, 2000; Szalai et al. 1999; Törnquist et al. 1999; Zarini et al. 2006).
- Both cause effects on receptor binding/neurotransmitter release involving one or more transmitters (Basu et al. 2008; Coccini et al. 2000; Cooper et al. 2003; Fonfria et al. 2001; Ida-Eto et al. 2011; Ndountse and Chan 2008; Yuan and Atchison 2003).
Despite this stark rejection of a decade of CDC
safety claims, CDC's public relations machine is still bucking the new
scientific consensus; the article concludes with a telling disclaimer in
tiny font:
"The findings and conclusions in this report
are those of the authors and do not necessarily represent the views of
the Agency for Toxic Substances and Disease Registry."
The original study
John F. Risher and Pamela Tucker,
Alkyl Mercury-Induced Toxicity: Multiple Mechanisms of Action, Rev. Environ. Contam. Toxicol., 240 (2016) 105-149.
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