Molar mass 229.65 g/mol | ||
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Ethylmercury (sometimes ethyl mercury) is a cation composed of an organic CH3CH2- species (an ethyl group) bound to a mercury(II) centre, making it a type of organometallic cation, and giving it a chemical formula is C2H5Hg+. It is one of the metabolites of thiomersal (sodium ethyl mercuric thiosalicylate), which is used as a preservative in some vaccines. The term "ethylmercury" is also sometimes used as a generic term to describe organomercury compounds which include the ethylmercury functional group, such as ethylmercury chloride and ethylmercury urea.
Contents
The toxicity of ethylmercury as derives from thimerosal is not well studied, and for may years, studies of the analog methylmercury were used as a basis to predict the safety and estimate the risk of thimerosal use. Both the methyl- and ethylmercury species distribute to all body tissues, and cross the blood–brain barrier and the placental barrier. However, more recent evidence that appears summarised in an NIAID fact sheet on the use of thimerosol suggests that methylmercury is an inadequate reference compound for evaluating the toxicology of ethylmercury, because the two compounds differ significantly in the ratio of organic to inorganic mercury each produces in the brain, as well as in their individual tissue distributions and clearance rates.
Chemistry
Given the comparable electronegativities of mercury and carbon, the mercury-carbon bond is best described as covalent. In its parent and related compounds (e.g., thimerosal), the bond angle of RHgX species are linear (due to sp or dz2s hybridization of Hg).
The route to the toxicity of inorganic mercury lies through its biological methylation, and the extreme toxicity of the methylmercury cation, CH3Hg+, the relevance to ethylmercury, CH3CH2Hg+ having yet to be established, but with clear evidence for conversion of ethylmercury to inorganic mercury (see below).
Toxicity
The toxicity of ethylmercury, for instance as it derives in vivo from thimerosal, is not well studied, and for may years, studies of methylmercury were used as a basis to predict the safety and estimate the risk of thimerosal use. Methylmercury and ethylmercury distribute to all body tissues, crossing the blood–brain barrier and the placental barrier, and ethylmercury also moves freely throughout the body. Risk assessment for effects on the human nervous system have been made by extrapolating from dose-response relationships for methylmercury. Clifton has offered the estimate that ethylmercury clears from blood with a half-life of seven to 10 days in adult humans.
However, preliminary direct evidence from a 2005 animal study, subsequently summarised in an NIAID fact sheet on the use of thimerosol, suggested that methylmercury is an inadequate reference compound for evaluating the toxicology of ethylmercury, because the two compounds differ significantly in the ratio of organic to inorganic mercury each produces in the brain, as well as in their individual tissue distributions and clearance rates. Taken together, the researchers conclude from their monkey study of ADME for inoculated thimerosal-derived ethylmercury and the stomach-administered methylmercury that past and ongoing studies of methylmercury are unsuitable as a basis for evaluating thimerosal toxicity, and that thimerosal risk assessments "based on blood mercury measurements may not be valid" [emphasis added].
Public health concerns
Concerns based on extrapolations from methylmercury caused thiomersal to be removed from U.S. childhood vaccines, starting in 1999. Clarkson has argue that risk assessments based on methylmercury were overly conservative, in light of observations that ethylmercury is eliminated from the body and the brain significantly faster than methylmercury. Moreover, Clarkson has argued that inorganic mercury metabolized from ethylmercury, despite its much longer half-life in the brain, is much less toxic than the inorganic mercury produced from mercury vapor, for reasons not yet understood.
Exposure standards based on methylmercury (such as those currently recommended by the United States Environmental Protection Agency) have not been demonstrated to be equivalent for ethylmercury.
Environmental accumulation
Unlike methylmercury, ethylmercury has not been found to bioaccumulate.