C. Organic Compounds of Mercury
As mentioned in the introduction to section B, the mercuric ion readily forms covalent bonds with carbon; some organic mercurials contain Hg-N bonds, as in mercuric succinimide. A large number of carbon-mercury compounds have been synthesized, and some are extensively used in agriculture and industry, for instance, for seed dressing and as antifungal compounds in paints and other materials.
Some organic mercurials have also been employed for therapeutic purposes, although especially systemic administration is rare now. Mercurials such as mercurochrome
(dibromohydroxymercurifluorescein) and merthiolate (ethylmercurithiosalycilate) have been applied as topical antiseptics. Among widely used systemic mercurial drugs were the diuretic isopropyl alcohol derivatives whose general structure is R • CH2 • CH(OY) • CH2 • HgX, where X stands for chloride, thioacetate, or other residues, and Y is commonly a methyl group. The Hg • X bond is ionic, leaving one mercury valence free to react with tissue components. The same important property is found in the monoaryl or monoalkyl derivatives of mercuric mercury. Thus, parachloromercuribenzoate is a common sulfhydryl reagent, extensively used in biochemical laboratories, and monomethylmercury forms biologically active complexes with L-cysteine or glutathione (see section 4.1).
Although monosubstituted organic mercurials may react directly in this manner with, and thereby inhibit the function of, critical sulfhydryl groups, much of their toxic potential is associated with the likelihood of degradation to free mercuric ions. The highly neurotoxic dimethylmercury, which is very lipid-soluble and rapidly absorbed through the skin and taken up across the blood-brain barrier into the brain, is presumably demethylated at least to the monomethyl compound before exerting any effects. Risks of exposure to dimethylmercury are exacerbated by the ability of the compound to pass through latex gloves; this apparently permitted the fatal poisoning of a laboratory worker, as reported recently in the news media.
The mercurial diuretics and many of the other organic mercurials are relatively nontoxic and possess little occupational relevance; they will therefore not be discussed in detail here. However, exceptions to the generally low toxicity of many organic mercurials are the extremely toxic short-chain alkylmercurials. Of primary interest from the environmental and toxicological points of view are monomethylmercury salts, as further discussed in section 4.0. The dimethyl compound is a volatile and extremely potent neurotoxin; it is very easily absorbed through the skin or by inhalation, but poses little threat outside the laboratory. As already mentioned in section 1.4, the high neurotoxicity of methylmercury compounds, in contrast to the primarily nephrotoxic action of inorganic mercury, can be explained by their respective abilities rapidly to cross the blood-brain barrier. The alkylmercurial selected here for more detailed discussion is the chloride salt of monomethylmercury (see section 4). Other salts, like the nitrate, are biologically indistinguishable from the chloride. Ethyl mercury, another short-chain monosubstituted alkylmercury compound, has also been used as a biocide and exhibits toxic effects similar to those of the methyl derivative.
A large number of aryl and alkoxyalkyl mercury compounds are known. The aryl compounds especially are used in occupational settings. A well-known example is the application of phenylmercury as a biocide in paints and agricultural products. Parachloromercuribenzoate (PCMB) is an aryl compound frequently employed in biochemical laboratories as a sulfhydryl reagent. The arylmercurial selected here for more detailed consideration is the acetate salt of phenylmercury (see section 5). The nature of the alkyl or aryl residue in organic mercury compounds largely determines their ability to reach especially intracellular target sites in selected organs and the stability of the mercury-carbon bond. Generally, long-chain alkyl mercurials are more readily dealkylated than short-chain compounds. 4.0 Methylmercuric Chloride 4.0.1 CAS Number: [115-09-3]
4.0.2 Synonyms: Chloromethylmercury; MMC; methylmercury chloride; monomethyl mercury chloride; caspan; methylmercury (II) chloride
4.0.3 Trade Name: Caspan
4.0.4 Molecular Weight: 251.10
4.0.5 Molecular Formula: CH3 • HgCl
4.0.6 Molecular Structure:
4.1.1 General Monomethylmercuric mercury is a univalent cation. Despite its ionic nature, however, it is significantly more lipid-soluble than inorganic mercury compounds. The chloride salt is a white crystalline solid at room temperature and has a melting point of 170°C. Its specific gravity is 4.06, and its vapor pressure at 25°C is 0.0085 mmHg. At higher temperatures, it volatilizes with a disagreeable odor. It is slightly soluble in water (<0.1 mg/mL at 21°C). The free mercury valence bond in monomethylmercury reacts with sulfhydryl groups in biological molecules. Thus, biliary secretion of methylmercury involves a complex with glutathione, the most common endogenous and low molecular weight thiol compound in cells (37). An L-cysteine complex of methylmercury is transported across membranes by the neutral amino acid carrier system (38). The direct binding of methylmercury, for example, to protein sulfhydryl groups through the free valence bond of mercury may, in part, explain the toxicity of methylmercury. At the same time, demethylation to the mercuric ion in the body also contributes to the toxic effects observed.
4.1.2 Odor and Warning Properties In spite of its disagreeable odor, methylmercury is not readily recognized at low concentrations. In biological matrices the compound is set free from sulfhydryl complexes at pH 1 and can then be measured by gas chromatography and other convenient techniques (see section 2.1.2). The procedure of Magos (25) for determining methylmercury in the presence of inorganic mercury was referred to in section 2.1.2. It avoids the need for separating inorganic from organic mercurials before chemical reduction, based on the fact that stannous chloride does not reduce mercury bound covalently to carbon. Another sensitive method (lower detection limit 15 ng/g) for estimating methylmercury takes advantage of its enzymatic conversion to methane (39).
4.2 Production and Use
Alkylmercury compounds, like methylmercury, are generally prepared by a Grignard reaction in ethyl ether:
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