Mercury-sulfhydryl complexes

The hybridization potential of a probe remains intact after mercuration. Sulfhydryl compounds bind strongly to mercurated nucleic acids and a variety of sulfhydryl-(primary label) ligands can be introduced in the complex after hybridization. Simpson (1961) observed a typical association constant of mercury for sulfhydryl compounds of 10 M but the mercury-sulfhydryl bond in polynucleotide complexes may be less stable (Hopman et al., 1986a). An interesting application... 

Stability Constants of Mercury(II) Complexes With Sulfhydryl Containing Donors... 

Literally hundreds of complex equilibria like this can be combined to model what happens to metals in aqueous systems. Numerous speciation models exist for this application that include all of the necessary equilibrium constants. Several of these models include surface complexation reactions that take place at the particle-water interface. Unlike the partitioning of hydrophobic organic contaminants into organic carbon, metals actually form ionic and covalent bonds with surface ligands such as sulfhydryl groups on metal sulfides and oxide groups on the hydrous oxides of manganese and iron. Metals also can be biotransformed to more toxic species (e.g., conversion of elemental mercury to methyl-mercury by anaerobic bacteria), less toxic species (oxidation of tributyl tin to elemental tin), or temporarily immobilized (e.g., via microbial reduction of sulfate to sulfide, which then precipitates as an insoluble metal sulfide mineral). 

Not much is known about the toxic effects of ethylmercury and most toxicologists have assumed that the toxic changes would be similar to that caused by methylmercury. These alterations in turn are very complex and depend on duration of exposure, dose, and the age of the individual. Mercury salts have a strong affinity for sulfhydryl groups and this is likely to play a role in effecting their neurotoxicity. Some in vitro studies indicate that oxidative stress leading to lipid peroxidation and DNA damage may also underlie the mechanism of toxicity. 

In vitro evidence suggests that organic mercury is also readily absorbed in the gastrointestinal tract and that methylmercuric chloride is absorbed to a greater extent than phenylmercuric chloride (Endo et al. 1989). Complexing of methylmercury with nonprotein sulfhydryls also may play a role in intestinal absorption and reabsorption (Urano et al. 1990). Phenylmercuric salt in the diet was completely... 

The metals discussed in this chapter—lead, arsenic, mercury, and iron—frequently cause significant toxicity in humans. The toxicity profiles of metals differ, but most of their effects appear to result from interaction with sulfhydryl groups of enzymes and regulatory proteins. Chelators are organic compounds with two or more electronegative groups that can form stable covalent-coordinate bonds with cationic metal atoms. As emphasized in this chapter, these stable complexes can often be excreted readily, thus reducing the toxicity of the metal. 

In order to understand the biochemistry of mercury more completely, it is important to describe the chemistry of this element with sulfhydryl donors in the most common coordination environments. Ideally, one would examine mercuric complexes within a construct that was basically invariant, but which allowed for preparation of mercury compounds in the most common structures. The desired coordination modes include linear (or slightly bent) for 2-coordinate complexes, trigonal planar (or slightly T-shaped) for 3-coordinate compounds and tetrahedral for 4-coordinate complexes. Figure 1. Unfortunately, diere are no known native systems that allow for this diversity of metal coordination geometry. However, given advances in peptide synthesis and the prediction of... 

As ATP is often the substrate in the case of enzyme-substrate-metal complexes, most metals are active for they mostly bind to the triphosphate. Copper (II), mercury (II), and other very strong Lewis acceptors are inhibitors as they bind to the ring nitrogens of ATP and in enzymes they could also block essential sulfhydryls. 

Gregus et al. (1992) recently demonstrated that acute administration of high doses of dihydrolipoic acid, a lipid-soluble coenzyme containing two sulfhydryl groups, stimulates hepatic uptake and biliary excretion of mercury. The enhanced uptake presumably results from diffusion of the hydro-phobic metal complex. However, because lipoic acid is normally localized to mitochondria, it may not play a major role in metal uptake under physiological conditions. Mercury and cadmium can also form lipid-soluble complexes with selenium (Magos and Webb 1980), but the mechanisms involved have not been elucidated. 

Schwartz ER, Reed LJ (1970) a-Keto acid dehydrogenase complexes XIII. Reaction of sulfhydryl groups in pymvate dehydrogenase with organic mercurials. J Biol Chem 245 183-187... 

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