Biotransformation, mercury

Pan-Hou HSK, Hosono M, Imura N. 1980. Plasmid-controlled mercury biotransformation by Clostridium cochlearium T-2. Appl Environ Microbiol 40 1007-1011. 

Barkay, T., Turner, R., Saouter, E. Horn, J. (1992). Mercury biotransformations and their potential for remediation of mercury contamination. Biodegradation, 3, 147—59-... 

Lind B, Friberg L, Nylander M. 1988. Preliminary studies on methyhnercury biotransformation and clearance in the brain of primates. II. Demethylation of mercury in brain. J Trace Elem Exp Med 1 49-56. 

Omata S, Toribara TY, Cemichiari E, Clarkson TW. 1988. Biotransformation of methyl-mercury in-vitro in the tissues of wild and laboratory animals. Fifty-ninth Annual Meeting of the Zoological Society of Japan, Sapporo, Japan, Zool Soc Tokyo, Japan. 

Windom, H.L. and D.R. Kendall. 1979. Accumulation and biotransformation of mercury in coastal and marine biota. Pages 301-323 in J. 0. Nriagu (ed.). The Bio geochemistry of Mercury in the Environment. Elsevier/North-Holland Biomedical Press, NY. 

Biotransformation An organism changing one substance into another form often to increase excretion or reduce toxicity Bacteria changing mercury into methyl mercury... 

Dimercaprol is FDA-approved as single-agent treatment of acute poisoning by arsenic and inorganic mercury and for the treatment of severe lead poisoning when used in conjunction with edetate calcium disodium (EDTA see below). Although studies of its metabolism in humans are limited, intramuscularly administered dimercaprol appears to be readily absorbed, metabolized, and excreted by the kidney within 4-8 hours. Animal models indicate that it may also undergo biliary excretion, but the role of this excretory route in humans and other details of its biotransformation are uncertain. 

What biotransformation might inorganic mercury undergo in a biological system ... 

Organic mercury compounds, especially phenyl and methoxyethyl mercury may also be biotransformed into inorganic mercury by cleavage of the carbon-mercury bond. Although such compounds are more readily absorbed than inorganic mercury compounds, the toxicity is similar. 

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). 

Guilherme, S., M. Valega, M.E. Pereira, M.A. Santos, and M. Pacheco. 2008. Antioxidant and biotransformation responses in Liza aurata under environmental mercury exposure—relationship with mercury accumulation and implications for public health. Mar. Pollut. Bull. 56 845-859. 

Enterohepatic circulation provides an example of a special case of intestinal absorption. Certain chemicals, like methyl mercury, after undergoing biotransformation in the liver, are excreted into the intestine via the bile. They then can be reabsorbed in the intestine, sometimes after enzymatic modification by intestinal bacteria. This process can markedly prolong the stay of chemicals in the body. It can be... 

In tissues, its monomethylated metabolite may undergo further biotransformation. Ultimately, conversion into inorganic mercury enables the metal to bind to glutathione for biliary excretion. However, much of this complex can also be reabsorbed by the gastrointestinal tract. Such bile-hepatic recycling permits redistribution of mercury. 

Organic mercurials are capable of inducing nephrotoxicity in S2 and S3 segments of the proximal tubule. Part of the S3 damage results from the biotransformation of the organic mercurial to release mercuric ions. Methylmercury (CH3Hg + ) readily concentrates in renal proximal tubular cells and alters mitochondrial function and lysosomes. At least part of methylmercury-induced nephrotoxicity may be due to homolytic scission of methylmercury to release methyl radicals and to lipid peroxidative toxicity. 

It should not be forgotten that a great deal of biotransformation does occur, especially for metals such as mercury and for many organics. In many cases, the result is a less toxic form of the original input, but occasionally more toxic materials are created. 

The demethylated inorganic mercury accumulates in the kidney and liver. Suda et al. (1991) evaluated the transformation of methylmercury to inorganic mercury by phagocytic cells. The liver and kidneys are also potential sites of biotransformation (Lind et al. 1988 Magos et al. 1976 Norseth and Clarkson 1970). 

The most important transformation process in the environmental fate of mercury in surface waters is biotransformation. Photolysis of organomercurials may also occur in surface waters, but the significance of this process in relation to biotransformation is not clear (Callahan et al. 1979). 

Gallagher PJ, Lee RL. 1980. Role of biotransformation in organic mercury neurotoxicity. Toxicol 15 129-134. 

Norseth, T. and Clarkson, T.W. (1970) Biotransformation of methylmercury salts in the rat studied by specific determination of inorganic mercury. Biochem. Pharmacol., 19, 2775-2783. 

It is not known what amounts of gaseous metallic mercury are contributed by biotransformation and evaporation from the sea. We may assume that the lowest concentration (0.6 ng/m ) observed by Williston (7) with an airborne instrument over the ocean is an equilibrium value. With an onshore breeze greater values up to 1.5 ng/m observed near shore may have been formed by photolysis (3) of alkyl mercurials. The... 

---