Toxic metal resistance system

It is frequently thought that these resistances arose as a result of human pollution in recent centuries. It seems, however, more likely that toxic metal resistance systems arose soon after life began, in a world which was already polluted by volcanic activities and geological sources (Ji et al. 1995). 

Errzymatic conversion of the metal to a form which is less toxic for the bacterium, e.g. CHsHgand Hg (Misra 1992). Many of these metal-resistance mechanisms are encoded by genetic systems which have been extensively studied and are well understood. Perhaps the best-studied metal-resistance system is encoded by genes of the mer, or mercury resistance, operon. In this system, Hg(II) is transported into the cell via the MerT transporter protein, and detoxified by reduction to less toxic volatile mercury by an intracellular mercury reductase, MerA (see Osborn et al. 1997, Hobman etal. 2000). 

Some metals can be converted to a less toxic form through enzyme detoxification. The most well-described example of this mechanism is the mercury resistance system, which occurs in S. aureus,43 Bacillus sp.,44 E. coli,45 Streptomyces lividans,46 and Thiobacillus ferrooxidans 47 The mer operon in these bacteria includes two different metal resistance mechanisms.48 MerA employs an enzyme detoxification approach as it encodes a mercury reductase, which converts the divalent mercury cation into elemental mercury 49 Elemental mercury is more stable and less toxic than the divalent cation. Other genes in the operon encode membrane proteins that are involved in the active transport of elemental mercury out of the cell.50 52... 

Toxie and biologically resistant materials will require special consideration for their treatment. You will need to adjust the nutrient stream to aeeommodate the bacteria in the system and aid in the hydrolysis of the eompounds or even wash or chelate the toxic metals out of the way. In one waste stream where nitroalcohols were being treated, the system required 42 days of detention in order to provide sufficient dilution and residence time to allow speeialized enzymes to develop in the bacterial population. 

Arsenic resistance is not the only toxic heavy metal ion resistance system found in bacteria. Bacteria have known plasmid and chromosomal genes for resistances to Ag+, As02, ASO4 -, Cd ", Co ", CrO/-, Cu ", Hg +, Ni +, Pb ", Sb ",... 

Bacteria have specialized transport systems for exporting toxic metals (Silver et al. 1989 Kaur and Rosen 1992). Metal ion resistance in bacteria is commonly associated with the induction of membrane ATPases that function to export toxic metals as either anions or cations, including Hg " ", Ag, AsO, Cd, Cr02. Recent evidence suggest that in humans Menkes disease is caused by a mutation in a gene that encodes a copper-transporting ATPase (Vulpe et al. 1993). 

Silver S (1992) Bacterial heavy metal detoxification and resistance systems. In Mongkolsuk S, Lovett PS, Trempy J (eds) Biotechnology and environmental science molecular approaches. Plenum, New York, pp 109-129 Silver S, Ji G (1994) Newer systems for bacterial resistances to toxic heavy metals. Environ Health Perspect 102/4 (in press)... 

They were studied in the context of chemical processes occurring in soils and aquatic systems, but also considering their importance in other circumstances. Citric acid as a complexing agent is often used in the separation of actinides, lanthanides and other toxic metals from wastes, sediments and contaminated soils. Citric complexes of molybdenum and other metals are components of electrolytic baths used for electrodeposition and cleaning of corrosion resistant alloys. 

Zinc is a component of several enzymes and other chemicals that are essential to our survival. Zinc deficiencies have been associated with many effects in the body, including effects on the skin (dermatitis, hair loss) and immune systems (lowered resistance to infections). Zinc supplements are common in stores, and we don t think about taking too much. However, in very high levels zinc is a toxic metal that can affect the digestive tract (e.g., intestinal cramping, diarrhea). Many environmental cleanup sites must set safe levels of zinc that can remain in soil and won t lead to toxic effects from possible future exposure. 

Phenolic-neoprene contact cements are used for structural metal-metal bonding. especially where fatigue resistance and low temperature performance are important [209]. They are also used for bonding textiles, wood, rubbers, plastics, ceramics, and glass to metal and to one another. Solvent toxicity and flammability has greatly reduced the use of contact cements in the wood products industry. Water-based contact cements persist, but generally do not perform as well as the solvent systems, thus allowing market erosion by alternative binders. 

To circumvent some of the above-mentioned drawbacks of sulfur-based mercury chemodosimeters, a system based on the alkyne oxymercuration of 58 has been developed (Fig. 22) [146]. 58 shows high selectivity, a limit of detection of ca. 8 ppm, resistance against strong oxidants, and a positive reaction even in the presence of cysteine, which is known to form stable mercury complexes and is used for the extraction of mercury from tissue samples. Another metal that is well-known for its catalytic ability is palladium, catalyzing different reactions depending on its oxidation state. Since this metal is toxic, assessment of the maximum allowable concentration of Pd in consumer products such as pharmaceuticals requires highly sensitive and selective detection schemes. For this purpose, indicator 60 was conceived to undergo allylic oxidative insertion to the fluorescein... 

Several systems have been established which can regulate gene expression. For instance, there are systems which rely on the addition of steroid hormones or heavy metal ions. However, certain physiologic or toxic effects may result and high basal-transcriptional activity may limit their usefulness (Furth et al., 1994). Another approach has been developed which is based on the tetracycline-resistance operon tet from E. coli transposon Tn70. 

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