Mercury reductase

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... 

Keywords mercury immobilization biotechnology bacteria enzyme purification mercury reductase volatilization heavy metals... 

The bound mercury is delivered to the mercuric reductase in the cytosol, which requires intracellular NADPH and thiol as cofactor for enzymatic activity. Mercury reductase reduces toxic Hg2+ to less toxic Hg° which is volatilized out of the system due to its high vapor pressure (Ghosh, 1996). 

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 bacteria including the common E. coli have on their plasmid (extranuclear gene) Hg-resistant factor. This gene, when faced with the presence of an excess level of mercury, produces an enzyme called mercury reductase. It reduces Hg(II) to metallic mercury Hg(0). Metallic mercury is not toxic and is somewhat volatile [remember that it is a liquid at room temperature]. Hence, the reduced mercury can diffuse out of the bacterial cell in the form of mercury vapor, and hence mercury is removed. 

Rotenone is a common insecticide that strongly inhibits the NADH-UQ reductase. Rotenone is obtained from the roots of several species of plants. Tribes in certain parts of the world have made a practice of beating the roots of trees along riverbanks to release rotenone into the water, where it paralyzes fish and makes them easy prey. Ptericidin, Amytal, and other barbiturates, mercurial... 

Steingrube VA, RJ Wallace, LC Steele, Y Pang (1991) Mercuric reductase activity and evidence of broad-spectrum mercury resistance among clinical isolates of rapidly growing mycobacteria. Antimicrob Agents Chemother 35 819-823. 

LYSOPINE DEHYDROGENASE MALATE DEHYDROGENASE MERCURY(II) REDUCTASE... 

Olson, G. J., F. D. Porter, J. Rubinstein, and S. Silver. 1982. Mercuric reductase from a mercury-volatilizing strain of Thiobacillus ferrooxidans. J. Bacteriol. 151 (3) 1230-1236. 

Another flavoprotein constructed on the glutathione reductase pattern is the bacterial plasmid-encoded mercuric reductase which reduces the highly toxic Hg2+ to volatile elemental mercury, Hg°. A reducible... 

Mercapturic acid 550s Mercuric reductase 787 Mercury 317... 

In-vitro approach Data are available in abundance concerning metal effects on isolated chloroplasts (for a review, see Clijsters and Van Assche, 1985). All the metals studied were found to be potential inhibitors of photosystem 2 (PS 2) photosystem 1 (PS 1) was reported to be less sensitive. From the in-vitro experiments, at least two potential metal-sensitive sites can be derived in the photosynthetic electron transport chain the water-splitting enzyme at the oxidising side of PS 2, and the NADPH-oxido-reductase (an enzyme with functional SH-groups) at the reducing side of PS 1 (Clijsters and Van Assche, 1985). Moreover, in vitro, non cyclic photophosphorylation was very sensitive to lead (Hampp et al., 1973 b) and mercury (Honeycutt and Korgmann, 1972). Both cyclic and non-cyclic photophosphorylation were proven to be inhibited by excess of copper (Uribe and Stark, 1982) and cadmium (Lucero et al, 1976). 

A very important conclusion was reached based on the effect of p-mer-curibenzoate on the NADPH oxidase and the NADPH-cjrtochrome c reductase activities of microsomes, namely, that the natural acceptor might be a component reactive with oxygen and involved in hydroxyla-tions or demethylations (11). It was found that in the absence of cytochrome c, the oxidase activity was largely inhibited by p-mercuri-benzoate. In the presence of cytochrome c, NADPH oxidation exceeded cytochrome c reduction in the absence of p-mercuribenzoate and the two rates equaled each other in the presence of p-mercuribenzoate. Thus, a mercurial sensitive oxidase distinct from the reductase was indicated, and this component was hypothesized to be connected with hydroxylation and/or demethyktion (11). 

The influence of mercurials on the NADPH-cytochrome c reductase activity is complex. The activity in microsomes is stimulated about 50% by p-mercuribenzoate (11). Mersalyl inhibits the NADPH-cytochrome c reductase activity (S87). 

The lipase-solubilized reductase is inhibited by p-mercuribenzoate, is protected from this inhibition by NADPH, and the inhibition is relieved by thiols (10). Careful titration of this enzyme with p-mercuribenzoate at pH 6.5 results in an almost 3-fold stimulation upon addition of 2 moles of mercurial per flavin the control activity is again observed when 7 equivalents have been added. At pH 7.7, a stimulation of 70% is seen with 1 equivalent and loss of activity is complete (extrapolated) with 6 equivalents (245). The protection of the enzyme by NADPH against mercurial inhibition is reminiscent of the effects with NADH cytochrome 63 reductase (360). 

The nitrite reductase of Torulopsis nitratophila is specific for NADPH and FAD, and can utilize reduced benzyl or methyl viologen as electron donor, but not reduced flavins (34S). With NADPH as electron donor, nitrite reduction is inhibited by cyanide and mercurials. Michaelis constants for FAD and nitrite have been reported to be 45 nM and 19 ftiW, respectively. Unlike the Neurospora enzyme, the nitrite reductase of T. nitratophila could not reduce hydroxylamine in the presence of NADPH and FAD. 

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