Toxic Metal Ions And Mechanisms Of Resistance 1161 Mercury

With the strongest toxicity, mercury does not have any beneficial function. The major form of mercury in the atmosphere is elemental mercury (Hg0), which is volatile and is oxidized to mercuric ion (Hg2+) photochemically. Hg2+, the predominant form entering aquatic environments, readily adsorbs to particulate matter. The major microbial reaction observed is the methylation of mercury, yielding methylmercury, CH3Hg+. Because bacteria confront toxic Hg2+ and CH3Hg+, several methods of detoxifying mercury species exist. An NADPH-linked enzyme called mercuric reductase, which is related to glutathione reductase and other proteins [23], transfers two electrons to Hg2+, reducing it to Hg0, which is essentially nontoxic and leaves the cell through passive diffusion [2,24,25].

In Gram-negative bacteria, mercury resistance genes, called mer genes, are arranged in an operon and are under control of the regulatory protein MerR, which can function as a repressor and an activator [26-41]. In the absence of Hg2+, MerR binds to the operator region of the mer operon and prevents transcription of mer genes. However, when Hg2+ is present, it forms a complex with MerR, which then functions as an activator of transcription of the mer operon [42,43]. MerD, the product of merD, also plays a regulatory role by preventing an overshot during induction [44].

The mercuric reductase is the product of the merA gene, whereas merP encodes a periplasmic Hg2+-binding protein. This MerP binds Hg2+ and transfers it to a membrane protein, MerT, the product of merT, which transports Hg2+ into the cell for reduction by mercuric reductase [45,46]. Alternatively or in addition to MerTP, an alternative uptake route exists that involves the MerC protein [47,48]. The product of the reduction, Hg0, which is volatile, escapes from the cell to the environment.

Methylmercury, as stated earlier, is soluble and can be concentrated in the aquatic food chain, primarily in fish, or further methylated by microorganisms to yield the volatile compound called dimethylmercury. Metabolically, methylation of mercury occurs by donation of methyl groups from CH3- B12. Methylation has been observed for arsenic, mercury, tin, lead, selenium, and tellurium [49]. Methylmercury and dimethylmercury bond to proteins and tend to accumulate in animal tissues, especially muscle. Organomercurials, which are always much more toxic than the Hg2+ is (e.g., methylmercury is about 100 times more toxic than Hg0 or Hg2+), may also be detoxified if the mer resistance determinant encodes a MerB organomercurial lyase in addition to the other proteins [35,38-40,50-54]. After cleavage by MerB, MerA reduces the resulting Hg2+. The high toxicity of organomercurials and other methylated and alkylated heavy metal compounds makes it very unlikely that these kinds of chemical modifications of heavy metals are metal resistance mechanisms [11].

11.6.2 Lead

Use of lead and its toxicity has been well known for a long time [55,56]. It is no transition element, but belongs to the element group IVa, C-Si-Ge-Sn-Pb. In seawater, it is even more rare than mercury [10]. Due to its low solubility, especially, lead phosphate is insoluble with a solubility product of 10-54; its biologically available concentration is low. Molecular information on lead uptake is not available but Pb-tolerant bacteria have been isolated [57], and a process involving precipitation in intracellular lead phosphate granules in Staphylococcus has been reported [58,59].

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