Metals, nonessential elements

Reduction of Metals and Nonessential Elements by Anaerobes 225 Table 16.4. Growth coupled to reduction of metals and metalloids. 

Several proteins were reported to function as enzymes for the dissimilatory reduction of metals and nonessential elements. As Usted in Table 16.4, the most frequently reported proteins involved in metal reduction are the cytochromes from sulfate-reducing bacteria. The focus on these cytochromes supports the initial papers by Lovley and colleagues in which they reported that reduced cytochrome Cs from Desulfovibrio vulgaris Hildenborough reduces uranyl salts (Lovley et al. 1993a) and chromate (Lovley and PhUhps 1994). 

Certainly, considerable flexibility and adaptability of electron flow is expected in bacteria, and many new strains are expected to be found that obtain energy from these chemical reductions. The natural gene flow over the years in the anaerobic ecosystems has produced microorganisms of considerable physiologic diversity. These anaerobic organisms continue to provide numerous biochemical challenges in the areas of anaerobic reduction of metals, metalloids, and nonessential elements by microorganisms. 

Specific considerations of metabolism and accumulation are not necessary in the application of a TTC provided that the substances are not likely to show very large species differences in accumulations such as, e.g., polyhalogenated-dibenzo-p-dioxins, dibenzofurans, and biphenyls and related compounds, as well as nonessential heavy metals in elemental, ionic or organic forms. Such substances are known to accumulate in the body, and the traditionally employed safety factors (Section 5.2.1) may not be high enough to account for species differences in rates of elimination of such chemicals. Therefore, the TTC approach should not be used for such substances. 

Once inside the organism, organic chemicals and metals are dealt with differently. Organic chemicals generally distribute based on their chemical properties (e.g., molecular size, lipophilicity, stereochemistry) and are eliminated through metabolism (phases I and II) or excretion (for example, renal excretion in mammals) of either the parent compound or the metabolites. Metals, on the other hand, can be split into essential and nonessential elements. Biochemical mechanisms and pathways have evolved to regulate essential metals with physiological functions. However, nonessential metals due to physicochemical similarities also use some of these pathways and thus affect the homeostasis of essential metals. 

Although copper is an essential element, it is much more toxic to cells than such nonessential elements as nickel and cadmium. Acute poisoning from ingestion of excessive amounts of copper salts, most frequently copper sulfate, results in nonspecific toxic-symptoms, a metallic taste, nausea, and vomiting (with vomitus possibly a blue-green color). The gastrointestinal tract can be damaged by ulceration. 

Many nonessential trace elements are found in the body. Depending on the local environment, at least 43 elements are normally incorporated into developing teeth another 25 elements are seen less frequently. The rest, notably the heavy metals, have never been detected in teeth. Many trace elements, particularly the heavy metals, are considered when testing for metal poisoning. Many plants concentrate essential and nonessential elements from soil and water, including aluminum (several species of subtropical plants), selenium (many plants), strontium (mesquite beans), and lithium (wolfberries, used by Native Americans in the southwestern United States for jam). Ingestion of these plants can cause toxicity for the element involved. 

A wide variety of interactions of selenium with essential and nonessential elements, vitamins, xenobiotics, and sulfur-containing amino acids have been demonstrated in numerous studies. Selenium has been reported to reduce the toxicity of many metals including mercury, cadmium, lead, silver, and to some extent, copper (Frost 1972). Most forms of selenium and arsenic interact to reduce the toxicity of both elements (Levander 1977). Because of selenium s role in the antioxidant glutathione peroxidase enzymes, selenium also reduces the toxicity of metals in vitamin E-deficient animals (Diplock et al. 1967). 

Many of the trace metals occur in animals in quantities that reflect the contact of the animal with its environment. It has been suggested that the essential elements can be distinguished from nonessential elements by observing the distribution patterns... 

There are two isomers of metallothionein in mammalian cells MT-1 and MT-2. There are several metallothionein genes. The transcription of each is controlled by several heavy-metal response elements or by hormone response elements [14,15]. The mechanism of control of the metallothionein genes by its promoters is a subject of intense investigation. Despite all the knowledge gained so far, the function of the metallothionein is still uncertain. However, it appears that the metallothionein may play key roles in the homeostasis of zinc within cells and in the detoxification of excess copper or toxic nonessential metals like cadmium and mercury. 

This is a soft, silvery metal with physical and chemical properties similar to those of calcium. There is no evidence as yet that strontium plays an essential role in man and other higher animals so, it must te considered as a nonessential element at this time. It is noteworthy, however, that it has E)een reported that the omission of this element from the diet of rats and guinea pigs on a purified diet resulted in (1) growth depression, an impairment of the calcification of tenes and teeth (but this report has E)een neither confirmed nor invalidated) and (2) a higher incidence of carious teeth. 

Inorganic or metal-containing medicinal compounds may contain either (a) chemical elements essential to life forms—iron salts used in the treatment of anemia—or (b) nonessential/toxic elements that carry out specific medicinal purposes—platinum-containing compounds as antitumor agents or technetium... 

We are at the discovery stage for determining the ability of various bacteria to reduce metals and nonessential compounds. Mechanisms for these reductions generally have not yet been established, and it is apparent that much is unknown. A number of questions pertaining to reduction are raised Which elements and compounds are reduced at the cell surface Why are some of the compounds not reduced at the cell surface but become reduced at the plasma membrane or in the cytoplasm What is the nature of the nonenergetic reactions in the cytoplasm of the bacterial cell What are the physiologic substrates for the cytochromes and which reactions occur because of substitution of chemicals due to similar structural features ... 

The majority of this chapter focuses on the toxicology of nonessential metals. Cadmium, lead, and mercury are three nonessential metals that have been investigated in great detail over the years, and they will be highlighted below. Other nonessential metals such as aluminum, beryllium, and nickel have not received as much attention but can pose toxicity issues. Arsenic and selenium are technically not metals, but are often included in discussions of metal toxicology. These elements will not be discussed here. 

Element-specific detection by ICP-MS has been widely used in the characterization of metallothioneins (MTs). The biological importance of these proteins is due to their role in homeostatic regulation of essential heavy metals like Cu and Zn. On the other hand, MT protects the cells from harmful chemicals, like nonessential and excessive essential heavy metals, reactive oxygen species, radicals, and alkylating agents. Fararello et reviewed different chromatographic approaches with ICP-MS detection for the multielemental speciation in MTs and MT-like proteins. 

Essential and Nonessential Metals. It is well known that elements in the biological systems may vary a great deal in their concentration from organ to organ and from species to species, but for the purpose of this chapter, the following classification of elemental concentrations has been adopted (17) major, > 1% minor, 0.10-1% micro, 0.01-0.1% trace, 0.01-0.001% ultratrace, < 0.001%. Since total copper in the average, "standard man (18) is approximately 150 mg (2), its classification would fall between trace and ultratrace concentration. However, as is the case with any other element, what is a trace in one organ may be an ultratrace in another, but for serum copper concentration, which is about 100 /xg%, the definition of copper as an ultratrace metal by the above classification may not be justifiable. If the criteria of the "standard man is taken into account, however, the definition seems appropriate. 

The tentative tolerable daily intakes proposed for certain metals provide a guideline for maximum tolerable exposure. In the case of essential elements, these levels exceed the daily requirements, but this should not be construed as an indication of any change in the recommended daily requirements. In the case of both essential and nonessential metals, the tentative tolerable intake reflects permissible human exposures to these substances as a result of natural occurrence in foods or various food processing practices, as well as exposure from drinking water. 

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