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. 

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. 

Antimony, which is considered a nonessential element, is comparable in its toxicological behavior to arsenic and bismuth. In analogy to arsenic, trivalent antimony compounds generally are more toxic than the pentavalent compounds. Poisoning with antimony and its compounds can result from acute and chronic exposure, especially from exposure to... 

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. 

Bones are actually living protein networks to which minerals attach themselves. Not aU of the minerals deposited on bones are essential to bone building. There are at least two dozen elements in bones that have no known function in the human body, as well as a handful of nonessential elements, such as boron, strontium, silicon, barium, bismuth, and arsenic (yes, arsenic), that are believed to do some good. Five toxic elements—lead, cadmium, mercury, polonium, and radium—are often found in human bones. As long as they are stabilized in the bones, they do no apparent harm. 

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... 

Copper interacts with numerous compounds normally found in natural waters. The amounts of the various copper compounds and complexes present in solution depend on water pH, temperature, and alkalinity and on the concentrations of bicarbonate, sulfide, and organic ligands. In animals, copper interacts with essential trace elements such as iron, zinc, molybdenum, manganese, nickel, and selenium and also with nonessential elements including silver, cadmium, mercury, and lead interactions may be either beneficial or harmful to the organism. The patterns of copper accumulation, metabolism, and toxicity from these interactions frequently differ from those produced by copper alone. Acknowledgment of these interactions is essential for understanding copper toxicokinetics. 

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