Cadmium biological roles

No biological role has been demonstrated for mercury, other than its interaction with essential nutrients and toxicity (Natl. Research Council, 1978). Moderate growth stimulation by low dietary levels of cadmium has been described by Schwarz (1977) but not independently confirmed. Therefore, the following discussion will be limited to two trace elements, arsenic and lead, until recently known only for their toxicity. Arsenic deficiency has been produced in goats, minipigs, rats and chicken (Anke et al., 1976 Nielsen and Schuler, 1978 Uthus and... 

Cadmium and zinc are related transition metals with contrasting biological roles. Zinc is an essential ion (functioning catalytically and structurally in proteins) and probably has a specific transport mechanism for entering all cells, whereas cadmium is a toxic ion with no known biological functions. Cd " needs to be excluded, if possible, or to be extruded when found inside the cell. Because of the chemical similarity between Zn and Cd, it is difficult to exclude cadmium specifically from zinc uptake systems. The cell uses an efflux transporter as the strategy for keeping a low cytoplasmic concentration of cadmium. 

According to Greek Mythology, Cadmus, son of a Phoenician king, founded Thebes, and gave the Greeks the letters of their alphabet. But he and his wife Harmonia ended their lives as serpents. It is this dichotomy of achievement and fatality that are reminiscent of cadmium s role in industrial chemistry and biology. 

There are at least three major types of natural proteins and peptides which are rich in cysteinyl residues metallothioneins, zinc finger proteins, and phytochelatins. The biological role and structural characteristics of metallothioneins are dealt with in Chapter 11 of this book. Zinc finger peptides are possible targets for cadmium(ll) toxicity, while phytochelatins play an important role in the defense mechanisms of plants (Chapter 13). The multicysteine peptides are frequently used to mimic the cadmium(ll) binding ability of all these natural substances. 

Zinc has a role in biological systems second in importance only to iron cadmium has a more restricted biological chemistry but its toxicology and physiological effects (Section 56.1.13.2) have several interesting features.1470... 

The determination of trace metal impurities in pharmaceuticals requires a more sensitive methodology. Flame atomic absorption and emission spectroscopy have been the major tools used for this purpose. Metal contaminants such as Pb, Sb, Bi, Ag, Ba, Ni, and Sr have been identified and quantitated by these methods (59,66-68). Specific analysis is necessary for the detection of the presence of palladium in semisynthetic penicillins, where it is used as a catalyst (57), and for silicon in streptomycin (69). Furnace atomic absorption may find a significant role in the determination of known impurities, due to higher sensitivity (Table 2). Atomic absorption is used to detect quantities of known toxic substances in the blood, such as lead (70-72). If the exact impurities are not known, qualitative as well as quantitative analysis is required, and a general multielemental method such as ICP spectrometry with a rapid-scanning monochromator may be utilized. Inductively coupled plasma atomic emission spectroscopy may also be used in the analysis of biological fluids in order to detect contamination by environmental metals such as mercury (73), and to test serum and tissues for the presence of aluminum, lead, cadmium, nickel, and other trace metals (74-77). 

Future trends in trace element analysis will put even greater pressure on the need to provide unbiased determinations. Increasing interest in the role of trace elements in health and disease will provide the stimulus for the better provision of quantitative determinations on which important decisions are made (Centers for Disease Control, 1991 Moukarzel et al., 1992). Increased public awareness and legislation are likely to bring about substantial reductions in the currently acceptable levels of occupational and environmental exposure to some non-essential elements such as lead, cadmium and aluminium. Determinations that are made as part of the assessment of such exposure will need to be both carefully validated and reproducible over many years or decades (Braithwaite and Brown, 1988 Brown, 1991), which will have a serious impact on laboratory costs. However, reproducible trace element determination with a low bias in biological fluids represents the cornerstone of any proper understanding of the role of trace elements in human health and disease. 

Even though MTs exist naturally with zinc and/or copper bound to them, the discovery of the first MT in 1957 from horse kidney was the result of a search for a cadmium protein. Since then, MTs have continuously challenged the interest of chemists and life scientists. A search in the SciFinder database with metallothionein as the entry yields about 15,000 publications and reveals more than 700 articles per year over the 1991-2001 decade. It also shows that developments in MT research have been covered by about 300 reviews. The widespread occurrence of MTs in nature suggests that they serve an important biological function not yet completely established. It would appear that MTs have no enzymatic activity, nor do they perform any catalytic role in known metabolic processes. Precise identification of the function of MTs accounts for the outstanding number of works available (as indicated by the search results) and prompts most of the research currently being undertaken. 

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