Essential trace elements metal complexes

Tn recent years there has been an increased interest in assessing the human health effects from environmental exposure to trace metals. Studies of occupational exposures and dietary intakes of trace metals have required the refinement and development of analytical techniques for the analyses of low elemental concentrations in complex matrices. Molybdenum is one of the trace metals that has been the subject of intensive study because it is an essential trace element in both plant and animal nutrition. It is an integral constituent of several metalloenzymes including xanthine oxidase, which is the last enzyme in the catabolic pathway of purines. Extensive ingestion of molybdenum has been shown to cause molybdenosis in cattle. Many of the features of this condition can be ascribed to induced copper deficiency. Whether biochemical changes or adverse health effects in humans can be attributed to excessive exposure to molybdenum is not known. 

By and large, of the 86 naturally occurring elements in nature, some 20 or so are deemed to be essential as trace elements and approximately 10 are envisaged as toxic. Most of these essential trace elements are metals and are able to form simple ionic compounds or complex coordination compounds. However, some of the metallic elements such as cobalt, palladium, platinum, gold, mercury, germanium, tin, lead. 

The role of Cu as an essential trace element has focused attention on possible roles for copper chelation of biologically active ligands, with subsequent interference of normal transport and distribution, as well as the role of the metal in redox reactions due to the accessible oxidation states of (I) and (II). Similarly, the physiological response of copper levels in disease conditions [50] and the overall role of trace metals in health and disease [51, 52] are relevant and of considerable importance. The increase in serum copper content in infections, arthritic diseases, and certain neoplasms is well documented and, in fact, the subsequent decrease in level upon treatment has been used successfully as an indicator of cancer remission [50]. Copper complexes may be effective in therapy due in part to their ability to mimic this physiological response of elevated copper [53] and, clearly, the interplay of introduced copper with pre-existent bound copper and effects on copper—protein mediated processes will affect the ultimate biological fate of the complex. Likewise, while the excess accumulation of free Cu, and indeed Fe and Zn, caused by malfunction or absence of normal metabolic pathways is extremely damaging to the body, the controlled release of such metals may be beneficially cytotoxic. The widespread pharmacological effects of copper complexes have been briefly reviewed [54]. 

Ingestion. The absorption of swallowed materials is a complex phenomenon and during the course of evolution elaborate systems have developed to control the uptake of substances from the gut, principally the so-called essential trace elements which are themselves toxic if their concentration in the blood exceeds certain limits. There are thus a number of specific transport mechanisms for the transfer of essential metals across the gut wall, and some toxic metals, such as lead and cadmium, can take advantage of these to gain access to the body. Even so, it is necessary for the metals to be in the correct chemical state for absorption to take place. Highly soluble metal compounds may cross the gut wall by simple diffusion, and organic compounds which are fat soluble can also... 

Of the ten trace elements known to be essential to human nutrition, seven are transition metals. For the most part, transition metals in biochemical compounds are present as complex ions, chelated by organic ligands. You will recall (Chapter 15) that hemoglobin has such a structure with Fe2+ as the central ion of the complex. The Co3+ ion... 

The reddish metal was already known in prehistoric times. It occasionally occurs as a native metal, but mostly in conspicuous green ores, from which it is extracted relatively easily. It is convenient to work, but not very hard. Not very optimal as a tool ("Otzi the Iceman" had a copper axe with him). Only through the addition of tin is the more useful bronze obtained. Its zinc alloy is the versatile and widely used brass. Copper is one of the coinage metals. Water pipes are commonly made of copper. Its very good thermal and electrical conductivity is commonly exploited (cable ), as well as its durability (roofs, gutters), as the verdigris (basic copper carbonate) protects the metal. Cu phthalocyanines are the most beautiful blue pigments. Seems to be essential to all life as a trace element. In some molluscs, Cu replaces Fe in the heme complex. A 70-kg human contains 72 mg. 

Soil pH is the most important factor controlling solution speciation of trace elements in soil solution. The hydrolysis process of trace elements is an essential reaction in aqueous solution (Table 3.6). As a function of pH, trace metals undergo a series of protonation reactions to form metal hydroxide complexes. For a divalent metal cation, Me(OH)+, Me(OH)2° and Me(OH)3 are the most common species in arid soil solution with high pH. Increasing pH increases the proportion of metal hydroxide ions. Table 3.6 lists the first hydrolysis reaction constant (Kl). Metals with lower pKl may form the metal hydroxide species (Me(OH)+) at lower pH. pK serves as an indicator for examining the tendency to form metal hydroxide ions. 

Most of the attention regarding amino acid complexation has been centred on animal fluids. There is considerable evidence, for example, that amino acid complexes are involved in the (active) transport of metal ions across various biological membranes.39 Complexation of the trace elements is also considered essential in reducing concentrations of the free or hydrated metal ions and hence preventing the formation of unwanted hydroxy species and limiting the toxicity of the metal ions. 

Iron is an essential micronutrient (see Nutritional Aspects of Metals Trace Elements) but in order to obtain sufficient iron and then to handle it in a safe way, biological organisms have evolved complex transport proteins for iron. A major reason for this is that iron bound to low molecular weight ligands can be highly toxic owing to its ability to catalyze Fenton-type reactions, which produce free radicals, as in reactions (1) and (2) below. 

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