Chelation in Biological Systems

One of the most important consequences of chelation is that the biochemical, chemical, and physical properties of the metal chelate complexes are quite different from those of the uncomplexed metal ion. In almost all environments a metal cation will form bonds to other species which can share 

The clinical use of chelation is to transform the toxic metal complex with, say, an enzyme into a toxic metal complex with the administered chelating agent. Subsequently we want the enzyme to be reactivated and the toxic metal chelate complex to be excreted from the body in either the urine or the feces. When we use a chelating agent to treat metal intoxication, we transform the coordination sphere of the toxic metal ion and form a new complex with very different properties from those of the complex from which it was formed. One result of complexing a toxic metal ion with the therapeutically useful chelating agents is a considerable decrease in the 

Sites which actively transport anions are present in those two organs in which toxic metals frequently concentrate the liver (Meier 1988) and the kidneys (M0ller and Sheikh 1983 Pritchard and Miller 1992). Using these criteria we can sort out those chelating agents used in the clinic (Fig. 5). BAL (British Anti-Lewisite or 2,3-dimercapto-l-propanol) is electrically 

Highly hydrophobic Extracellular sites, most cells, brain, fatty tissue, etc. Single negative charge Extracellular spaces, cells with appropriate monoanionic 

Double negative charge Extracellular spaces, cells with appropriate transport 

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A. Shulman and F. P. Dwyer Metal Chelates in Biological Systems (Chelating Agents and Metal Chelates, Eds. E. P. Dwyer and D. P. Mellor), pp. 383—431. Academic Press (1964). 

The interest in colour indicators has recently increased as they are used for the direct determination of pH (acid-base indicators) and free calcium ions (fluorescent derivatives based on the calcium chelator EGTA as metallochromic indicators) in biological systems at cellular level. 

As mentioned previously, siderophores must selectively bind iron tightly in order to solubilize the metal ion and prevent hydrolysis, as well as effectively compete with other chelators in the system. The following discussion will address in more detail the effect of siderophore structure on the thermodynamics of iron binding, as well as different methods for measuring and comparing iron-siderophore complex stability. The redox potentials of the ferri-siderophore complexes will also be addressed, as ferri-siderophore reduction may be important in the iron uptake process in biological systems. 

Another factor that can possibly affect the redox potential in biological systems is the presence of secondary chelating agents that can participate in coupled equilibria (3). When other chelators are present, coupled equilibria involving iron-siderophore redox occur and a secondary ligand will cause the siderophore complex effective redox potential to shift. The decrease in stability of the iron-siderophore complex upon reduction results in a more facile release of the iron. Upon release, the iron(II) is available for complexation by the secondary ligand, which results in a corresponding shift in the redox equilibrium toward production of iron(II). In cases where iron(II) is stabilized by the secondary chelators, there is a shift in the redox potential to more positive values, as shown in Eqs. (42)—(45). 

For most living bacteria (lactobacilli being the only notable exception [154]) iron is an essential nutrient. Iron is not readily available under normal conditions, although it is the fourth most abundant metal on earth. In the environment it is mainly found as a component of insoluble hydroxides in biological systems it is chelated by high-affinity iron binding proteins (e.g. transferrins,... 

There are many examples of the analytical use of 1,10-phenanthroline and its derivatives in biological systems by virtue of their chelating properties. Uses that depend solely on these properties are outside the scope of this review. The biological effects of metal complexes of 1,10-phenanthroline are also excluded. These have been reviewed.394... 

Sec also Chelates and Chelation Cobalt Copper Gold Hydrate Iron Manganese and Molybdenum (In Biological Systems). 

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