Magnesium biological importance

The number of elements that are known to be biologically important comprises a relatively small fraction of the 109 known elements. Natural abundance limits the availability of the elements for such use. Molybdenum (Z = 42) is the heaviest metal, and iodine (Z = 53) is the heaviest nonmetal of known biological importance. The metals of importance in enzymes are principally those of the first transition series, and the other elements of importance are relatively light sodium, potassium, magnesium, calcium, carbon, nitrogen, phosphorus, oxygen, chlorine, and, of course, hydrogen. 

Willis (W12) has recently summarized the principles and applications of this method. A short note appeared recently regarding the use of atomic absorption spectrometry for serum and urine copper analysis (B15). The sensitivity of this method for copper is rather less than for such other biologically important trace metals as magnesium, zinc, and sodium. The sensitivity can be improved by extracting the copper as dithiocarbamate or pyrollidinedithiocarbamate complex (A7) into methyl isobutyl ketone. While this method is less sensitive than some others, it is nevertheless very specific and the apparatus is only moderately expensive. 

L. J. Henderson has developed the concept of environmental fitness, and has shown that the unique properties of the primary elements, carbon, hydrogen, and oxygen, endow their compounds with the maximal fitness for the manifestation of life. This concept has not yet been extended to the other elements, although it is interesting to speculate on the biological importance of magnesium and iron. Without the former there would be no chlorophyll, without the latter there would be no haemoglobin. 

The side chains of the 20 different amino acids listed in Panel 1.1 (pp. 6-7) have very different chemical properties and are utilized for a wide variety of biological functions. However, their chemical versatility is not unlimited, and for some functions metal atoms are more suitable and more efficient. Electron-transfer reactions are an important example. Fortunately the side chains of histidine, cysteine, aspartic acid, and glutamic acid are excellent metal ligands, and a fairly large number of proteins have recruited metal atoms as intrinsic parts of their structures among the frequently used metals are iron, zinc, magnesium, and calcium. Several metallo proteins are discussed in detail in later chapters and it suffices here to mention briefly a few examples of iron and zinc proteins. 

Clinical chemistry, particularly the determination of the biologically relevant electrolytes in physiological fluids, remains the key area of ISEs application [15], as billions of routine measurements with ISEs are performed each year all over the world [16], The concentration ranges for the most important physiological ions detectable in blood fluids with polymeric ISEs are shown in Table 4.1. Sensors for pH and for ionized calcium, potassium and sodium are approved by the International Federation of Clinical Chemistry (IFCC) and implemented into commercially available clinical analyzers [17], Moreover, magnesium, lithium, and chloride ions are also widely detected by corresponding ISEs in blood liquids, urine, hemodialysis solutions, and elsewhere. Sensors for the determination of physiologically relevant polyions (heparin and protamine), dissolved carbon dioxide, phosphates, and other blood analytes, intensively studied over the years, are on their way to replace less reliable and/or awkward analytical procedures for blood analysis (see below). 

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