Cobalt biologically active chelates

A current area of interest is the use of AB cements as devices for the controlled release of biologically active species (Allen et al, 1984). AB cements can be formulated to be degradable and to release bioactive elements when placed in appropriate environments. These elements can be incorporated into the cement matrix as either the cation or the anion cement former. Special copper/cobalt phosphates/selenates have been prepared which, when placed as boluses in the rumens of cattle and sheep, have the ability to decompose and release the essential trace elements copper, cobalt and selenium in a sustained fashion over many months (Chapter 6). Although practical examples are confined to phosphate cements, others are known which are based on a variety of anions polyacrylate (Chapter 5), oxychlorides and oxysulphates (Chapter 7) and a variety of organic chelating anions (Chapter 9). The number of cements available for this purpose is very great. 

As shown in Figure 10.12, four of the six chelation sites of the cobalt atom of cobalamin are occupied by the nitrogens of the corrin ring and one by the nitrogen of the dimethylbenzimidazole side chain. The sixth site may be occupied by the following ligands in biologically active vitamers ... 

Recently the chelating agent ethylenediaminetetraacetic acid (EDTA) has been used in connection with this problem,and although the relationships are complex, it is possible to show biological activity for Ca++ and Co++. Slater has used 8-hydroxyquinaline to remove trace elements from the basal medium and has demonstrated a cobalt requirement in glucose-free medium. No such requirement was found, however, when glucose was present. 

I would suggest that the formation of metal chelate complexes, with a four or six-coordinate metal partly bound to an optically active protein and partly bound to a substrate molecule can explain this stereospecificity. The optically active coordination compounds of metals, such as cobalt, have extraordinarily high molecular rotation, and so the difference in chelation powers of the d and I forms of a substrate may be very great. As Dr. Chaberek has pointed out (Lecture 33), this chelation may involve both metals of constant valency, e.g.. Mg, Zn, and those of variable valency. Metallic ions of both types are proven essential trace metals in biological systems. 

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