CHAPTER 27 



TRACE ELEMENTS KNOWN TO BE ESSENTIAL 

 IN ANIMALS AND PLANTS 



Ellsworth C. Dougherty 



27.1. Introduction. The elements so far considered have major metabolic 

 roles in some or all living systems. In addition to these macronutrient con- 

 stituents of protoplasm, there are a number of elements that have been 

 recognized as being essential in "trace" amounts in the metabolism of certain 

 organisms at least. Certain of these micronutrient elements, at least iron 

 and probably also manganese and zinc, are universally required for living 

 systems. A number of others are known to be essential for certain forms of 

 life but not for others. 



The elements accepted as essential trace nutrilites for purposes of this 

 volume are, in order of increasing atomic weight, boron, aluminum, silicon, 

 vanadium, manganese, iron, cobalt, copper, zinc, molybdenum, and iodine. 

 They are arranged somewhat arbitrarily in the following subsections accord- 

 ing to their importance in mammalian nutrition; the last five elements 

 (molybdenum, boron, aluminum, silicon, and vanadium) are known to be 

 essential either to plants or to certain of the lower animals, but not to mam- 

 mals. Comar [Gen28] has recently reviewed in detail trace elements in 

 mammalian nutrition. 



It will likely prove very difficult to carry out satisfactory tracer studies on 

 trace elements for which no sufficiently long-lived isotope is available to 

 permit more than very short experiments. Silicon and particularly boron 

 and aluminum are in this category. The use of separated stable isotopes in 

 the case of silicon and boron does not offer much advantage theoretically over 

 the natural element. 



27.2. Iron. Iron, which is regarded as a universally essential trace 

 element in the oxidative enzyme systems of cells, exists in two radioactive 

 forms that have found tracer application: Fe 55 (about 4-year half-life) and 

 Fe 59 (47-day half-life). A mixture of these isotopes can be produced in the 

 cyclotron by the deuteron bombardment of iron: Fe 54 (d, p)Fe 55 and Fe 58 (d, 

 p)Fe 59 ; in addition Fe 55 can be made by the reactions Mn 55 (p, n)Fe 55 or 

 Mn 55 (d, 2n)Fe 55 , and Fe 59 by the reaction Co 59 (n, p)Fe 59 . In the nuclear- 

 pile reactor a mixture of both isotopes can be produced by the bombardment 

 of iron Fe 54 (n, 7)Fe 55 and Fe 58 (n, 7)Fe 59 . Counters sensitive to Fe 55 and 



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