Sec. 27.5] TRACE ELEMENTS ESSENTIAL TO ANIMALS AND PLANTS 527 



Radioactive iodine has also provided a therapeutic tool in the treatment 

 of hyperthyroidism (see Chap. 29). 



The use of the iodine label is the best illustration of the invaluable role that 

 radioactive isotopes can play in the study of trace-element metabolism. 



27.4. Manganese. Manganese, which is known to activate a number of 

 enzymes, but of which the biological role is not yet clearly defined, is never- 

 theless thought to be a probable micronutrient requirement for all living 

 systems. It has been well established as essential to the growth and repro- 

 duction of higher animals. At least three useful radioactive species exist: 

 Mn 52 (6.5-day half-life) made in the cyclotron by the reaction Cr 52 (d, 2n)Mn 52 , 

 which also produces small amounts of Mn 54 by the reaction Cr 54 (d, 2n)Mn 54 ; 

 Mn 54 (310-day half-life), made in the cyclotron by the reaction Fe 56 (d, 

 a)Mn 54 , which is accompanied by the reaction Fe 54 (d, a)Mn 52 ; and Mn 56 

 available from the cyclotron by the reaction Mn 55 (d, p)Mn 56 . 



Tracer experiments have been reported with' all three of the isotopes 

 mentioned. Mn 52 (admixed with small amounts of Mn 54 ) has been applied 

 to the study of the distribution of colloidal manganese oxide injected intrave- 

 nously for therapeutic purposes in man (see Chap. 29). Mn 54 (admixed with 

 small amounts of Mn 52 ) has had wider use, having been employed to show 

 that tracer amounts of manganese are rapidly excreted in the rat almost 

 entirely in the feces [Mn5,6 (in the latter reference Mn 54 is erroneously refer- 

 red to as Mn 56 )] and that, even in the case of perosis in the chicken produced 

 by manganese deficiency, the element is not deposited to a greater than nor- 

 mal extent in the bone, although it accumulates in the liver in greater 

 amounts than in normal animals [Mn8]. Mn 56 has also been used for the 

 study of manganese distribution in the mammal [Mnl]. 



The use of the tracers of manganese has been very limited, and much 

 further work may be anticipated. 



27.5. Copper. Copper, known to be an essential micronutrient require- 

 ment at least of higher animals and functioning in a number of protein and 

 enzyme complexes, has two potentially useful radioactive species for biological 

 experimentation: Cu 61 (3.4-hr half-life) and Cu 64 (12.8-hr half-life) made 

 as a mixture by the cyclotron reactions Ni 60 (d, n)Cu 61 , Ni 61 (d, 2n)Cu 61 , 

 and Ni 64 (d, 2n)Cu 64 . In addition, Cu 64 can be made in the uranium pile 

 by the reaction Cu 63 (n, 7)Cu 64 . The two stable species, especially the rarer 

 one Cu 65 , may also find tracer application, particularly in view of the rela- 

 tively short half-life of Cu 64 . 



So far the few biological studies published have been done with Cu 64 , any 

 associated Cu 61 having decayed away or having been ignored. Studies on 

 the uptake and elimination of tracer amounts of copper fed to rats show that, 

 of that absorbed, most goes to the liver in absolute amount, but that it con- 

 centrates relatively more in the kidneys than in other organs, although little 



