528 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 27 



is eliminated in the urine [Cu4]. Following ingestion it appears rapidly in 

 the plasma [Cu7]; the uptake is relatively greater in animals having hemato- 

 poietic activity increased over normal [Cu4,7]. Most of the copper present 

 in the plasma is bound in some way to protein. 



The existence of a longer lived radioisotope, presumed to be Cu 67 (56-hr 

 half-life), promises considerable extension in tracer work with copper pro- 

 vided that this species can be produced in sufficient yield. 



27.6. Zinc. Zinc, like manganese, is important in enzymatic functions 

 and may well be a universal micronutrient requirement of living systems. 

 It is known to be essential in the metabolism of the vertebrate. It occurs in 

 several stable and radioactive species of which only two have had biological 

 application: Zn 65 (250-day half-life), produced in the cyclotron by the reac- 

 tion Zn 64 (n, 7)Zn 65 and Cu 65 (d, 2n)Zn 65 and in the uranium pile by the reac- 

 tion Zn 64 (n, 7)Zn 65 ; and Zn 63 (38-min half-life), produced in the cyclotron 

 by the reactions Cu 63 (d, 2n)Zn 63 and Cu 63 (p, n)Zn 63 . Other radioisotopes 

 exist of short half-life: Zn 69 (13.8-hr), Zn 69 (57-min — isomer of preceding), 

 and Zn 72 (49-min). These, with Zn 63 , offer possible alternatives if it is 

 desired to avoid the long half-life of Zn 65 . 



So far the biological experiments with Zn 65 have been limited to a few 

 studies on the fate of zinc administered by various routes [Znl,6,7]. Like 

 manganese, but unlike cobalt, zinc is excreted largely by way of the gastro- 

 intestinal tract, with relatively little in the urine. It accumulates in the 

 liver from which it is rapidly lost in the bile; it also concentrates to varying 

 degrees in the pancreas, kidney, and spleen. 



Zn 63 has been used [Zn2-4] to obtain localized irradiation of tissues injected 

 with labeled colloidal zinc. 



Further fundamental advances in the understanding of zinc metabolism 

 quite evidently depend on further application of zinc tracers. 



27.7. Cobalt. Cobalt appears according to recent evidence to be an 

 essential trace element in the nutrition of higher animals. There are a 

 number of radioactive isotopes of which four are formed by the deuteron 

 bombardment of iron: Co 55 (18.0 hr) by Fe 54 (d, n)Co 55 ; Co 56 (80 days) by 

 Fe 56 (d, 2n)Co 56 ; Co 57 (270 days) by Fe 56 (d, n)Co 57 ; and Co 58 (72 days) by 

 Fe 57 (d, n) Co 58 . From the nuclear-pile reactor, Co 60 (5.3-year half-life) is 

 available by the reaction Co 59 (n, 7) Co 60 . 



With one exception the few studies reported have been carried out with 

 mixtures of Co 56 , Co 57 , and Co 58 , of which the first and last make up the 

 more important component; Co 55 decays away in a few days, and con- 

 sequently targets are allowed to age so that this species can be disregarded. 

 The chief route of excretion of cobalt is the urine, in contrast to manganese 

 and zinc [Co5,7,9]. Ingested cobalt is only partly absorbed, and the bile 



