Sec. 27.3] TRACE ELEMENTS ESSENTIAL TO ANIMALS AND PLANTS 525 



Fe 59 individually are known and make possible the use of these two isotopes 

 in multiple-labeling experiments. 



Fe 59 has been the more widely used isotope. It has been employed largely 

 in the study of iron metabolism and in the determination of the red-cell 

 survival time and the total red-cell volume in the mammal, especially man. 

 This use has recently been reviewed by Hahn [Fe32]. A number of impor- 

 tant new facts about iron metabolism have been demonstrated with Fe 59 . 

 Particularly striking has been clear-cut proof of the ability of the mammal 

 to conserve iron supplies by limiting uptake and excretion [Fe24,37,40, etc.]. 

 The uptake of iron is controlled by the presence of a protein that is capable 

 of combining stoichiometrically with 23 per cent of its weight of iron [Fe40,45] ; 

 this is ferritin (or, in its iron-free state, apoferritin). Ferrous iron is better 

 absorbed than ferric by man [Fe46,47,58]. The loss of iron from the body is 

 largely through wear and tear rather than any of the usual routes of excretion. 



The most important practical result of studies 'on the survival of red cells 

 has been demonstration of the best method of in vitro storage of blood to be 

 used for transfusions [Fel5-17, 19,67-69]; this involves refrigeration, the 

 addition of dextrose to the citrate anticoagulant, the maintenance of a 

 slightly acid reaction of the diluted plasma or resuspension fluid, and optimal 

 dilution. In certain acid-citrate solutions whole blood can be preserved 

 with up to 70 per cent viability for 3 weeks, whereas cells preserved by the 

 simple addition of citrate are almost entirely broken down within 24 hr in the 

 new host after only 2 weeks of preservation in vitro [Fe65]. 



The Fe 59 -labeled red cell has been used to follow the distribution of blood 

 [Fe20,21] and to determine the total red-cell volume by the intravenous 

 administration of tagged cells of known activity and the recovery of blood 

 samples after allowing time for mixing to approach completion [Fel8,21,22,29, 

 etc.]. 



Fe 55 has had relatively little application, but because of its availability in 

 high specific activity from the cyclotron by the reaction Mn 55 (d, 2n)Fe 55 , it 

 has been useful for studies on the distribution of tracer amounts. Work with 

 this isotope has shown the rapid turnover of iron in the bone marrow, its 

 storage in major part in the liver, and its rapid turnover in the small intestine 

 [Fe7], as well as its minimal excretion in the urine, bile, and feces [Fe7,24]. 

 An ingenious study with Fe 55 has shown that the parasite of benign tertian 

 malaria {Plasmodium vivax) selectively invades young erythrocytes [Fel4]. 



Fe 55 and Fe 59 have been used in double-labeling experiments on red-cell 

 survival [Fel6,17, etc.]. Conceivably the rarer stable species of iron Fe 54 , 

 Fe 57 , and Fe 58 may find use in multiple-labeling experiments now that they 

 have been prepared in enriched form. 



27.3. Iodine. Iodine, an essential element in vertebrate metabolism, 

 has a number of useful and potentially useful radioactive isotopes. Three 



