Sec. 28.2] ELEMENTS NOT KNOWN TO BE ESSENTIAL TO LIFE 531 



c. Chromium. No tracer work has been carried out for chromium, which 

 has been suggested as a possible trace requirement in certain plants and 

 animals. The radioactive species Ci 51 (26.5-day half-life) is available for 

 such studies. 



d. Nickel. Nickel has been found concentrated in the mammalian pan- 

 creas and in the bodies of several invertebrates and may be involved in 

 enzyme systems. Its metabolism has not yet been subjected to tracer 

 studies, although the radioactive isotopes Ni 57 (36-hr half-life), Ni 59 (12-year 

 half-life), and Ni 66 (56-hr half-life) are available and should prove useful. 



e. Gallium. There is strong evidence for the essential role of gallium 

 in certain plants. It has not yet been studied with tracer methods, however. 

 Several potentially useful radioactive species exist, the most promising of 

 these for tracer studies being Ga 67 (83-hr half-life). 



/. Bromine. Bromine is present in small amounts as an electrolyte in the 

 vertebrate body, primarily extracellularly like chlorine. Although an essen- 

 tial role has been postulated, definitive evidence is lacking. 



Tracer studies with the radioactive species Br 28 (34-hr half-life) have been 

 fairly extensive. The bromide ion has been found to behave in general 

 distribution very much like Na + [Br6]. It is completely distributed in the 

 dog body within a period of 1 to 2 hr. 



Br 82 has also been used to label diazo dyes in studying the escape of colloids 

 from the blood stream [Br3] and the concentration and detection of dyes 

 in abscesses [BrlO-12,17]. Certain dyes become bound to plasma proteins 

 and, when labeled with Br 82 , have been used in dogs to show the great increase 

 in capillary permeability to colloids after burns [Br3]. 



The significant selective uptake of bromine by the thyroid has also been 

 shown with Br 82 [Brl3]. 



g. Rubidium. It has been claimed that rubidium may actually replace 

 potassium in the nutrition of certain bacteria. The element is present in 

 trace amounts as an ion in the vertebrate body fluids. Evidence for an 

 essential role is, however, lacking in either case. 



There have been a few studies on the behavior of ionic rubidium in biologi- 

 cal systems with the use of the radioactive isotope Rb 86 (19.5-day half-life). 

 In the dog it closely resembles potassium in distribution after intravenous 

 injection [Rbl]. The uptake of the element and its concentration from very 

 dilute solutions of Rb + by the roots of plants has also been demonstrated 

 [Rb3], 



h. Strontium. Strontium has been of considerable interest because of the 

 similarity of its fate in vertebrate metabolism to that of calcium. Because 

 of the poor cyclotron yields of Ca 45 , considerable study on bone physiology 

 has been carried out using the radioactive isotope Sr 85 (65-day half-life) in 

 place of a calcium isotope. Despite its well-known metabolic behavior in 



