Sec. 28.3] ELEMENTS NOT KNOWN TO BE ESSENTIAL TO LIFE 533 



a. Arsenic. Arsenic is an important agent in syphilotherapy and the 

 treatment of leukemia. It is also used in antiprotozoal and anthelminthic 

 drugs. 



Studies with radioactive arsenic have been limited to the isotope As 74 

 (16-day half-life). Tracer experiments have demonstrated the uptake of 

 subcutaneously administered arsenites in the body proteins of various 

 mammals, but have not supported the view that arsenic may replace phos- 

 phorus in proteins [As7,ll]. In concentration per unit weight of tissue 

 arsenic administered as arsenite is found in largest amounts in the kidney 

 and in decreasing amounts in the liver, epidermis, spleen, lung, and muscle 

 [AslO]. In experimental filariasis (Litomosoides carinii infection in the 

 cotton rat) the concentration in the adult worm is only less than in the kidney 

 and liver [AslO]. 



b. Silver. Potentially valuable radioactive species for tracer work with 

 silver are particularly the isotopes Ag 106 (8.2-day half-life), Ag 110 (225-day 

 half-life), and Ag m (7.5-day half-life). 



c. Antimony. Antimony, important in anthelminthic and antiprotozoal 

 agents, has had some tracer application in the form of Sb 124 (60-day half-life). 

 When administered to dogs in the form of tartar emetic, it is found to con- 

 centrate most highly in the liver and in decreasing amounts in the thyroid, 

 kidney, cortex, pancreas, intestine, spleen, etc. In dogs with filariasis (the 

 heart worm, Dirofilaria immitis) Sb 124 concentrated more highly in the adult 

 worm than in any other tissues except the liver and thyroid [Sb2,3]. 



d. Gold. A single tracer study has been reported for gold with the use of 

 Au 198 (2.7-day half-life). The highest uptake of the element is in the kidney 

 and liver [Aul]. 



e. Mercury. Mercury, an important industrial poison and therapeutic 

 agent, has had but one tracer study — with the radioactive species Hg 197 (23- 

 and 64-hr half-lives). It was used to establish the concentration of mercury 

 vapor in a particular industrial operation suspected of causing chronic mer- 

 curialism [Hgl]. 



/. Lead. Lead is an important industrial poison. It was the first element 

 to be studied biologically by the "tracer" technique, namely, that of sub- 

 stituting for a normally stable element one of its radioactive species [Pb7] and 

 thus tracing its fate in a biological system (in this case a plant). 



Two lead isotopes, both belonging to naturally radioactive series, have 

 been used in the study of the uptake and fate of lead in organisms: Pb 210 or 

 RaD (22-year half-life) and Pb 212 or ThB (10.6-hr half-life). Pb 210 belongs 

 to the uranium series; Pb 212 , to the thorium series. 



Studies with Pb 210 have included the uptake of lead in erythrocytes [Pb9], 

 the absorption of lead tetraethyl [Pb8], the effect of pectin on the retention 



