256 A SYMPOSIUM ON RESPIRATORY ENZYMES 



dehydrogenase. If carbon dioxide was concerned only in a non-essen- 

 tial exchange, it would not be required. 



R. H. BuRRis, Columbia University: 



The criteria that must be met in the use of isotopes as tracers have 

 been well covered by Dr. Wood in the preceding discussion, so we 

 may consider certain other problems connected with isotopic tracers. 

 The question is frequently heard, "Which tracers are more suitable 

 for biological problems, radioactive or stable isotopes?" There is no 

 blanket answer, for the suitability of a given tracer depends not only 

 upon the properties and availability of the given element, but also 

 upon the type of experiment in which the tracer is to be employed. 



In quahtative studies the radioactive isotopes of reasonably long 

 half -life possess many advantages; they can usually be detected in 

 greater dilution and with greater facility than can the stable isotopes. 

 In addition, they may often be traced directly in vivo without la- 

 borious fractionation of the organism. For example, shortly after 

 the ingestion of a radioactive sodium salt radioactivity may be de- 

 tected in any part of the body if it is brought into proximity to a 

 Geiger counter chamber. Radioactive substances with sufficiently in- 

 tense radiations will reveal their distribution by forming radiographs 

 on photographic plates. In this manner the pattern of radioactive 

 substances in bones has been demonstrated following the feeding of 

 isotopes. 



Radioactive isotopes have been prepared in much greater variety 

 than have the stable isotopes. More than two hundred radioactive 

 isotopes have been produced. Many, of course, are of no biological 

 significance; others have not been prepared in suitably high con- 

 centrations for tracer studies; and unfortunately some of the ele- 

 ments of greatest biological interest form isotopes of very short half- 

 life. The following radioactive elements have been used in biological 

 investigations: 



Isotope Half-life Isotope Half-life 



,W 85 days i,CP« 37 minutes 



eC^i 20.5 minutes j^K" 12.4 hours 



eC^^ 100-1000 years 2oCa*^ 2.5 hours 



tN^^ 10.5 minutes scFe^^ 47 days 



9F18 112 minutes 3,Br8o 44 j^q^j-s 



nNa^* 14.8 hours g.Br^^ 34 j^q^^s 



15P'' 14.3 days ggP^s 25 minutes 



leS^^ 88 days 



