Discussion on Bacterial Respiration 



W. H. PETERSON, University of Wisconsin, Chairman 



CRITERIA FOR EXPERIMENTS WITH ISOTOPES 

 H. G. Wood, Iowa State College: 



The criteria that are to be used in considering the rehabihty of 

 isotopic work will vary considerably with the type of investigation. 

 For example, in studies with carbon of atomic weight 13 in which 

 one wishes to determine qualitatively whether or not carbon dioxide 

 is fixed in a biological reaction, the criteria are relatively simple. 

 The reaction is simply conducted in an atmosphere containing car- 

 bon dioxide in which the content of C^^ has been increased artifi- 

 cially above that of carbon found in nature. Carbon in nature 

 contains about 1.1 per cent C^^. The carbon dioxide used for a tracer 

 usually contains from 5 to 15 per cent C^^, the concentration 

 depending on the method used to obtain the heavy isotope. To 

 determine whether or not carbon dioxide is incorporated into an 

 organic compound, it is necessary only to free the reaction mixture 

 of carbon dioxide, convert the organic compounds to carbon dioxide, 

 and determine its content of C^^ in a mass spectrometer. If the C^^ 

 content of the reaction mixture is significantly above 1.1 per cent 

 (the natural complement of C^^), carbon dioxide has been incor- 

 porated into organic compounds. This is obviously the case, since 

 there is no other source of carbon than carbon dioxide with a content 

 of C^^ above the normal. 



The question is, what reliable conclusion can be drawn from such 

 an experiment? Clearly, the only conclusion that can be made is 

 that carbon dioxide is fixed, but no idea is given as to the reaction 

 involved or the compounds concerned. In the early isotopic work 

 such experiments were given undue significance. For example, this 

 type of experiment was conducted by Ruben and Kamen with pigeon 

 liver, Escherichia coli, and a number of other heterotrophic systems 

 and it was found that a small amount of radioactive carbon dioxide 

 was fixed. These experiments have been cited as examples of assimila- 

 tion of carbon dioxide by heterotrophic organisms. There is no general 

 agreement on a definition of assimilation, but it is certain that to 



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