THREE DECADES PROGRESS IN MICROBIOLOGY 



One might rightly ask for the reasons which are responsible for the 

 fact that such a far-reaching discovery which amongst others notice- 

 ably affects our views on the carbon cycle in nature has been made so 

 late in the development of our science. Yet this is easily understood, if 

 one realizes that in all these organisms metabolism is characterized by 

 a net production of carbon dioxide thus rendering it virtually impossible 

 to detect any simultaneous reassimilation of carbon dioxide with the 

 usual chemical or manometrical techniques. It is only the application 

 of a new and powerful tool to the problem, namely the use of 'tracer' 

 or 'labelled' elements which has made this progress possible. 



On the principle of this method which even in the short time of its 

 application has revolutionized several aspects of metabolism I need 

 not digress, since it is again in Copenhagen that it was first introduced 

 into biochemistry by von Hevesy. The remarkable results obtained 

 by this scientist and his followers on introducing compounds contain- 

 ing radioactive phosphorus into metabolic investigation are universal- 

 ly known. But the importance of these studies have perhaps even been 

 surpassed when Schoenheimer and Rittenberg applied the same prin- 

 ciple in their researches on nitrogen metabolism by making use of 

 nitrogen compounds containing the stable heavy N 15 isotope. It now 

 has been the application of radioactive C 11 carbon by Ruben, Kamen 

 and their co-workers, and of the stable heavy C 13 isotope by Wood, 

 Werkman and collaborators that has yielded the incontestable proof 

 for the assimilation of carbon dioxide by heterotrophic cells. 



However, it should not be inferred that the purport of the recent 

 investigations with the carbon isotopes is restricted to the mere estab- 

 lishment of this carbon dioxide assimilation. 



In the first place it has been possible to identify in some instances 

 the primary reactions in which the carbon dioxide is fixed. Thanks to 

 the use of the labelled carbon there is nowadays no longer any doubt 

 that Bacterium coli contains enzyme-systems capable of bringing about 

 the carboxylation of pyruvic acid leading to the formation of oxal- 

 acetic acid which acid then is converted into succinic, lactic and acetic 

 acids. It was further proved that in accordance with expectation in 

 all these products the labelled carbon was located in the carboxyl 

 group. But more recent studies by Lipmann and Werkman have 

 shown that carboxylation of pyruvic acid is not the only way in which 

 Bacterium coli can fix carbon dioxide. The second way is especially 



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