48 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



Steric Relations and Enzyme Action. In a very important paper 

 Ogston (108) postulated that compounds that are chemically symmetrical 

 may exist in two spatial configurations, one of which could combine with 

 an enzyme and undergo certain reactions, whereas the other configuration 

 could not. Support for this concept was found in the studies of Potter 

 and Heidelberger (109), as follows: 0^*02 was incorporated into citric acid 

 in a rat liver homogenate, and the citric acid then purified by chromatog- 

 raphy and converted to a-ketoglutarate in another rat liver homogenate. 

 This compound was purified and chemically degraded to yield CO2 and 

 succinic acid. The C^^ label was found entirely in the CO2 and not at all 

 in the succinic acid. This reaction may then be represented schemat- 

 ically as follows: 



iCOOH 



1 

 2CH2 



iCOOH 



I 

 2CH2 



rat liver 

 I homogenate I 



HO— ^C— ^COOH > 'CH5 



^CH2 



1 

 6C*00H 



citric acid 



KMn04 



I 



6C*00H 



a-ketoglutaric 

 acid 



iCOOH 



■-CH2 



^CH2 + *'C*02 



»COOH 



succinic acid 



(1-41) 



It is clear from the experimental results that carbon atoms 1 and 6 in 

 Eq. (1-41) were not identical so far as the enzyme action was concerned. 

 However, there was the question as to whether this result may not have 

 been due to the presence of the radioactive carbon atom. In other words, 

 was there an isotope effect? This matter was settled by the work of 

 Wilcox et al. (110), who prepared the l- derivative of an asymmetric com- 

 pound which was then converted to citric acid and carried through the 

 same procedures as above. The reactions are indicated below, and in 

 this case all the C^* was found in the succinic acid, none being present in 

 the CO2: 



CI 

 2CH5 



iC*OOH 



I 

 2CH2 



rat liver 

 1 KC^^N I homogenate 



HO— ^C— ^COOH > HO— ^C— ^COOH > 



^CH2 



I 

 6C00H 



^CH2 

 «COOH 



citric acid 



