DISCUSSION ON BACTERIAL RESPIRATION 255 



dioxide has not been accomplished, but Krampitz in our laboratories 

 has done the next best thing. With a preparation of Micrococcus, 

 which decarboxylates oxalacetate to pyruvate, he has shown the 

 catalysis of the exchange of C^'^-carbon dioxide with the carboxyl 

 group of oxalacetate, i.e., incubation of oxalacetate with C^^-carbon 

 dioxide and the enzyme gave oxalacetate containing C^^ in the 

 carboxyl group. During this exchange the oxalacetate is apparently 

 broken down to a three-carbon compound and again resynthesized, 

 permitting the entrance of C^ ^-carbon dioxide. It is suggested that 

 the three-carbon compound involved in the fixation reaction is not 

 pyruvic acid, as such, but a derivative of this compound that is 

 foraied during the decarboxylation of oxalacetate. This is the first 

 direct evidence, i.e., evidence with oxalacetate as such, that has been 

 obtained as proof of the reaction. 



Finally, I should like to comment on one further consideration. 

 Assuming that we know the mechanism of the reactions concerned 

 in fixation of carbon dioxide, we are still faced with one very impor- 

 tant question, namely, is the fixation reaction an essential step in the 

 dissimilation or is it the result of an exchange reaction, for example? 

 Time does not permit full development of this subject. Furthermore, 

 since the problem has not yet been given much consideration in most 

 investigations, satisfactory criteria have not been devised for de- 

 termining whether a reaction is essential or not. An example may 

 serve to illustrate the point. In the Krebs cycle, as Dr. Evans pointed 

 out in his lecture, the pyruvate is believed to be oxidized after 

 union with oxalacetate through a cyclic conversion. The oxalacetate, 

 it is believed, arises from the fixation reaction through union of pyru- 

 vate and carbon dioxide. The fixation reaction would then be an 

 essential reaction in the oxidation of pyruvate, since it would supply 

 the necessary oxalacetate. It is possible, however, that the oxalacetate 

 cannot be formed by this reaction but is formed by some other 

 reaction of pyruvate. The oxalacetate thus formed might react with 

 carbon dioxide by an exchange reaction as studied by Krampitz. In 

 this case one would arrive at oxalacetate containing heavy carbon 

 just as he would if the oxalacetate was formed by union of pyruvate 

 and carbon dioxide. Isotopic analysis would not serve to differentiate 

 the mechanisms. It is evident that a false importance may be ascribed 

 to carbon dioxide fixation reactions, for fixed carbon dioxide may 

 result from a non-essential exchange reaction. Frankly, I do not 

 believe this to be the case, but I can cite only indirect criteria to 

 support my opinion, such as the fact that carbon dioxide is necessary 

 for growth of bacteria and in the reduction of methylene blue by 



