254 A SYMPOSIUM ON RESPIRATORY ENZYMES 



peared that the carbon dioxide was equally distributed among the 

 three carbons in the chain. In other words, it seeemed that the pro- 

 pionic acid might be synthesized entirely from carbon dioxide, which 

 would be a most remarkable accomplishment for a typically hetero- 

 trophic propionic acid organism. It was soon proved, however, that 

 the reaction was not a reliable method of decarboxylating propionic 

 acid. This was done by checking the reaction with synthetic pro- 

 pionic acid, which contained C^^ in the carboxyl group. Further, by 

 use of a rehable degradation reaction, all the fixed carbon in the 

 biologically formed acid was shown to be in the carboxyl group. 

 Carson and his co-workers obtained the same results when they re- 

 checked their previous procedures. 



With this example we may pass on to consideration of the case, in 

 which it will be assumed the criteria described above have been 

 met, i.e., ( 1) a reliable determination of C^^ has shown there is fixa- 

 tion of carbon dioxide; (2) the experimental procedure has been a 

 good one for demonstrating the desired results; and (3) the com- 

 pound or compounds containing the fixed carbon have been isolated 

 and degraded by reliable chemical reactions; thus the location of the 

 fixed carbon in the respective compounds is known. 



It is true that only by the use of isotopic carbon could such in- 

 formation be obtained, but even then there may be much uncer- 

 tainty respecting the mechanism of fixation of carbon dioxide. For 

 example, in bacterial glucose fermentations by Staphylococcus, 

 Streptococcus, and Proteus, it has been shown by Slade et al. that the 

 carbon dioxide is fixed in the carboxyl groups of the lactate and suc- 

 cinate; with Aerohacter and Clostridium welchii, there is fixation in 

 the carboxyl group of the acetic acid as well. With pigeon liver on 

 pyruvate, there is fixation in the carboxyl groups of malate, fumarate, 

 succinate, lactate, and alpha-ketoglutarate. Many of these fixations 

 are believed to occur initially by three- and one-carbon addition 

 through the following reaction: 



CO2 -1- CH3 • CO • COOH = COOH • CH2 • CO • COOH 



Particularly it is believed that this reaction is instrumental in the 

 formation of four-carbon dicarboxylic acids. The mechanism of the 

 fixation in lactate and acetate is still largely unknown. But what 

 criteria can be used in determining the reliability of the suggested 

 fixation by three- and one-carbon addition? At present there is no 

 completely reliable criteria; but this represents our interpretation of 

 the present known experimental facts. 



Thus far the synthesis of oxalacetate from pyruvic acid and carbon 



