SEVERO OCHOA 



cal. and the calculated equilibrium constant, K^ = (/-malate)/(/(+)- 

 lactate)(C02), approximately 0.1. 



Step 3 could also involve dismutation with any other diphos- 

 phopyridine nucleotide dehydrogenase system having an oxidation- 

 reduction potential lower than that of the malic system (see Table I). 

 We have considered the lactic dehydrogenase in this connection because 

 there is evidence that it can participate in carbon dioxide fixation by 

 pyruvic acid. In the presence of the enzyme fumarase, part of the 

 /-malic acid formed in reaction He would be dehydrated to fumaric 

 acid. Fumarase is widely distributed in plant and animal cells. Re- 

 action lie would then be replaced by reaction Ilf: 



pyruvate + CO2 + /(+) -lactate ^ 



fumarate + H2O -f pyruvate (Ilf) 



Experimental support for the occurrence of the over-all reaction 

 (Ilf) is gained from the observation that fumarate, when added to an 

 enzyme preparation from liver in the presence of pyruvate, DPN, and 

 manganese ions, is converted to lactate and carbon dioxide (5). This 

 indicates that reaction Ilf can proceed from right to left. That it also 

 proceeds from left to right is indicated by the presence of isotopic carbon 

 in the carboxyl groups of the residual fumarate when the reaction is 

 carried out in presence of isotopic carbon dioxide (29). The pyruvic 

 oxalacetic system of carbon dioxide fixation has not yet been investi- 

 gated by the methods used in the study of the ketoglutaric-oxalosuccinic 

 system. 



A considerable shift of the equilibrium of reaction Ilf in the 

 direction of carbon dioxide fixation can be brought about by reduction 

 of the fumarate to succinate, a reaction that occurs in bacteria and 

 liver tissue. In fermentation of glucose or glycerol by propionic acid 

 bacteria, succinate is found to be one of the end products. Experi- 

 ments with isotopic carbon dioxide have shown that the carboxyl 

 groups of succinic acid become labeled. The carboxyl groups of mal- 

 ate, fumarate, and succinate, formed by pyruvate fermentation with 

 Escherichia coli in the presence of carbon dioxide containing isotopic 

 carbon, also show excess of heavy carbon. This is also the case when 

 pyruvate is incubated with liver preparations. Some bacteria, e. g., 

 E. coli, can use molecular hydrogen for fumarate reduction and, in the 



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