METABOLIC CYCLES AND DECARBOXYLATION 205 



considered additional evidence for the nonparticipation of citric 

 acid in the synthesis of alpha-ketoglutaric acid from pyruvate and 

 carbon dioxide. 



Table 2.— The e£Fect of citrate on the radioactivity of synthesized 

 alpha-ketoglutaric acid* 



* Experimental conditions were similar to those in the experiments reported in 

 Table 3. The sodium citrate was added simultaneously with the pyruvic acid. 



Wood and Werkman and their collaborators (12) have suggested 

 that isocitric acid rather than citrate serves as an intermediate, such 

 a scheme retaining in skeleton form the original suggestion that 

 alpha-ketoglutarate was produced by way of the citric acid cycle. 

 In view of the demonstrated equilibrium, in most tissues, between 

 citrate and isocitrate (16), additional evidence is required to estab- 

 lish this hypothesis. The exclusion of citric acid as an intermediate 

 in alpha-ketoglutarate synthesis might well lead to an examination 

 of whether oxalacetate itself is formed as the primary product of 

 carbon dioxide assimilation in pigeon liver. The great chemical 

 reactivity and instability of this compound make it improbable that 

 it should accumulate to any appreciable extent under our experi- 

 mental conditions. Though we have studied a wide variety of 

 conditions, we have failed to obtain any evidence that the compound 

 was present during the course of alpha-ketoglutarate synthesis; or, 

 to express the matter more precisely in experimental terms, we have 

 failed to observe the accumulation of any substance that would 

 release radioactive carbon dioxide on treatment with aniline citrate. 

 Certainly it would be desirable to have more convincing evidence 

 that oxalacetic acid is the primary product of carbon dioxide as- 

 similation in pigeon liver. 



There seems little doubt that the utilization of carbon dioxide by 

 pigeon liver is a process of considerable magnitude. Table 3 gives 

 data from a series of typical experiments which indicate that as 

 much as 5 per cent of the original activity of the added inorganic 



