DAVID SHEMIN 



3 times more radioactive than that made from the a-carbon atom 

 of glycine (9). 



The cycle also provides a mechanism by which the carbon 

 atoms of glycine can be oxidized to carbon dioxide. The for- 

 mation of a-amino-jS-ketoadipic acid and subsequent decarbox- 

 ylation of this /3-keto acid would yield 1 mole of carbon dioxide. 

 This mole of carbon dioxide was originally the carboxyl group 

 of glycine. If the postulated ketoglutaraldehyde or some other 

 derivative can be converted to a-ketoglutaric acid, a pathway 

 is provided for the conversion of the oi-carbon atom of glycine to 

 carbon dioxide, for the a-carboxyl group of the ketoglutarate 

 was originally the a-carbon atom of glycine. In this mech- 

 anism, it will be noted, formate is not an obligatory intermediate 

 for the conversion of the a-carbon atom of glycine to carbon 

 dioxide. a-Ketoglutaric acid isolated from a cell-free extract of 

 duck red blood cells which were incubated with 5-aminolev- 

 ulinic acid-5-G^^ contained radioactivity (29). 



Although the evidence for the condensation of succinate and 

 glycine appears to be rather conclusive, it does appear that this 

 reaction may not be unique but rather a prototype of a more 

 general reaction. We observed a few years ago that the utiliza- 

 tion of glycine for porphyrin formation is markedly inhibited by 

 the addition of either acetate or pyruvate (13). These com- 

 pounds appear to inhibit the condensation of succinate and 

 glycine, since the conversion of 5-aminolevulinic acid to por- 

 phyrin is not influenced by the addition of these acids. It 

 would appear that "active" acetate may compete with succinate 

 for the glycine, since the inhibition can be overcome by the 

 addition of other members of the citric acid cycle. If acetate 

 condenses with glycine, the product would be a-aminoaceto- 

 acetic acid. Upon decarboxylation this j8-keto acid would 

 yield aminoacetone, an analogue of 5-aminolevulinic acid. 

 We have carried out some studies with aminoacetone labeled 

 with C^^ in the carbon atom bearing the amino group. We 

 have found that this radioactive carbon atom is utilized for the 

 ureido groups of uric acid, that it can be converted both to for- 



534 



