60 David Shemin and Jonathan Wittenberg 



would result in a-ketoglutaric acid labelled only in the y-car- 

 boxyl group in the first turn of the cycle. The a-ketoglutaric 

 acid produced after repeated cycles would contain ^^C in both 

 carboxyl groups but in no other position (Fig. 5 and Table V) 

 (Lifson et al,, 1948; Wood, 1948). If a-ketoglutaric acid were 

 directly utilized, with the elimination of the a-carboxyl group, 

 the protoporphyrin made from carboxyl labelled acetate 

 would contain ^*C only in the carboxyl group. Actually, 

 however, carbon atoms numbered 3 and 5 in the porphyrin 

 contain some ^*C. These atoms correspond to the carbonyl 

 caibon atom of a-ketoglutaric acid, and should contain no 

 ^*C if a-ketoglutaric acid were the immediate precursor, unless 

 the conversion of a-ketoglutaric acid to succinic acid were 

 reversible. From the best evidence to date it would appear 

 that this reaction is irreversible in higher animals, and until 

 this reaction is shown to be reversible another intermediary 

 compound must be postulated. ' 



The postulated unsymmetric four carbon intermediate 

 must take into account the finding of some activity in carbon 

 atoms numbered 3 and 5 in the experiment using carboxyl 

 labelled acetate. The low activity in carbon atoms 3 and 5 

 in conjunction with the high activity in the carboxyl groups 

 of haem (ClO, DlO) produced from carboxyl labelled acetate 

 can be explained by presupposing that the compound utilized 

 is derived in greater part from an unsymmetrical compound 

 and in lesser part from a symmetrical compound. A four 

 carbon atom unsymmetric intermediate arising from both 

 a-ketoglutaric and succinic acids would explain the findings 

 (Fig. 6). The detailed mechanism of the conversion of 

 a-ketoglutarate to succinate is at present unknown. It is 

 conceivable that a four carbon atom unsymmetric compound 

 may be an intermediate in this conversion. This compound 

 may be the semialdehyde of succinic acid or more likely a 

 succinyl coenzyme complex. The succinyl coenzyme com- 

 plex may be formed in a manner analogous to the formation 

 of acetyl coenzyme A from both pyruvate and acetate. 

 a-Ketoglutaric acid labelled in either the y-carboxyl group or 



