248 



Essays in Biochemistry 



also indicated that the succinyl intermediate is formed from succinate 

 as well as from a-ketoglutarate; that is, reaction C occurs (Fig. 5). 

 In Fig. 6 the labeling pattern which should be found in protoporphyrin 

 synthesized from carboxyl-labeled succinate is given. On degradation 

 of protoporphyrin synthesized from carboxyl-labeled succinate, it was 



C'OOH 



I 



CH, 

 I : 



CH, 

 I ' 

 C'OOH 



C'OOH 

 I 



CH, 

 I 

 CH 2 



C'OR 



+ Glycine 



C'OOH 



C'OOH CH 2 

 I I 



CH 2 CH2 



CH T C'OR 



I. ! 

 C'OR CH 2 COOH 



H 2 



10 C'OOH 

 I 

 7 C'OOH 9CH, 

 I I ' 



6CH, 8CH, 



COOH 



* Radioactive 

 carbon atoms 



positions 

 7 and 10 



9CH, 



I 

 10 C'OOH C'OOH 10 



Fig. 6. The position of succinate in protoporphyrin and the labeling pattern 

 obtained in protoporphyrin synthesized from succinate-l,4-C 14 . 



found that the indicated 10-carbon atoms contained the C 14 . 11 In order 

 to demonstrate that reaction C occurs, a study of the utilization of 

 both raethylene-labeled and carboxyl-labeled succinate for porphyrin 

 formation in the absence and presence of malonate was carried out. 

 Theoretically carboxyl-labeled succinate cannot produce labeled por- 

 phyrin by entering the oxidative pathway of the citric acid cycle 

 (reaction F, Fig. 5), for in this direction the 4-carbon-atom compound 

 formed from a-ketoglutarate would no longer contain any of the orig- 

 inal carboxyl groups of the labeled succinate. Therefore the formation 

 of labeled porphyrin from carboxyl-labeled succinate would be evidence 

 for the occurrence of reaction C. These theoretical considerations and 



