The Biosynthesis of Porphyrins '247 



from a-ketoglutarate, which would have a relative C 14 -distribution 

 pattern similar to that found in the porphyrin synthesized from methyl- 

 Labeled acetate. For example, if one starts with methyl-labeled acetate 

 with a relative activity of 10 in the methyl group, the a-ketoglutarate 

 formed on the first turn of the cycle would contain C 14 activity only in 

 the y-carbon atom and the relative activity would be 10 also (Table 1 i . 

 On formation of symmetrical succinate, the activities of the methylene 

 carbon atoms would be 5 and 5, and those of the oxaloacetate eventu- 

 ally formed would contain half of the C 14 activity of the y-carbon atom 

 of a-ketoglutarate. The recycling of this newly formed oxaloacetate 



(D) I 



(C) 

 + Glycine 



(E) 

 Pyrroles +~ Protoporphyrin 



Fig. 5. The relationship of the citric acid cycle and protoporphyrin formation. 



with the labeled acetate would now result in a specimen of a-keto- 

 glutarate having the relative activities shown in Table 1 for the second 

 cycle. In Table 1 the relative activities found in a-ketoglutarate 

 formed from methyl-labeled acetate are given after a number of cycles. 

 It can be seen that a 4-carbon-atom compound arising from a-keto- 

 glutarate after a finite number of cycles would have the same C 14 - 

 distribution pattern as is found in the 4-carbon-atom unit in the por- 

 phyrin synthesized from methyl-labeled acetate; three adjacent carbon 

 atoms are radioactive, and the one arising from the y-carbon atom has 

 the highest activity. The relationship between the citric acid cycle 

 and porphyrin formation is shown in Fig. 5. 12 



Direct documentation of these conclusions was obtained by studying 

 the utilization of C 14 -succinate, 14 C 14 -a-ketoglutarate, 15 and C 14 -labeled 

 citrate 15 for porphyrin formation. In each case the predicted carbon 

 atom of the porphyrin molecule contained C 14 . For example, a-keto- 

 glutarate-5-C 14 and primary carboxyl-labeled citrate produced the 

 same labeling pattern as was found for carboxyl-labeled acetate. 



The studies with C 14 -labeled succinate furnished direct evidence for 

 the participation of a 4-carbon compound in porphyrin formation and 



