242 



Essays in Biochemistry 



porphyrin, is synthesized. This conclusion was well supported by the 

 subsequent experimental results. 



It appeared reasonable to expect that, since the nitrogen atom of 

 glycine is specifically utilized for porphyrin synthesis, the carbon atoms 

 of this amino acid might also be involved. It was soon found that, 

 whereas the a-carbon atom of glycine is indeed utilized for porphyrin 

 synthesis/ 5 ' 9 the carboxyl group was not. 6 - 10 This latter negative find- 

 ing was an important clue in the elucidation of the mechanism by 



9CH 2 



II 

 6CH 3 8CH 



HC5 



V 



9CH. 



II ' 

 6CH 3 8CH 



I) 



6CH, 



C 



8CH, 



I 

 9CH 2 



10COOH 



HC/rf 



8CH0 6CH; 



I 

 9CH, 



I " 

 10COOH 



Fig. 1. Protoporphyrin IX. The above numbering system is the same as that 



previously employed. 8 ' 12 



which glycine and succinate condense. However, on incubation of duck 

 erythrocytes 11 with doubly labeled glycine (N 15 H 2 — C 14 H 2 — COOH), 

 it was found 7 - 9 that the dilution for the nitrogen atom was twice that 

 for the a-carbon atom; that is, for every nitrogen atom utilized, two 

 carbon atoms from the a-carbon atom of glycine entered the porphyrin 

 molecule. Therefore, it would appear that eight carbon atoms of the 

 porphyrin molecule arise from the a-carbon atom of glycine, since the 

 four nitrogen atoms of the porphyrin are derived from glycine. In 

 order to establish definitely that eight carbon atoms of the porphyrin 

 are indeed derived from the a-carbon atom of glycine, and, if so, to 

 locate the positions of these carbon atoms in the porphyrin molecule, 

 and to gain some insight into the mechanism of porphyrin synthesis, 

 we developed a chemical degradation procedure of protoporphyrin 

 whereby each carbon atom from a particular position in the porphyrin 

 could be unequivocally isolated 8 ' 12 (Fig. 2). On degrading protopor- 

 phyrin synthesized from glycine-2-C 14 it was indeed found that eight 

 carbon atoms are derived from the a-carbon atom of glycine. The 



