PORPHYRINS 



that its 5-carbon atom (originally the a-carbon atom of glycine) 

 is utilized for the synthesis of the ureido groups of purines, for 

 the j8-carbon atom of serine, and for methyl groups, while the 

 remaining four-carbon atom residue is reconverted to succinate. 

 Since this succinate-glycine cycle was arrived at from our study 

 of porphyrin synthesis, it may then be worth while to consider 

 the pertinent facts of porphyrin synthesis which led to the elab- 

 oration of this cycle, and the findings which lend their support 

 and broaden the implications of the postulated series of reac- 

 tions. Furthermore, data will be presented to support the 



■ <^ 



(F) 



Tricarboxylic Acid Cycle < 



(F) (^) 



(A) JB)^ 

 —^ a-Ketoglutarate ^ Succinyl derivative ^_ Succinate. >- 



+ Glycine 

 Pyrroles ^^K Protoporph\Tin 



Fig. 3. The relationship of the citric acid cycle and protoporphyrin formation. 



Statement that 5-aminolevulinic acid, one of the intermediates, is 

 formed from glycine and "active" succinate and is the source of 

 all the atoms of the porphyrin molecule. 



It has been previously demonstrated that although the four 

 nitrogen atoms of protoporphyrin are derived from glycine 

 (15,22,26,27,33) eight carbon atoms of the porphyrin molecule 

 are derived from the a-carbon atom of glycine (16,18,34); 

 four of these carbon atoms are still attached to the nitrogen atom 

 and four are not. The carbon atoms of protoporphyrin derived 

 from the a-carbon atom of glycine are shown in Figure 2 (34). 

 Furthermore, it has been found that the carboxyl group of 

 glycine is not utilized for any of the carbon atoms of the por- 

 phyrin (12,18). This latter negative finding was an important 



521 



