24G 



Essays in Biochemistry 



tate are equal to those found in the carbon atoms (1130) adjacent to 

 these groups in the porphyrin synthesized from methyl-labeled acetate 

 (Fig. 4). This equality, i.e., the same degree of dilution, demonstrates 

 that the acetic acid enters as a unit and that the utilization of acetic 

 acid for pyrrole formation is via a 4-carbon-atom unsymmetrical com- 

 pound. Therefore the common precursor pyrrole originally contained 

 acetic and propionic acid side chains in its (3 positions; the methyl 

 groups in the porphyrin arose by decarboxylation of the acetic acid 

 side chains, and the vinyl groups arose from decarboxylation and de- 

 hydrogenation of propionic acid side chains. 



The data obtained in these experiments can readily be explained by 

 assuming the participation of the tricarboxylic acid cycle in porphyrin 

 formation. In the light of the relative distribution of the C 14 activities 

 among the carbon atoms of the porphyrin derived from acetate, it 

 appeared that the acetate was converted to the 4-carbon unsymmetrical 

 compound via this cycle. The entrance of methyl-labeled acetate in 

 the citric acid cycle can give rise to a 4-carbon-atom compound, derived 



Table 1. Relative Distribution of C 14 Activity in Carbon Atoms of 



a-Ketoglutaric Acid Resulting from Utilization of C 14 -Labeled Acetate 



in Tricarboxylic Acid Cycle * 



The results are expressed in counts per minute. 



