54 David Shemin and Jonathan Wittenberg 



compound. On examination of the structure of protopor- 

 phyrin and noting the quantitative distribution of ^^C among 

 the carbon atoms in the experiments, it can be seen that a 

 three carbon atom compound would satisfy the data as the 

 precursor of the methyl sides of the pyrrole units (carbon 

 atoms 6, 4 and 5) and the same compound would also be 

 consistent with the data as the precursor of the vinyl sides of 

 pyrrole units A and B (carbon atoms 9, 8 and 3), excluding 

 carbon atom 2, which is derived from the a-carbon atom of 

 glycine. However, it would appear that a four carbon atom 

 compound would be necessary as the precursor for the pro- 

 pionic acid sides (carbon atoms 10, 9, 8 and 3) of pyrrole 

 units C and D, again exclusive of carbon atom 2. If a three 

 carbon atom compound were utilized, subsequent carboxyla- 

 tions must have occurred on positions C9 and D9. On the 

 other hand, if a four carbon atom compound Avere utilized, 

 decarboxylations must have occurred on all 6 positions and 

 on positions A9 and B9 subsequent to pyrrole formation. It 

 can be decided which of these two alternative mechanisms 

 operates in the synthesis of protoporphyrin by correlating 

 some of the data obtained in the experiments using methyl 

 labelled and carboxyl labelled acetate. This correlation is 

 valid since the experiments were so carried out that the 

 synthesis of hsem from carboxyl labelled acetate proceeded 

 to the same extent as with methyl labelled acetate. 



The activities of carbon atoms ClO, DlO (1170 cpm) in the 

 carboxyl groups in protoporphyrin made from carboxyl 

 labelled acetate are equal, with the limits of error, to those of 

 carbon atoms C9, D9 (1128 cpm), adjacent to these groups, 

 in the porphyrin made from methyl labelled acetate (Fig. 4). 

 This equality, i.e. the same degree of dilution, makes it 

 appear that the acetic acid enters as a unit and that the 

 utilization of acetic acid for 2jyrrole formation is via a four 

 carbon atom compound. If ClO and DlO had been introduced 

 by carboxylation the activity of these carbon atoms would 

 have been much lower. Moreover, it has been shown that 

 carboxyl labelled acetate gives rise to labelled carbon dioxide 



