44 



David Shemin and Jonathan Wittenberg 



merits that glycine nitrogen was utilized for the formation of 

 all four pyrrole structures. In protoporphyrin (Fig. 2) two 

 of the pyrrole rings (A and B) contain methyl and vinyl side 

 chains and two (C and D) contain methyl and propionic acid 

 side chains, and it was conceivable that these two different 

 pyrrole structures are synthesized in the organism from 

 different compounds. To elucidate this point ^^N labelled 

 glycine was administered to a human and a duck (Wittenberg 

 and Shemin, 1949), and the resulting labelled h?em was 

 degraded in a manner such that Rings A and B were separated 

 from Rings C and D. It was found that the ^^N concentrations 

 of the porphyrin, pyrrole rings A and B and pyrrole rings C 

 and D were equal (Table I) (Muir and Neuberger, 1949; 



Table I 



Distribution of ^^n in Protoporphyrin Synthesized from ^^N Labelled 



Glycine 



(Wittenberg and Shemin, 1949) 



The results are expressed as atom per cent excess ^^N 



Wittenberg and Shemin, 1949). Therefore glycine nitrogen 

 is utilized equally well for both types of pyrrole units in 

 protoporphyrin. This finding suggested that a common 

 precursor pyrrole is first synthesized and that this pyrrole 

 derivative is subsequently utilized for both types of pyrrole 

 units found in the porphyrin. As will be seen, later work will 

 more than support this suggestion and will elucidate the 

 structure of the common precursor pyrrole. 



In this degradation and the subsequent procedures 

 developed to isolate each carbon atom of each pair of pyrrole 

 rings, the pyrrole units are isolated as pairs. Rings A and B 



