Biosynthesis of Porphyrins 45 



and Rings C and D, since pyrrole A is identical with B, and 

 pyrrole C is identical with pyrrole ring D. However, the 

 conclusions which will be drawn from the data are concerned 

 with each of the carbon atoms of each of the pyrrole rings. 

 This appears to be valid since, as shown below, the biosyn- 

 thetic mechanism for the dissimilar pairs of pyrrole rings is 

 the same and it is reasonable to expect that each pyrrole of 

 each pair is made in the same manner. With this assumption 

 we may then conclude that all four nitrogen atoms of the 

 porphyrin are derived equally from the nitrogen atom of 

 glycine. 



Unless the glycine nitrogen atom were involved in porphyrin 

 formation by some sort of transamination reaction, it could 

 be predicted that the a-carbon atom would also be utilized 

 for porphyrin formation. Indeed it was later demonstrated 

 that the a-carbon atom of glycine is found in the porphyrin 

 (Altman, Casarett, Masters, Noonan and Salomon, 1948; 

 Radin, Rittenberg and Shemin, 1950a). However, carboxyl 

 labelled glycine did not form labelled hsem (Grinstein, Kamen 

 and Moore, 1948; Radin et al., 1950a). 



Incubation of duck erythrocytes with ^^NHg^^CHgCOOH 

 resulted in hsem containing twice as much ^*C as ^^N (Muir 

 and Neuberger, 1950; Radin et ciL, 1950a), indicating that 

 eight carbon atoms of the porphyrin are derived from the 

 a-carbon atom of glycine since the four nitrogen atoms are 

 known to be derived from this source. 



In order to locate the positions in the porphyrin of these 

 eight carbon atoms, methods then had to be developed 

 whereby one could systematically degrade the porphyrin 

 molecule. We have utilized methods by which one can 

 isolate unequivocally each carbon atom of each pair of 

 pyrrole rings from known positions in the molecules (Witten- 

 berg and Shemin, 1950; Shemin and Wittenberg, 1951). As 

 can be seen in Fig. 2, hsemin obtained from haemoglobin was 

 converted to protoporphyrin and the latter reduced to meso- 

 porphyrin. The mesoporphyrin was then oxidatively cleaved 

 with chromic acid to yield methylethylmaleimide from Rings 



