Biosynthesis of Porphyrins 49 



thirty carbon atoms and containing four tagged carbon 

 atoms, is 2614 cpm. Therefore the four methene bridge 

 carbon atoms must have contained the remaining 2620 cpm. 

 The average activity of the individual radioactive carbon 



atoms in the rings was 654 cpm, i.e. ( — - — ) while the average 



activity of the methene bridge carbon atom was 655 cpm, i.e. 



( ). This demonstrated the equal utilization of the 



a-carbon atom of glycine for carbon atoms numbered 2 and 

 for the methene bridge carbon atoms and that eight carbon 

 atoms of protoporphyrin are derived from the a-carbon 

 atom of glycine (Fig. 3). 



Since glycine accounts for eight carbon atoms of proto- 

 porphyrin, the origin of the remaining twenty-six carbon 

 atoms remained to be determined. It was previously 

 demonstrated that on the administration of deuterioacetic 

 acid (CD3COOH) to a rat, the haemin isolated contained 

 deuterium (Bloch and Rittenberg, 1945). This indicated that 

 some of the side chain carbon atoms of the porphyrin were 

 derived from the methyl group of acetic acid, since these are 

 the only carbon atoms bonded to hydrogen. Subsequent to 

 this finding it was indeed demonstrated that both the methyl 

 group and the carboxyl group of acetate were utilized for 

 hsem formation (Muir and Neuberger, 1950; Radin, Rittenberg 

 and Shemin, 19506). It was found that methyl groups of 

 the porphyrin and the j8-carbon atoms of the pyrrole to which 

 they are attached are derived from the methyl group of acetic 

 acid, whereas the carboxyl group of acetate is utilized for the 

 carboxyl group of the porphyrin (Radin et ah, 1950&). 



In order to determine the extent of utilization of acetate for 

 porphyrin formation and to locate all the carbon atoms in the 

 porphyrin which may be derived from acetate, duck blood 

 was incubated separately with ^*C methyl labelled and with 

 ^*C carboxyl labelled acetate (Shemin and Wittenberg, 1951), 

 and the resulting labelled hsemin degraded as outlined above. 

 The activities of both samples of acetate were the same. The 



ISOTOPES 5 



