Biosynthesis of Cholesterol 51 



Wittenberg, 1951) that the total activity of the porphyrins 

 produced from ^*C methyl labelled and carboxyl labelled 

 acetate resides in the carbon atoms other than the methene 

 bridge carbon atoms, since the sum of the -total activities of 

 the methylethylmaleimide and haematinic acid samples is 

 equal to the total activities of the porphyrin samples. This 

 is consistent with the previous finding that the a-carbon 

 atom of glycine is the source of the methene bridge carbon 

 atoms. 



In Table III it can be seen that the total activity of pyrroles 

 A and B is equal to that of pyrroles C and D in haemin made 

 from methyl labelled acetate. This comparison holds for 

 pyrroles A and B and pyrroles C and D in haemin made from 

 carboxyl labelled acetate if one excludes, for the moment, 

 those carboxyl groups in C and D which are not found in A 

 and B. Also, as will be seen later, in both of these experiments 

 the carbon atoms of Rings A and B that occupy similar posi- 

 tions in Rings C and D have the same activities. These 

 findings support the previously suggested hypothesis that in the 

 biosynthesis of protor porphyrin a pyrrole is formed which is the 

 common precursor of both types of pyrrole structures found in 

 protoporphyrin. This was first suggested by Turner (1940-41) 

 but no evidence has heretofore been offered. 



Another conclusion concerning the mechanism of porphyrin 

 formation can be drawn from the activities of the keto-acid 

 fragments of the pyrrole rings. On cleavage of the pyrrole 

 rings in the methyl labelled acetate experiment the activities 

 of the pyruvic acid and a-ketobutyric acid samples of Rings 

 A and B are equal to those of Rings C and D (Table IV) 

 (Shemin and Wittenberg, 1951). The same relationship was 

 found to exist in the experiment using carboxyl labelled 

 acetate. On further degradation of these keto-acids it was 

 found that, not only did the same number of carbon atoms 

 contain ^*C, but the activities of comparable carbon atoms 

 were similar (Fig. 4). In the porphyrin made from methyl 

 labelled acetate, not only did the methyl group carbon atoms 

 (A6, B6, C6, D6) of each pair of pyrroles have similar activities, 



