Biosynthesis of Porphyrins 



61 



in both carboxyl groups would on decarboxylation yield a 

 succinyl derivative labelled only in the carboxyl group. This 

 compound in turn would be converted to succinic acid, a 

 symmetrical compound. If the latter reaction is reversible 

 (XOC-CHs-CHsCOOH^HOOC-CHa-CHs-COOH) the succinyl 

 derivative arising from the symmetrical succinate would 

 contain equal activity in the carboxyl group and in the other 

 terminal carbon atom. However, since the succinyl derivative 



14 



C OOH 



I 



CHo 



I 



CH2 



C = 



I 

 COOH 



14 

 C^OOH 



C"*OOH 



C'^OOH 

 I 



CH? 

 1 



CHp 

 I 

 COX 



C"^00H 



CH2 

 CH2 



' 14 



c'^ox. 



/ 



PYRROLES 



Fig. 6. Formation of succinyl derivative and distribution of ^*C 

 in this compound in experiment using carboxyl labelled acetate. 



is presumably formed more extensively from a-ketoglutarate 

 than it is from succinate, the carboxyl group of the pooled 

 intermediate arising from both processes would contain more 

 activity than the other terminal carbon atoms.* Two molecules 



♦According to the formulation of the functioning of the tricarboxylic cycle 

 in porphyrin formation, carboxyl labelled succinate cannot give rise to 

 labelled haem unless this reaction occurs. It has recently been found by Shemin 

 and Kumin that carboxyl labelled succinate produced labelled haem in which 

 pyrrole Rings A and B contained 40 per cent of the activity and Rings C and 

 D contained 60 per cent. The carboxyl group of Rings C and D contained one- 

 third of the activity of these rings. This distribution agrees with the theory 

 and therefore further supports the above reaction and the utilization of a four 

 carbon compound for porphyrin formation. 



