Biosynthesis of Porphyrins 



57 



have the relative activities shown in Table V. If a-keto- 

 glutarate or an un symmetric compound derived from 

 a-ketoglutarate were utilized for hsem synthesis, after a finite 

 number of cycles most of the carbon atoms of the hsem would 

 contain ^^C. Moreover, the carbon atoms of the porphyrin 

 derived from the y-carbon atom of the a-ketoglutarate would 

 have the highest activity and the two adjacent carbon atoms 

 in the haem would theoretically have somewhat lower but 



Table V 



Relative Distribution of ^^C Activity in the Carbon Atoms of a-KETO- 

 GLUTARic Acid resulting from Utilization of i*C Labelled Acetate in 



THE Tricarboxylic Acid Cycle 



equal activities. It can be seen from Fig. 4 that a pattern 

 exists on both sides of each pyrrole unit consisting of three 

 carbon atoms, one with the highest activity and two adjacent 

 carbon atoms having somewhat lower activity. The com- 

 parable carbon atoms numbered 6 and 9 have the highest 

 activities and the carbon atoms numbered 4 and 5 on the 

 methyl side and 8 and 3 on the opposite side have somewhat 

 lower activities. However, the comparable carbon atoms 

 numbered 4 and 8 are slightly more active, on the average, 

 than those numbered 5 and 3. The inequality of activities 

 of these carbon atoms (numbered 4 and 8; 5 and 3) would 



