The Biosynthesis of Porphyrins 245 



pyrrole rings had the same C 14 -distribution pattern, support was ob- 

 tained for the suggestion that a common precursor pyrrole is first 

 formed. Furthermore, since both the methyl side of the pyrrole units 

 and the vinyl propionic acid sides of the pyrrole units had the same 

 C 14 -distribution pattern, it was concluded that each side of each pyr- 

 role unit is made from the same compound. It would appear that the 

 compound which condenses with glycine to form the pyrrole unit must 

 be either a 3- or 4-carbon-atom compound. On examination of the 

 structure of protoporphyrin and noting the quantitative distribution 



CH 3 C l4 OOH experiment C " H ., COOH experiment 



(lb) COOH (H70) (80) COOH @ 



I I 



CH 2 © V \^ VCH 2 ® 



© H 3 C CH 2 ® © H 3 C^>CH 2 © 



® feFf ® n 'If'' n 



H H 



Fig. 4. Average activities of comparable carbon atoms in all pyrrole units. The 



activities are given in parentheses. The pyrrole unit represented contains a car- 



boxyl group which is found only in rings C and D of protoporphyrin. 



of C 14 among the carbon atoms in the experiments, it can be seen that 

 a 3-carbon-atom compound would satisfy the data as the precursor 

 of the methyl sides of the pyrrole units (carbon atoms 6, 4, and 5) and 

 that the same compound would also be consistent with the data as 

 the precursor of the vinyl sides of the pyrrole units (carbon atoms 9, 

 8, and 3), excluding carbon atom 2, which is derived from the a-carbon 

 atom of glycine. However, it would appear that a 4-carbon-atom com- 

 pound would be necessary as the precursor for the propionic acid sides 

 (carbon atoms 10, 9, 8, and 3), again exclusive of carbon atom 2. If a 

 3-carbon-atom compound were utilized, subsequent carboxylations 

 must have occurred to give rise to the propionic acid side chains in 

 pyrrole rings C and D. On the other hand, if a 4-carbon-atom com- 

 pound were utilized, decarboxylations must have occurred to give rise 

 to the methyl and vinyl groups. It can be decided which of these two 

 alternative mechanisms operates in the synthesis of protoporphyrin by 

 comparing the data obtained in the experiments using methyl-labeled 

 and carboxyl-labeled acetate. The C 14 activities of the carboxyl groups 

 (1170 c.p.m.) in protoporphyrin synthesized from carboxyl-labeled ace- 



