Biosynthesis of Porphyrins 59 



Therefore the distribution of activities among the carbon 

 atoms of the porphyrin fits the utihzation of an unsymmetric 

 compound with a minimum of four carbon atoms, as concluded 

 above, rather than a three carbon compound. 



The finding that in the carboxyl labelled experiment the 

 activities of ClO, DlO are much greater than carbon atoms 

 numbered 5 and 3 eliminates many members of the tricar- 

 boxylic acid cycle, such as all the four carbon atom dicar- 

 boxylic acids, as immediate precursors. Once succinic acid, 

 a symmetrical compound, is formed, the two terminal or 

 carboxyl atoms would have equal activities. If succinate 

 or the fumarate, malate or oxaloacetate subsequently formed 

 were utilized, the carboxyl groups of protoporphyrin (carbon 

 atoms ClO, DlO) and carbon atoms 3 and 5 would have equal 

 activities in hsem made from carboxyl labelled acetate. Also 

 if any of these dicarboxylic acids were utilized, the carboxyl 

 groups of the porphyrin would have had the same activity 

 as carbon atoms C3 and D3 in the experiment in which methyl 

 labelled acetate was used. 



From all these considerations it would appear that two 

 molecules of an unsymmetrical compound arising from the 

 tricarboxylic acid cycle condense with glycine to form pyrrole 

 units containing acetic and propionic acid side chains (Fig. 7). 



Lemberg and Legge (1949) have suggested that 2 mols of 

 a-ketoglutaric acid condense with glycine with the elimination 

 of the a-carboxyl group of the keto-acid to form a pyrrole 

 bearing acetic acid and propionic acid side chains. Muir and 

 Neuberger (1950) have adopted the suggestion of Lemberg 

 and Legge with a modification. They believe that the 

 keto-acid condenses with hydroxyaspartic acid, since it has 

 been shown that eight a-carbon atoms of glycine are utilized 

 in porphyrin formation. However, this view is incompatible 

 with the distribution of the a-carbon of glycine in the por-. 

 phyrin (Wittenberg and Shemin, 1950). 



Our data also appear to eliminate a-ketoglutaric acid as an 

 immediate precursor of the porphyrin. The mode of utiliza- 

 tion of carboxyl labelled acetate in the tricarboxylic acid cycle 



