Biosynthesis of Porphyrins 55 



in the system used and that radioactive carbon dioxide is not 

 incorporated into hsemin (Radin et at., 19506; Bufton, 

 Bentley and Rimington, 1948). 



As stated above, in the methyl labelled acetate experiment 

 the activities of the methyl groups (A6, B6, C6 and D6) and 

 the activities of the terminal carbon atoms of the vinyl 

 groups (A9, B9) are equal to that of the corresponding carbon 

 atoms of the propionic acid side chains (C9, D9). Therefore 

 the activities of A6, B6, C6 and D6 and A9, B9 are also equal 

 to that of the carboxyl groups of haem (ClO, DlO) made from 

 carboxyl labelled acetate (Fig. 4). It would appear, therefore, 

 from this distribution and the evidence of the utilization of 

 acetic acid as part of a four carbon atom unit, that in some 

 intermediate stage in the formation of protoporphyrin a pyrrole 

 or porphyrin was formed hearing carboxyl groups attached to the 

 four methyl carbon atoms and to the terminal carbon atoms of 

 the vinyl side chains-, in other words, it would appear that the 

 common precursor pyrrole originally formed contained acetic 

 and propionic acid side chains in its ^-positions (Fig. '7). 

 Although the evidence presented thus far makes it seem 

 highly probable that each of the four pyrrole rings bore two 

 carboxyl groups in some stage of synthesis and that a four 

 carbon atom compound was utilized for pyrrole synthesis, 

 more evidence will be furnished later for these conclusions. 



The data obtained from these acetate experiments can 

 readily be explained by assuming the participation of the 

 tricarboxylic acid cycle in porphyrin formation. In the light 

 of the relative distribution of the activities among the carbon 

 atoms of the porphyrin derived from acetate, it would appear 

 that acetate was utilized via a member of the tricarboxylic 

 acid cycle. It has been demonstrated that the acetic acid 

 carbon atoms on entering the tricarboxylic acid cycle are the 

 direct source of the y-carbon atom and the y-carboxyl group 

 of a-ketoglutaric acid (Buchanan and Hastings, 1946; Wood, 

 1946; Lifson, Lorber, Sakami and Wood, 1948; Wood, 1948). 

 The use of a-ketoglutarate in the following argument is not 

 intended as evidence for the participation of this compound 



