DAVID SHEMIN 



give rise to a prophyrin of the III series. In the formation of the 

 porphyrin of the III series it can be seen from Figure 6 that it is 

 necessary to lose a one-carbon atom compound, since there are 

 three aminomethyl side chains and only two are required to 

 condense the two dipyrroles to the porphyrin structure. If the 

 mechanism similar to that outlined in Figure 6 is concerned with 

 porphyrin synthesis, it would appear that this one-carbon atom 

 compound given off could well be formaldehyde. Consistent 

 with this idea is our finding (29) that on the conversion of por- 



Ac 



NH2CH2'\ 



/" 



/ \ CHgNHj 



Ac 



CH2-NH2 



'Acelic ocid side chom 

 Propionic acid side chain 

 °(- carbon atom of glycine and 



J - carbon atom of /{-aminolevulinic acid 



Fig. 6. A mechanism of porphyrin formation from the monopyrrole. 



phobilinogen to porphyrins either by heating under acid con- 

 ditions (32) or by enzymatic conversion in cell-free extracts 

 (23,29) formaldehyde was formed. This was established by 

 heating or incubating porphobilinogen, labeled with C^"* in 

 the aminomethyl group, and subsequently isolating radioactive 

 formaldehyde as the dimedon derivative. 



It would appear that on conversion of porphobilinogen to 

 porphyrins, formaldehyde from the aminomethyl group is 

 formed and that any postulated mechanism should take this 

 into consideration. It is difficult at present to establish the 

 structure of the intermediate tetrapyrrole compounds which 

 are formed prior to the formation of protoporphyrin. However, 

 we would like to suggest that these intermediate tetrapyrrole 

 compounds may be in a more highly reduced state, containing 



532 



