PORPHYRINS 



c.p.m. and 487 X 10^ c.p.m., respectively. This finding demon- 

 strates experimentally the utilization of 2 moles of 5-amino- 

 levulinic acid for porphobilinogen formation. Further evidence 

 that porphobilinogen is an intermediate in protoporphyrin 

 synthesis was obtained by incubating equal volumes of the cell- 

 free extract of duck erythrocytes with equimolar amounts of 

 5-aminolevulinic acid (0.018 mc./mM) and with the enzymati- 

 cally synthesized radioactive porphobilinogen (0.036 mc./mM), 

 and subsequently isolating the hemin and determining its 

 radioactivity. The radioactivities of the hemin samples syn- 

 thesized from 5-aminolevulinic acid and from the porphobilino- 

 gen were 92 c.p.m. and 85 c.p.m., respectively, after a 2-hour 

 incubation, and 350 and 336 c.p.m., respectively, after a 15- 

 hour incubation period (21). This latter result is in agreement 

 with the findings of Falk, Dresel, and Rimington (7) and of 

 Bogorad and Granick (2). 



Although no evidence has yet been obtained concerning the 

 biological mechanism of conversion of the monopyrrole to the 

 tetrapyrrole structure, several suggestions have been advanced 

 (2,4). We would like to suggest still another possibility which 

 may explain the distribution of the a-carbon atom of glycine or 

 the 5-carbon atom of 5-aminolevulinic acid in the porphyrin 

 molecule of the I and III series. This mechanism is based on 

 the synthetic mechanism of dipyrrole and tetrapyrrole formation 

 demonstrated by Corwin and Andrews (5) and by Andrews, 

 Corwin, and Sharp (1). 



Condensation of 3 moles of the precursor pyrrole (porpho- 

 bilinogen) or of a closely related derivative, would lead to a 

 tripyrrylmethane compound, as schematically represented in 

 Figure 6. The tripyrrylmethane then breaks down into a 

 dipyrrylme thane and a monopyrrole. The structure of the 

 dipyrrylmethane is dependent on the place of splitting. An A 

 split would give rise to dipyrrylmethane A, and a B split would 

 give rise to dipyrrylmethane B. Condensation of 2 moles of 

 dipyrrylmethane A would give rise to a porphyrin of the I 

 series, and condensation of a mole of A and a mole of B would 



531 



