47fi 



L. nor.OTlAD 



cules of PBG is one which could give rise to uroporphyrin I (Fig. ]). 

 The tetrapyrroHc precursor of this porphyrin could be visualized as 

 being formed by the linear condensation of four PBG molecules, 

 followed by a head-to-tail condensation of the linear tetrapyrrole, 

 leading to cyclization. ITroporphyrin I has been found in nature but 

 thus far only in abnormal situations like congenital porphyria in man 

 and its equivalent in other animals; in these cases the compound is 

 excreted by the organism. 



COOH 



I 



c=o 



CNHo 

 "2 



DAL 



COOH 



CH 



I 



CH, 



CH_ COOH 



H 



a. at.' 



Ac 



CHjNH^ 



H 



PORPHOBILINOGEN 



HC . CH 



P H P 



Uropopphypin 111 



Ac « -CHg-COOH 



p = -CH2-CH2-C00H 

 Fig. 1. 



HC 



Ac H p 



Upopopphypin I 



In the biosynthetic chain of chlorophyll and protoporphyrin the 

 most probable first tetrapyrrole is one which could give rise to 

 uroporphyrin III (Fig. 1). This compound could not be formed as a 

 product of the linear condensation of four molecules of PBG. Thus 

 this is the "fork in the road" in the utilization of PBG — one branch 

 leads to a product which is useful to the organism as a raw material 

 for the synthesis of porphyrins important in its metabolism; the other 

 leads to a product which, so far as is now known, is of no use to the 

 organism. 



Porphobilinogen deaminase has been purified from aqueous ex- 

 tracts of spinach-leaf acetone powder; this enzyme catalyzes the con- 

 sumption of PBG. (The quantitative estimation of PBG is accom- 

 plished by the Ehrlich test in which the concentration of a complex 

 between the pyrrole and p-dimethylaminobenzaldehyde is measured 

 colorimetrically. The p-dimethylaminobenzaldehyde couples with the 

 pyrrole at the unsubstituted a-position; thus a "consumption of 

 PBG" really means a decrease in the number of a-positions capable 

 of coupling with the p-dimethylaminobenzaldehyde.) Concomitantly, 



