BIOSYNTHESIS OF UROPORPHYRIN PRECURSORS 477 



a mole of ammonia is released for each mole of PBG consumed. 

 The reaction proceeds aerobically or anaerobically ; and, at the time of 

 exhaustion of the PBG, there is present in the reaction mixture a color- 

 less compound which does not react with p-dimethylaminobenzalde- 

 hyde, plus, usually, a small amount of porphyrin. 



This colorless compound may be spontaneously converted to 

 uroporphyrin I on standing in air (i.e., on oxidation), or it may be 

 converted to this compound by an enzyme of undetermined speci- 

 ficity which is also present in extracts of spinach-leaf acetone powder. 

 The enzymatic conversion proceeds via a short-lived intermediate 

 characterized thus far only by its strong absorption at about 500 m^. 



The uroporphyrin present in the reaction mixtures at the ter- 

 mination of the experiments accounts for 50% to 75% of the PBG 

 consumed ; the fate of the remaining PBG has not been determined. 

 The most active PBG deaminase preparations obtained to date have 

 been capable of catalyzing the consumption of approximately 1.3 

 /iM of PBG per milliliter per hour per milligram of protein, at 37°C. 

 The porphyrin produced was characterized as uroporphyrin I by its 

 absorption spectrum, by paper chromatography of the free por- 

 phyrin, by paper chromatography of the methyl ester, and by de- 

 terminations of the melting points of the uroporphyrin methyl ester 

 and of the methyl ester of coproporphyrin prepared by the partial 

 decarboxylation of the uroporphyrin (8). Crystals of the uropor- 

 phyrin methyl ester melted at 288° to 289°C., and those of the co- 

 proporphyrin methyl ester softened at 235 °C. and all birefringence 

 was gone by 252 °C. 



In contrast to the production of uroporphyrin I from the colorless 

 product of the action of PBG deaminase on PBG, very little of the 

 octacarboxylic porphyrin, uroporphyrin, can be reco^'ered after a 

 solution containing this colorless compound has been incubated with 

 a frozen and thawed preparation of Chlorella. Under these circum- 

 stances a tetracarboxylic porphyrin, coproporphyrin, is usually 

 found as the major product, and porphyrins with from seven to 

 three carboxyl groups per molecule have been identified on paper 

 chromatograms. These data indicate that this colorless compound 

 may serve as a substrate for the enzymes catalyzing the decarboxyla- 

 tion of precursors of porphyrins like coproporphyrin and protopor- 

 phyrin. That the colorless compound is not con\'erted to uropor- 

 phyrin prior to decarboxylation is indicated by the observation that 



