BIOSYNTHESIS Or UROPORPHYRIN PREPTTRSORS 



481 



consumption of PBG occurs in the presence of the deaminase alone, 

 that this enzyme catalyzes the "linear" condensation of molecules of 

 the pyrrole with the splitting out of ammonia. It is possible that a 

 linear tripyrrole (e.g., Fig. 5) formed through the action of the de- 

 aminase on PBG (E-l, Fig. 5) constitutes one of the substrates for 

 the isomerizing enzyme which then introduces a molecule of PBG 

 at point "G" on the trip3Trole to form a tetrapyrrole of the con- 

 figuration shown (Fig. 5). A split at B of the "T" tetrapyrrole would 



Fig. 4. Tracing of chromatogram of methyl esters of uroporphyrins recovered 

 from expt. W lOB described in Fig. 3. Circles of dotted lines show positions of por- 

 phyrins at the end of the first development. Circles of solid lines show positions of 

 the porphyrins at the end of the second development. Numbers correspond to those 

 for treatments in Fig. 3; "UF' and "UIIF' are reference compounds, uroporphyrins 

 I and III, respectively. 



result in the production of two dipyrrolic compounds which could 

 condense with one another only to form a precursor of a type III 

 porphyrin. This precursor would also serve as a substrate for certain 

 decarboxylating enzymes and thus as a precursor of porphyrins with 

 fewer than eight carboxyl groups per molecule, including proto- 

 porphyrin IX and chlorophyll. In the absence of the isomerizing en- 

 zyme a fourth molecule of PBG would be added to the one end of the 

 tripyrrole and uroporphyrin I would ultimately be produced. This 

 is one hypothesis which may account for the observations reported 

 here. 



