Pfizer Handbook of Microbial Metabolites 440 



to (D) (reaction 5). The linear tetrapyrrole (B) shown is 

 the intermediate proposed by Wittenberg. 



The enzyme, uroporphyrinogen isomerase, acting on 

 porphobihnogen, yields uroporphyrinogen III (E) as its 

 first detectable product. Wittenberg proposed that the 

 function of this enzyme is to condense 2 molecules of (B) 

 (reaction 2), creating the cychc octapyrrole (C). Model 

 studies indicate that such an intermediate could fold and 

 undergo rearrangement, spontaneously or under contin- 

 ued enzyme influence, to yield 2 molecules of uropor- 

 phyrinogen III (E) (reaction 4). 



The over-all result of this reaction sequence would be 

 the interchange of the pyrrole moieties destined to form 

 rings D of the porphyrins between two tetrapyrroles, with 

 consequent reversal of the positions of the D rings rela- 

 tive to the other pyrrole rings of the tetrapyrroles. 



This hypothesis seems to be in accord with aU other 

 known evidence concerning porphyrin biosynthesis, and 

 it would account for their pecuhar asymmetry. Many ref- 

 erences to related work are cited by Wittenberg. It is 

 notable that appropriate dipyrrome thanes were not con- 

 verted to porphyrinogens or porphyrins by porphobilino- 

 gen deaminase. ^^ 



Vitamin 6^2 is the only vitamin produced exclusively by 

 microorganisms, although not all microbes are capable 

 of elaborating it. Most seem to form little more than 

 enough for their own slight requirements, the best or- 

 ganisms for primary production by fermentation being: 

 Streptomyces olivaceus, S. griseus, Propionibacterium 

 shernianii and Bacillus megatherium. 



The nucleus of vitamin Bj^ differs somewhat from that 

 of porphyrins and is called the corrin ring:^"' 



17 -15. 13 

 Corrin 



18 D. S. Hoare and H. Heath, Biochim. et Biophys. Acta 39 167 

 (1960). 



