FRANK M. HUENNEKENS 



present discussion. Suffice it to say, even the most hardy co- 

 enzymes can be rent asunder by the concerted action of various 

 enzymes in the repertoire of the enzymologist. 



Enzymatic synthesis of coenzymes is a more complicated 

 affair, but satisfactory and accelerated progress is being made in 

 this area. The biosynthesis of vitamins and various other 

 nitrogen-containing heterocycles has been approached largely 

 with the aid of isotopic tracers. As an example of this, one may 

 cite the tour de force of Buchanan and his colleagues (9), who fed 

 various labeled molecules to pigeons, isolated and degraded the 

 excreted uric acid, and thus established that the purine ring has 

 carbon atoms 2 and 8 derived from formate, carbon atom 6 from 

 CO2, carbon atoms 4 and 5 and nitrogen atom 7 from glycine, 

 nitrogen atoms 3 and 9 from glutamine, and nitrogen atom 1 

 from glutamic or aspartic acids. Subsequent work, notably by 

 Buchanan and by G. R. Greenberg (16), has provided further 

 information as to the sequence in which these building blocks 

 unite and the nature of the intermediates preceding the intact 

 purine. For example, glycine amide ribotide has been proved 

 to be an intermediate, thus confirming the earlier finding that 

 the purine ribotide, rather than the free purine, was the molecule 

 initially synthesized. Likewise carbon atoms 2 and 8 are known 

 to serve as "ring-closers" under the influence of a folic-acid 

 coenzyme. It is to be hoped that these definitive studies will 

 encourage similar efforts on the biosynthetic pathways of other 

 important ring structures, e.g., riboflavin. 



The enzymatic synthesis of nucleosides takes place under the 

 influence of a phosphorylase, e.g. : 



nicotinamide + ribose-1-P ^ ^ nicotinamide riboside 4- P (8) 



or by the action of certain DPN-ase enzymes which catalyze the 

 reversible hydrolytic cleavage of DPN : 



DPN -|- H2O < nicotinamide + adenosine diphosphate ribose 



(9) 



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