608 G. SCHMIDT 



preparation from pigeon liver acetone powder on a mixture of ATP and 

 ribose-5-phosphate. 



Orotidyl nucleotide is decarboxylated by another enzyme fraction from 

 yeast to uridylic acid. It appears that the enzymic decarboxylation of orotic 

 acid requires its intermediate conversion to 5'-orotidylic acid. 



The complete enzyme system is therefore capable of converting adenine 

 or orotic acid in the presence of ribose-5-phosphate and ATP to 5'-ade- 

 nylic or 5'-uridylic acid, respectively. The presence of the complete enzyme 

 system in rat and pigeon liver has been demonstrated by Hurlbert and 

 Reichard.^i'^' 



Karger and Carter-'- obtained evidence for the enzymic conversion of 

 orotic acid to uracil by liver extracts and for the essential role of uridylic 

 acid as an intermediary step in this conversion. 



5. DPN Hydrolases 



A more detailed description of the enzymes which hydrolyze DPN to 

 nicotinamide and adenosine diphosphorylribose is beyond the scope of this 

 book since they are not enzymes of nucleic acid metabolism in the strict 

 sense. Two aspects, however, are of interest in correlation with the proper- 

 ties of the enzymes discussed in this chapter: (a) The group of DPN hydro- 

 lases is so far the only group of hydrolases known to cleave an A^'-glycoside- 

 like linkage in a compound whose structure resembles that of dinucleotides. 

 (b) Some DPN hydrolase preparations catalyze phosphate-independent 

 exchange reactions of the bound nicotinamide group of DPN. With regard 

 to these exchange reactions, the occurrence of three different types of DPN 

 hydrolases has so far been established: (1) DPN hydrolase of Neurospora 

 crassa was obtained as a soluble, highly purified preparation by Kaplan, 

 Colowick, and Ciotti.^^' It is competitively inhibited by nicotinamide in 

 relatively high concentration (1.45 X 10~^ M is required for 50 % inhibition) 

 and does not catalyze the exchange of the nicotinamide group of the sub- 

 strate with free nicotinamide. (2) DPN hydrolases of some animal organs 

 such as beef spleen, lung, and brain. The enzyme of beef spleen was obtained 

 as a particulate centrifugal fraction, the specific activity of which was ap- 

 proximately 1 % that of the purified Neurospora enzyme. It is inhibited to 

 a degree of 50% by a 1.5 X 10~^ M nicotinamide concentration. This inhi- 

 bition is noncompetitive with respect to DPN, and must be attributed to 

 the enzymically catalyzed exchange of the bound nicotinamide group with 

 free nicotinamide. The exchange was demonstrated with C'Mabelled nico- 

 tinamide.^'* (3) Whereas the action of some of the animal DPN hydrolases 



"ih R. B. Hurlbert and P. Reichard, Acta Chem. Scand. 8, 701 (1954). 



^"2 B. Karger and C. E. Carter, personal communication (1954). 



2" N. O. Kaplan, S. P. Colowick, and M. M. Ciotti, J. Biol. Chem. 194, 579 (1952). 



2'* L. J. Zatman, N. O. Kaplan, and S. P. Colowick, J. Biol. Chem. 200, 197 (1953). 



