ENZYMES ATTACKING NUCLEIC ACIDS 623 



HN^ ^CH +^0, HN "^CH. 



OC CH ^ OC^^/CO 



H H 



Uracil Barbituric acid 



HN^ ^C-CH3 +10, HN HC-CH, 



I II * I T 



OC^ /CH OC CO 



H H 



Thymine 5-Methylbarbituric acid 



Fig. 11. Bacterial oxidation of uracil and thymine. 







Fig. 12. Action of barbiturase. 



Hayaishi and Romberg succeeded in obtaining from cell-free extracts of 

 mycobacteria (obtained at pH 6.65) besides urease an enzyme which cata- 

 lyzes the cleavage of barbituric acid into urea and malonate. The enzyme 

 could be separated from most of the urease by purification with protamine 

 sulfate. In crude extracts, the urease acti\'ity could be selectively suppressed 

 by the addition of silver ions in 0.5 X 10~^ M concentration. 



Specificity. Barbiturase is a highly specific enzyme. 5-Methylbarbituric 

 acid, orotic acid, barbital, pentobarbital, 2-thiobarbituric acid, and iso- 

 barbituric acid are inert. 



pH Optimum. The optimal range of the activity is between pH 8 and pH 

 9. 



Michaelis Constant. The approximate Km value is 3.4 X 10^* M. 



The enzymic mechanism of the final degradation of 5'-methylbarbiturate 

 which is rapidly metabolized by the intact bacteria under aerobic conditions 

 is as yet unknown. 



X. Some Data Concerning the Intracellular Distribution of Enzymes of 

 Nucleic Acid Metabolism 



Owing to the significance of nucleic acids as constituents of vital struc- 

 tural elements of cells (chromosomes, nucleoli, mitochondria, Nissl bodies), 

 the intracellular distribution of the enzymes of nucleic acid metabolism is 

 of great interest and is discussed also in Chapters 18 and 21. The interpre- 



