ENZYMES ATTACKING NUCLEIC ACIDS 



571 



mononucleotides has been studied and these experiments were carried out 

 with mixtures of the 2'- and 3 '-isomers. 



Zittle"*^ found approximately 50 % inhibition in the presence of guanylic 

 acid in 0.04 M concentration. Adenylic acid inhibited less than guanylic 

 acid or than the mixture of the four nucleotides obtained by alkali hy- 

 drolysis from PNA. He defined the inhibition as noncompetitive, although 

 the range of substrate concentrations studied appears as too narrow to 

 permit this conclusion. He also found that commercial nucleic acid prep- 

 arations (particularly free ribonucleic acid) contained acid-soluble con- 



250 



250 



150 



100 



50 



7.0 



8,0 



pH 



5 10 15 



Time, min. 



Fig. 5. Fig. 6. 



Fig. 5. Effect of variation of substrate level on ribonuclease activity. [From Bain 

 and Rusch."] Beginning with the highest, the curves represent 200, 150, 75, 50, and 

 25 mg. of substrate per flask, respectively. 



Fig. 6. Effect of pH on the activity of ribonuclease I. [From Bain and Rusch.^^] 

 3.0-ml. reaction mixture containing 200 mg. of sodium ribonucleate, and varying 

 concentrations of sodium bicarbonate, together with 20 fig. crystalline ribonuclease. 



taminants which considerably inhibited ribonuclease action. This suggests 

 inhibitory effects of acid soluble ribooligonucleotides. The available data 

 must be considered as preliminary for reasons explained above. It should 

 also be mentioned that the manometric determination of ribonuclease 

 action, particularly in its initial phases, is not very accurate owing to the 

 necessity to apply considerable and complicated corrections for the reten- 

 tion of carbon dio.xide. 



The kinetic observations just discussed were made before the discovery 

 of cyclic nucleotides as intermediaries of the hydrolysis of PNA by ribonu- 

 clease. It is obvious that the understanding of the kinetics of ribonuclease 

 action requires supplementary data which will permit the interpretation of 



" C. A. Zittle, /. Biol. Chem. 160, 527 (1945). 



