ENZYMES ATTACKING NUCLEIC ACIDS 599 



thopterine, 6-hydroxymethyl pteridine, and 6-formyl pteridine) on the 

 enzjnnatic deamination of azaguanine were reported by Dietrich and 

 Shapiro. 1^^" 



pH Optima of Guanase. According to Roush and Norris, guanase de- 

 aminates guanine over a pH range between 5 and 9 with a rather flat opti- 

 mum at pH 8 ; the deamination of azaguanine has a much sharper optimum 

 at pH 6.5, and the activity decreases steeply at both sides of the optimum. 

 At pH 8 the activity toward azaguanine is only slight. 



Michaelis Constants. For Michaelis constants, Roush and Norris calcu- 

 lated for guanine as substrate Kjugu^nine = 5 X 10~^ M; for azaguanine, 

 ^i»/a^aguanine = 7 X lO"* M iu phosphate buffers of pH 6.5. The high dilution 

 of the substrates necessitated by their small solubilities, must be considered 

 in the evaluation of these constants. 



Assay Methods. Guanase may be assayed spectrophotometrically by de- 

 termining the changes of the extinction at 245 mju, or by determination of 

 the liberated ammonia. The latter procedure requires substrate solutions of 

 concentrations exceeding the solubility of guanine at the pH range of op- 

 timal activity. Sufficiently stable colloidal guanine suspensions can be 

 prepared by neutralizing guanine solutions in dilute sodium hydroxide in 

 the presence of 0.5 % gelatin. ^^^ 



3. Enzymes Deaminating the Cytosine Group 



Very little information is available regarding deaminases of the cytosine 

 group of nucleic acid derivatives. Highly active extracts of cytosine de- 

 aminase were obtained by Kream and Chargaff"^ from ground cells of yeast 

 and E. coli. The extracts had no activity toward adenine and guanine. 

 Cytidine and cytidylic acid (possibly after enzymic dephosphorylation) are 

 deaminated by extracts of mouse kidney."* 



4. Evidence for Transamination in the Purine and Pyrimidine Field 



One of the least explored questions in nucleic acid metabolism is that of 

 the formation of the amino groups of the nucleic acid purines and pyrimi- 

 dines. Rapid turnover rates for the amino group of the adenine compounds 

 of muscle were demonstrated by Kalckar and Rittenberg."^ Only a few 

 pertinent enzymic observations are so far available. 



Weil-Malherbe'*" reported that the formation of ammonia during the 



»^6» L. S. Dietrich and D. M. Shapiro, J. Biol. Chem. 203, 89 (1953). 



"7 E. Chargaff and J. Kream, /. Biol. Chem. 175, 993 (1948) ; J. Kream and E. Chargaff, 



J. Am. Chem. Soc. 74, 4274, 5157 (1952). 

 '^8 J. P. Greenstein, C. E. Carter, H. W. Chalkley, and F. M. Leuthardt, J. Nail. 



Cancer Inst. 7, 9 (1946). 

 '" H. M. Kalckar and D. Rittenberg, /. Biol. Chem. 170, 455 (1947). 

 '80 H. Weil-Malherbe, Biochem. J. 54, vi (1953). 



