ENZYMES ATTACKING NUCLEIC ACIDS C19 



of purified uricase in the region of 9.2. Since uricase action is usually assayed 

 by determination of the oxygen uptake or by that of uric acid disappear- 

 ance, these observations refer to the oxidative phase of uricase action. Felix 

 ct al.;-^^ who worked with much cruder enzyme preparations, found the opti- 

 mum pH of the oxidative phase at pH 9.2 and that of the decarboxylation 

 phase at pH 9.9. Owing to the still problematic nature of the "decarboxyl- 

 ation reaction," the question of the pH optimum of the decarboxylation 

 requires further clarification. 



Inhibitors. Uricase action is inhibited to an extent of 94 % in presence of 

 1 X 10-^ M potassium cyanide, and to an extent of 83 % in the presence of 

 3 X IQ-^ilf potassium cyanide.-^^'-'^^-^'^^ The cyanide inhibition is com- 

 pletely reversed by dialysis.-"*^ 



Carbon monoxide is without influence on uricase according to David- 

 son.2*s 



Many heavy metals (Cu, Fe, Mn, Zn, Co, Ni) have considerable inhibi- 

 tory effects at concentrations in the range of 0.001 il/.'-" 



Pyrophosphates, fluorides, and urethan have no inhibitory effect. In the 

 presence of sodium azide the activity decreases by 28 % at pH 8 and by 40 % 

 at pH 6.8. 



Role of Heavy Metals. The possible significance of the zinc (0.1 %) and iron 

 (0.2 %) contents of purified uricase preparations is not yet clear.-**^ Prae- 

 torius concluded from the failure of British Anti-Lewisite (BAL) to inter- 

 fere with uricase action that the zinc content of uricase preparations has no 

 connection with their activity .^^^ 



IX. Enzymes Involving the Opening of the Pyrimidine Ring 



Intact animals metabolize administered uracil and thymine with the for- 

 mation of urea'^°-'^^ or of /3-amino acids"^'' (see also Chapter 26). The catab- 

 olism of pyrimidines in cell-free systems has so far been studied mainly on 

 bacterial enzymes. No observations on cell-free systems from animal sources 

 are as yet available, but the results obtained with tissue slices offer interest- 

 ing correlations with the mechanisms demonstrated for the action of bac- 

 terial enzymes. 



2" H. R. Fosse, Compt. rend. 191, 1153 (1930). 



268 R. Truszkowski, Biochevi. J. 24, 1340 (1930). 



269 E. Praetorius, Biochim. et Biophys. Acta 2, 590 (1948). 

 27» H. Steudel, Z. phrjsiol. Chevi. 32, 285 (1901). 



271 L. R. Cerecedo, J. Biol. Chem. 93, 269 (1931). 



272 L. B. Mendel and V. C. Myers, Am. J. Physiol. 26, 77 (1910). 



273 D. W. Wilson, J. Biol. Chem. 56, 215 (1923). 



274 H. Deuel, /. Biol. Chem. 60, 749 (1924). 



274a K. Fink, R. B. Henderson, and R. M. Fink, J. Biol. Chem. 197, 441 (1952). 



