BIOSYNTHESIS OF NUCLEOSIDES AND NUCLEOTIDES 



317 



100% 



20 40 60 80 100 120 

 MINUTES 



Fig. 4. Assay of the enzymic hydrolysis of uridine by spectrophotometry;" 76 ng. 

 of uridine per ml.; -• — •-, phosphate; -A— A-, glycine; -O— 0-, arsenate buffer 

 (0.1 M, pH 7.0). Curves A, B, and C: 125, 63, and 32 ^g- of enzyme protein per ml.; 

 t = 26°. 



to such a generalization was reported by Carter.^^ He purified an enzyme 

 from plasmolyzed yeast by ammonium sulfate fractionation and found that 

 it degrades uridine to uracil and ribose (Fig. 4). Inorganic phosphate or 

 arsenate is not needed in this process, which goes to completion. The reac- 

 tion was measured by ultraviolet spectrophotometry as shown in Fig. 5. 

 A well-defined optimum for activity was found at pH 7.0 in phosphate, 

 glycine, and veronal buffers. The reaction follows first-order kinetics up to 

 83 % hydrolysis of the substrate. An excess of uracil and, to a lesser extent, 

 ribose was found inhibitory to the splitting of uridine. Adenosine, inosine, 

 guanosine, cytidine, and thymidine were not degraded by this enzyme. 

 Uridylic acid was not split, nor did it inhibit the degradation of uridine. 

 Reversibility of this process has not been reported, but Horecker^^ believes 

 that this may be due to the high concentration of water in the medium com- 

 pared with the phosphate concentration in phosphorolysis ; the equilibrium 

 constants for these two reactions may be nearly the same, but in presence of 

 such an excess of water the reversal cannot be observed. 



33 C. E. Carter, J. Am. Chem. Soc. 73, 1508 (1951). 



3^ R. D. Hotchkiss, J. Biol. Chem. 175, 315 (1948). 



36 H. M. Ploeser and H. S. Loring, J. Biol. Chem. 178, 431 (1949). 



36 B. L. Horecker, /. Cellular Com-p. Physiol. 41, Suppl. 1, 279 (1953). 



