ENZYMES ATTACKING NUCLEIC ACIDS 607 



Activity Determination. The activity of the hydrolytic uridine nucleosidase 

 can be determined spectrophotometrically by utilizing the facts that, at 

 pH 7, the molar extinction of uridine at 280 van is 3.5 X 10^ that of uracil 

 1.4 X 10^ or that in 0.01 A^ sodium hydroxide the molar extinctions of uri- 

 dine and uracil are 30 and 5.4 X lO'', respectively. 



c. Nucleoside Hydrolases of Lactobacillus pentosus 



Lampen and Wang'^'^ demonstrated that extracts of Lactobacillus pento- 

 sus contain nucleoside hydrolases catalyzing the cleavage of purine ribo- 

 sides (including uric acid riboside) and pyrimidine ribosides. The hy- 

 drolysis of the pyrimidine nucleosides was enhanced, that of the purine 

 nucleosides depressed, by phosphate and arsenate ions. The authors 

 demonstrated that the enhancing effects of phosphate and arsenate on the 

 cleavage of the pyrimidine nucleosides were caused by the stabilizing effect 

 of bivalent anions on the enzyme proteins, but not by the participation 

 of these ions in the enzyme reaction. The hydrolysis of the purine ribosides 

 was strongly inhibited by phosphate and arsenate ions. 



3. NUCLEOTIDE-I'-PHOSPHORYLASE 



The requirement of ribose-5-phosphate and of ATP for nucleotide syn- 

 thesis in pigeon liver was first recognized by Williams and Buchanan.^*^ 



The role of ribose compounds esterified with phosphoryl groups in the 1 

 as well as in the 5 position as intermediaries in the enzymic formation of 

 nucleotides was suggested by SafTran and Scarano'*' who found that a 

 phosphorylated ribose believed to be 1,5-ribose diphosphate was trans- 

 formed into 5'-adenylic acid by pigeon liver homogenates in presence of 

 C'*-labeled adenine. It is very possible, however, that the actual inter- 

 mediary in this and in similar reactions is 1-pyrophosphoryl, 5-phosphoryl- 

 ribose, which was isolated by Kornberg et al.^^^ (see next paragraph). 

 According to J. M. Buchanan (private communication to the author) this 

 substance is the intermediary involved in the enzymic synthesis of ino- 

 sinic acid in pigeon liver extracts in presence of ATP and ribose-5-phosphate. 



4. NUCLEOTIDE-I'-PYROPHOSPHORYLASE 



According to Kornberg et al.,^^^ a partially purified enzyme fraction from 

 yeast catalyzes the reversible condensation between adenine or orotic acid 

 and a pyrophosphoryl ribose compound of the probable strucure of 1-pyro- 

 phosphoryl-ribose-5-phosphate to 5'-adenylic acid or 5'-orotidyl nucleotide, 

 respectively. Detailed data regarding the equilibria and the kinetics of 

 these important enzyme reactions are not as yet available. The phosphoryl- 

 ated ribose compound was obtained by the action of a purified enzyme 



2"8 J. O. Lampen and T. P. Wang, J. Biol. Chem. 198, 385 (1952). 



