600 G. SCHMIDT 



incubation of dialyzed brain homogenates increased considerably over the 

 sum of the amounts of ammonia in all controls when glutamine and inosine 

 triphosphate were added to the system. The most plausible explanation of 

 this observation is the assumption that the amide group of glutamine was 

 enzymically transferred to the hypoxanthine group of inosine triphosphate, 

 which was in turn deaminated by the enzymes of the homogenate. 



Stephenson and Trim^^"^ found that the rate of deamination of adenine 

 by E. coli suspensions was strongly accelerated by catalytic amounts of 

 adenosine. The authors discussed— very cautiously— the possibility of the 

 deamination of the added adenosine to inosine and of the subsequent trans- 

 amination of the amino group of adenine to inosine. It is just as reasonable, 

 however, to assume a phosphorolytic or nonphosphorolytic exchange of the 

 whole adenine molecule with the hypoxanthine group of inosine. 



According to Gunsalus and Tonzetich,^^! adenine, guanine, or cytosine 

 are amino donors for glutamate formation from ketoglutaric acid by cell- 

 free extracts of E. coli. Pyridoxal is required for the reaction; ammonium 

 ions are without effect. 



VI. Enzymes Acting on the Linkages Between the Basic and the 

 Carbohydrate Groups of Nucleic Acid Derivatives 



The enzymic cleavage of the linkages between the basic and the carbo- 

 hydrate groups of polynucleotide derivatives can take place either in the 

 free nucleosides or in pyrophosphoryl dinucleotides (e.g., DPN). So far, no 

 such cleavage has been observed on nucleic acids or polynucleotides. 



The enzymic cleavage of iV-glycoside bonds of nucleic acid derivatives 

 was first observed in nucleosides, and the term "nucleosidases" introduced 

 by Levene and Medigreceanu^ is still used.^*^ i^ the current literature this 

 name tends to be replaced by the designations "nucleoside phosphorylases" 

 and "nucleoside hydrolases." Those terms are more satisfactory because of 

 their adaptability to similar enzymes acting on nucleotides as substrates. 

 In the case of the latter enzymes, it might be advisable to change the term 

 "nucleotide phosphorylases" (proposed by Saffran and Scarano^^^) to "nu- 

 cleotide-l'-phosphorylases." Although the term "phosphorylases" still im- 

 plied (in analogy to Cori's original designation) enzyme reaction of glyco- 

 sidic groups with phosphate, this cannot be said for pyrophosphorylases. 

 In fact, most of the known nucleotide pyrophosphorylases act on groups 

 other than glycosidic groups. 



The reversible phosphorolysis of nucleosides offers a plausible pathway 



180a M. Stephenson and A. R. Trim, Biochem. J. 32, 1740 (1938). 

 1" C. F. Gunsalus and T. Tonzetich, Nature 170, 162 (1952). 



182 M. Dixon and R. Lemberg, Biochem. J. 28, 2065 (1934). 



183 M. Saffran and E. Scarano, Nature 152, 949 (1953). 



