BIOSYNTHESIS OF NUCLEOSIDES AND NUCLEOTIDES 311 



polynucleotides/ It will be seen from the discussion which follows that little 

 progress has been made in verifying this assumption. Indeed, some note- 

 worthy exceptions to this scheme have been observed, indicating that the 

 ring systems of the bases may be closed after combination with the sugar 

 phosphates. On the other hand, all nucleotides, nucleosides, and the con- 

 stituent structural units have been obtained by enzymic degradation of 

 nucleic acids, and it would be surprising if one were dealing here ^\^th arti- 

 facts caused by an idle play of enzymes without biological significance. 

 Provisional acceptance of both possibilities may be advisable. Future re- 

 search may decide in favor of one of the alternatives or establish multiple 

 pathways. At present, nucleosides and nucleotides \\'ill have to be dealt 

 with as intermediates of catabolism and perhaps of anabolism. 



II. Biosjnnthesis of Nucleosides 



Early studies of the enzymes resolving the glycosidic bond between ribose 

 (or deoxy ribose) and an imidazole or pyrimidine nitrogen were lacking in 

 conclusiveness because of the cumbersome and inaccurate analytical tech- 

 niques employed. The concept emerged that in degradation the phosphoric 

 acid ester linkage has to be split before nucleosidase action can take 

 place :^' * 



>N-pentose-P03H2 -^ >N-pentose + H3PO4 (1) 



(Nucleotidase action) 



>N-pentose — > >NH + pentose (2) 



(Nucleosidase action) 



Gradually, it became apparent that purine and pyrimidine nucleosides are 

 split by different enzymes and that there exists a variety of enzymes in each 

 category.^ 



1. Purine Riboside Phosphorylase 



Dixon and Lemberg' showed that not all purine nucleosides are split by 

 the enzyme or enzyme complex termed purine nucleosidase by earlier in- 

 vestigators. In particular, the enzyme isolated by them from milk is active 

 on inosine. These authors pointed out that the specificity of nucleosidases 

 is determined mainly by the noncarbohydrate part of the molecule. Levene 



* P. A. Levene and L. W. Bass, "Nucleic Acids." Chemical Catalog Co., New York 

 1931. 



5 H. von Euler and E. Brunius, Ber. 60, 1584 (1927). 



* W. Klein, in "Methoden der Fermentforschung" (Bamann and Myrback, eds.). 

 Academic Press, New York, 1945. 



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



