320 F. SCHLENK 



pyrimidine base, the formation of the pyrimidine deoxyriboside could be 

 followed by acid hydrolysis of the mixture. Any purine deoxyriboside re- 

 maining at the end of the incubation period is readily hydrolyzed, while the 

 pyrimidine deoxyriboside is relatively resistant and can be discovered by 

 paper chromatography or by its bacterial growth promoting ability.^^ 



With dialyzed enzyme from L. helveticus, and with hypoxanthine deoxy- 

 riboside as the donor of the carbohydrate moiety, thymine, uracil, cytosine, 

 and 5-methylcytosine accepted the deoxyribosyl group yielding pyrimidine 

 deoxyribose nucleosides. In every instance, more than half of the purine 

 deoxyriboside was converted to pyrimidine deoxyriboside if an excess of 

 the pyrimidine base was provided. Transglycosidation of deoxyribose from 

 one purine to another purine or from one pyrimidine to another was also 

 observed. Likewise, transfer of the deoxyribose from pyrimidine nucleosides 

 to purine bases was accomplished. The following compounds were found 

 reactive with thymine deoxyriboside: adenine, guanine, hypoxanthine, 

 xanthine, and 4-amino-5-imidazolecarboxamide. Uric acid and 2,6-diamino- 

 purine did not enter into the exchange reaction. All these observations were 

 greatly facilitated by the fact that the rather crude bacterial enzyme prep- 

 arations were relatively free from deoxyribo- (but not ribo-) nucleosidases 

 and from deaminases. The discovery of uracil deoxyriboside by McNutt^" 

 among the reaction products of this enzyme system deserves special con- 

 sideration. This compound may be a precursor of both cytosine and thymine 

 deoxy ribosides. 



In cases where transfer of deoxyribose from one purine to another or 

 from one pyrimidine to another is observed, a transglycosidation might be 

 simulated by deamination, transamination, oxidation, or reduction of the 

 base with the sugar remaining linked to it. Thus, the formation of adenine 

 deoxyriboside from adenine and hypoxanthine deoxyriboside could be the 

 result of a transamination^^ shifting the 6-amino group of the free adenine 

 to hypoxanthine deoxyriboside (equation a) or the result of transglycosida- 

 tion (equation b). These alternatives can be tested by the use of labeled 

 material : 



(a) Adenine-8-C*^ + hypoxanthine deoxyriboside ^ hypoxanthine-8-C''' -(- adenine 

 desoxyriboside. 



(b) Adenine-8-C'^ + hypoxanthine deoxyriboside ;=^ adenine-8-C'^ deoxyriboside -1- 

 hypoxanthine. 



Similar formulations are possible for other purines and pyrimidines and their 

 deoxyribosides. Kalckar and co-workers'*^ tested the alternatives formulated 



« E. HofiF-j0rgensen, Biochem. J. 50, 400 (1950). 



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



« H. M. Kalckar, W. S. McNutt, and E. Hoff-j0rgensen, Biochem. J. 50, 397 (1952). 



