BIOSYNTHESIS OF NUCLEOSIDES AND NUCLEOTIDES 



PER CENT PER CENT 



313 



SYNTHESIS 



OSPLITTING 



-O- 



-o 



20 

 40 

 60 

 -|80 

 100 



15 20 

 MINUTES 



25 30 35 



Fig. 1. Splitting and synthesis of inosine.'^ Ordinates: Concentration of substrate 

 mixture in percentage of initial concentration; at the left, ribose-1 -phosphate + 

 hypoxanthine: -• — •-; at the right, inosine + phosphate: -O — -O-. 



the nucleoside.^* The optical changes which occur during the oxidation of the 

 base are visualized in Fig. 2. In the reactions, 



Hypoxanthine —* Xanthine -^ Uric Acid 

 Guanine -^ Xanthine —^ Uric Acid 



(5) 

 (6) 



the decrease in optical density at 248 m^ is attended by an increase first 

 at 270 m/x (xanthine), and then at 290 m^ (uric acid). This technique has 

 been applied in a micromodification to the investigation of brain tissue.^* 



The limited range of purine nucleoside phosphorylase is illustrated by the 

 lack of action, or insignificantly low effect, on such nucleosides as adenine 

 thiomethylriboside, isoguanosine, and hypoxanthine thiomethylriboside.'^ 

 Korn et a/.,^^'^* however, have recently reported that adenine may react 

 with ribose-1 -phosphate in the presence of a purified enzyme from beef 

 liver. Friedkin'* • found that some artificial nucleosides may also be split 

 by nucleoside phosphorylase.^^ 



" H. M. Kalckar, J. Biol. Chem. 167, 429 (1947). 



16 E. Robins, D. E. Smith, and R. E. McCaman, J. Biol. Chem. 204, 927 (1953). 

 '« M. L. Schaedel, M. J. Waldvogel, and F. Schlenk, J. Biol. Chem. 171, 135 (1947). 

 " E. D. Korn, F. C. Charalampous, and J. M. Buchanan, J. .4m. Chem. Soc. 75, 3610 



(1953). 

 "' W. J. Williams and J. M. Buchanan, J. Biol. Chem. 203, 583 (1953). 

 18 M. Friedkin, /. Biol Chem. 209, 295 (1954). 

 i8» I. Lasnitzki, R. E. F. Matthews, and J. D. Smith, Nature 173, 346 (1954). 



