HANS KLENOW 



The first enzyme which activates ribose-5-phosphate was found to be fairly heat 

 stable and was partly fractionated. This enzyme fraction was shown among a number 

 of sugars and sugar phosphate esters to utilize adenosine triphosphate in the presence 

 only of ribose-5-phosphate or fructose-6-phosphate (see Figure 3) (Scarano, 1953). 



All these experiments suggest that the nucleosides may to a great extent be by- 

 passed in the synthesis of the nucleotides, and that a special type of ribose phosphate 

 ester is an intermediate in this synthesis. Here it was natural to consider a ribose- 1, 



TIME IN MINUTES 



Figure 4. Phosphoribomutase activity in the presence of different amounts 

 of glucose- 1 ,6-diphosphate. 



Ribose- 1 -phosphate, 3 X io -3 m; magnesium sulphate, io -3 m; trihydrochloric 

 acid buffer, 2 X io -2 m, pH 7-3; 8-hydroxyquinoline, io -3 m; muscle enzyme, 

 60/xg. ; protein per ml.; glucose- 1 ,6-diphosphate, synthetic sample. O control; 

 # glucose-i,6-diphosphate2 X io -6 m; A glucose- 1 ,6-diphosphate 3 X io _5 m; 

 I I glucose- 1 ,6-diphosphate 8 X io -6 m. 



The 



5-diphosphate, as was already suggested by Leder and Handler (1951). 

 nucleotide formation should then proceed as follows: 



Ribose- 1, 5-diphosphate + base ^ nucleoside + orthophosphate. 

 The first indication of the existence of this di-ester was obtained from experiments 

 on the enzymatic conversion of ribose- 1 -phosphate to ribose-5-phosphate (Klenow, 

 1953). This reaction is analogous to the phosphoglucomutase reaction which was 

 shown by Gardini et al. (1949) to require glucose- 1,6-diphosphate as a coenzyme. 

 The mechanism of this reaction was found (Sutherland et al, 1949) to be the transfer 

 of the 1 -phosphate of the coenzyme to the six-position of glucose- 1 -phosphate, where- 

 by a new molecule of coenzyme and the reaction product, glucose-6-phosphate, are 

 formed. Therefore the possibility existed that the phosphoribomutase reaction pro- 

 ceeded in a similar way, i.e. that it required ribose- 1, 5-diphosphate as a coenzyme. 



74 



