PHOSPHORYLATION OF CARBOHYDRATES 185 



triphosphate and a corresponding amount of fructose disappear, but 

 practically no glucose is formed. The formation of glucose is clearly 

 dependent on the addition of phosphatase. The conversion of fruc- 

 tose to glucose is another example of the utilization of oxidative 

 energy in the cell by way of the phosphate cycle. 



Table 7.— Conversion of fructose to glucose by a purified enzyme 



system 



(The complete system consisted of 5 mg. of hexokinase, 0.05 mg. of "muscle factor," 

 10 mg. of phosphatase, 0.2 rag. of Mg++, 3 mg. of fructose, 0.33 mg. of labile phosphate 

 (as adenosinetriphosphate), and 0.025 M veronal buffer of pH 7.5, in a total volume of 

 1.3 cc. Incubated 60 minutes at 37° C.) 



It may be emphasized at this point that all the reactions of the 

 phosphate cycle except the phosphorylation of pyruvic acid by 

 adenosinetriphosphate have now been shown to be reversible. When 

 lactic or pyruvic acid is converted to carbohydrate, phosphopyruvic 

 acid is apparently formed in an indirect way, probably from a four- 

 carbon dicarboxylic acid such as malate or fumarate, both of which 

 are assumed to be intermediates in the oxidation of pyruvate. Kalckar 

 (16) has shown that when malate or fumarate is added to kidney 

 extract under aerobic conditions, phosphopyruvic acid is formed. 



Regeneration of Inorganic Phosphate 



The formation of phosphopyruvate has just been mentioned. The 

 reverse reaction, the dephosphorylation of phosphopyruvate, was 

 originally shown to consist in a transfer of phosphate from phospho- 

 pyruvate to adenylic acid, with formation of pyruvate and adeno- 

 sinetriphosphate. The reaction proceeds rapidly with catalytic 

 amounts of adenylic acid, provided the adenylic acid is regenerated, 

 either by phosphate transfer from adenosinetriphosphate to some 

 suitable phosphate acceptor such as creatine, or by dephosphoryla- 

 tion of adenosinetriphosphate by adenylpyrophosphatase. It was 



