PHOSPHORYLATION OF CARBOHYDRATES 183 



"myokinase" by Colowick and Kalckar and which, when added in 

 catalytic amounts, enables hexokinase to transfer the labile phos- 

 phate group of adenosinediphosphate to glucose. It may be seen 

 that with adenosinetriphosphate as phosphate donor approximately 

 half of the labile phosphate disappears when hexokinase alone is 

 added, and that with the further addition of a few micrograms of 

 muscle factor almost all the labile phosphate disappears. With 

 adenosinediphosphate as phosphate donor no reaction with glucose 

 takes place until the muscle protein is added. 



In extracts of mammalian tissues, such as kidney, heart, and brain, 

 the hexokinase reaction is generally followed by a reaction between 

 adenosinetriphosphate and fructose-6-phosphate, yielding fructose-1, 

 6-diphosphate or Harden-Young ester. In the intact cell, however, 

 particularly in muscle, where this has been studied in detail, 

 fructose-6-phosphate does not react rapidly with adenosinetriphos- 

 phate. This is borne out by the fact that hexosemonophosphate, the 

 equilibrium mixture of glucose- and fructose-6-phosphate, is a nor- 

 mal constituent of muscle and that it can increase considerably 

 under certain experimental conditions without any increase in the 

 formation of lactic acid (15). This indicates that the reaction between 

 fructose-6-phosphate and adenosinetriphosphate in intact muscle is 

 a limiting factor as regards the rate at which lactic acid is formed 

 and carbohydrate is oxidized. It is not yet known whether the system 

 for the direct oxidation of glucose-6-phosphate which has been 

 found by Warburg in yeast is significant for mammalian tissues; it 

 would in any case lead to the formation of triosephosphate and 

 hence of pyruvic acid and thus join the main path of carbohydrate 

 breakdown. 



The reaction between fructose-6-phosphate and adenosinetri- 

 phosphate has not been studied in detail, and the enzyme that 

 catalyzes this reaction has not been purified. The reaction has been 

 regarded as irreversible. Lohmann (11), however, has reported that 

 muscle extract splits oflF phosphate from position 1 when fructose-I, 

 6-diphosphate and magnesium ions are added. Recent experiments 

 carried out in our laboratory with Dr. Ochoa have shown that 

 Harden-Young ester added to liver extract is converted in quantita- 

 tive yield to glucose. This involves dephosphorylation in position 1, 

 conversion of fructose-6- to glucose-6-phosphate, and splitting of the 

 latter by liver phosphatase to glucose and inorganic phosphate. We 

 have repeatedly convinced ourselves that liver phosphatase forms 



