PHOSPHORYLATION OF CARBOHYDRATES 181 



was omitted, and it may be seen that no conversion of glucose-6- 

 phosphate to glycogen took place. 



Colowick and Sutherland have also been able to convert glucose 

 to glycogen in vitro by the addition of hexokinase and adenosinetri- 

 phosphate to the concentrated muscle extract. It can be shown that 

 in this case glucose is first converted to glucose-6-phosphate at the 

 expense of the labile phosphate groups of adenosinetriphosphate. 



Glucose-6-phosphate is thus an important intermediate of carbo- 

 hydrate metabolism; it is formed from glycogen via glucose-1- 

 phosphate and it can also be formed by direct phosphorylation of 

 glucose in position 6. The latter reaction is the so-called hexokinase 

 reaction which was described by Meyerhof (12) and v. Euler and 

 Adler (13). 



Transphosphorylation 



All known transphosphorylation reactions involve adenosinemono- 

 or adenosinediphosphate; these nucleotides act in catalytic amounts 

 as acceptors of phosphate from such substances as phosphopyruvate, 

 acetylphosphate, and 1,3-diphosphoglycerate and are thus converted 

 to adenosinedi- and triphosphate respectively. These polyphosphates 

 then serve in a second enzyme reaction as phosphate donors to a 

 number of organic molecules, such as glucose, fructose, mannose, 

 fructose-6-phosphate, glycerol, creatine, adenosine, and probably 

 others. The enzymes which transfer the phosphate group from the 

 polyphosphates are specific with respect to the acceptors. For ex- 

 ample, yeast contains enzymes that phosphorylate glucose, fructose, 

 mannose, and adenosine, but no enzyme that phosphorylates crea- 

 tine. 



Extract of skeletal muscle has little or no hexokinase activity. 

 Since this can hardly be true of intact muscle, it may be due to 

 destruction, inhibition, or poor extractability of the enzyme. Other 

 tissues, such as brain, heart, liver, kidney, and retina yield active 

 extracts. The hexokinase in these tissue extracts has not been 

 separated from other enzymes, but in the case of yeast such a 

 separation has been effected. 



Colowick and Kalckar (14) have recently studied the reaction 

 between adenosinetriphosphate and glucose with purified yeast 

 hexokinase. Only one of the labile phosphate groups of adenosine- 

 triphosphate is transferred to glucose; that is, the reaction products 

 that were identified are adenosinediphosphate and glucose-6-phos- 

 phate. When adenosinediphosphate is substituted for adenosine- 

 triphosphate, no reaction with glucose takes place, but when a heat- 



