DISCUSSION ON PHOSPHORYLATION 191 



triphosphate and it is therefore of interest to discuss briefly the 

 problems relating to adenylpyrophosphate utilization. The utilization 

 of adenylpyrophosphate takes place by easily reversed reactions or 

 by irreversible degradation. The phosphorylation of carboxylate or 

 amidine ions represents easily reversed dephosphorylations of adenyl- 

 pyrophosphate. The phosphorylations of the hydroxy groups of 

 monohexoses or glycerol and the mineralization of phosphate repre- 

 sent the irreversible degradations of adenylpyrophosphate. These 

 irreversible strongly exergonic reactions are probably essential in the 

 energy transformations. 



The phosphorylation of glucose by adenylpyrophosphate has been 

 observed by Euler and Adler in brewer's yeast, and by Meyerhof 

 in baker's yeast and in extracts of animal tissue. Strangely enough, 

 the enzyme called hexokinase does not occur in muscle extracts. 

 This may be related to the fact that the concentration of free glucose 

 inside the muscle cell is very low. Perhaps phosphorylation of glu- 

 cose in the skeletal muscles takes place in the so-called cell mem- 

 brane and only there. I think that there is every reason to look for 

 the hexokinase enzyme in the water-insoluble residue. Recently 

 Colowick and I found that phosphorylation of glucose by adenylpy- 

 rophosphate when adenosinerf/phosphate is the phosphate donor 

 needed, besides hexokinase, another protein which occurred only in 

 skeletal muscle and was therefore called myokinase. Myokinase is 

 active in amounts smaller than one microgram per milliliter and is 

 extremely resistant to acid treatment. I have found recently that my- 

 okinase is also necessary for the dephosphorylation of adenosine- 

 diphosphate in muscle tissue. The experiments were carried out with 

 suspensions of myosin as described by Engelhardt and Ljubimova. 



The exact function of myokinase is not known. Dr. Johnson has 

 suggested that myokinase catalyzes a transfer of phosphate from 

 one molecule of adenosinediphosphate to another molecule, thus 

 forming from two moles of adenosinediphosphate one mole of adeno- 

 sinetriphosphate and one mole of adenosinemonophosphate. One 

 might also consider the formation of a dinucleotide between adeno- 

 sinetriphosphate and adenylic acid of the type that has been isolated 

 by Kiessling and Meyerhof. Myokinase might function not only as a 

 catalyst but also as a phosphate transfer system. The inhibition of 

 myokinase by adenylic acid may also indicate a reversed reaction 

 of myokinase""with adenosinediphosphate. 



Adenylpyrophosphate phosphorylates glucose or fructose to 



