192 A SYMPOSIUM ON RESPIRATORY ENZYMES 



hexose-6-phosphate. Sutherland, Colowick, and Cori have shown 

 recently that this ester can be converted to hexose-1-phosphate and 

 thus to starch or glycogen. 



The phosphorylation of hexosemonophosphate to hexosediphos- 

 phate has been observed in several tissues and also in muscle 

 extracts. This reaction is probably strongly exergonic, hke the hexo- 

 kinase reaction. 



Meyerhof and Lohmann have shown that the dephosphorylation 

 of adenylpyrophosphate to adenosinedi- or monophosphate and 

 orthophosphate is one of the strongest exergonic reactions in bio- 

 logical systems. The enzyme is present in large amounts in muscle 

 extracts but the myosin fraction contains also large amounts of this 

 enzyme, and Engelhardt and Ljubimova have found that even 

 highly purified myosin contains adenosinetriphosphatase. 



The utilization of adenylpyrophosphate by these irreversible reac- 

 tions probably determines the extent of numerous important oxida- 

 tions. The great question is how the irreversible utilization of 

 adenylpyrophosphate is regulated. The existence of adenylpyro- 

 phosphate in resting muscles with very low metabolism clearly shows 

 that the enzymes which catalyze the breakdown of pyrophosphates 

 must be in a more or less inactivated state and are fully activated 

 only under certain conditions. The state of structural proteins such 

 as myosin might determine the extent of the activation of the en- 

 zymes which catalyze the degradations of adenylpyrophosphate. 

 Discharged structural proteins might even be enabled to use the 

 large amount of energy liberated by pyrophosphate degradations for 

 recharging by participating in the transfer of phosphate. 



We do not yet know how irreversible enzymatic reactions are 

 regulated or how they are directed in order to transfonn chemical 

 energy into mechanic^ energy. It has become clear, however, that 

 the irreversible dephosphorylations of adenylpyrophosphate repre- 

 sent some of the most important degradations in biological systems. 



Otto Meyerhof, University of Pennsylvania: 



May I discuss three of the points that have been proposed by Dr. 

 Kalckar: 



1. Because of the specific structure of the living cell, the metab- 

 olism can be regulated differently from the metabolic processes in 

 extracts or other preparations of dead cells. At least one such dif- 

 ference can easily be explained by the insufiicient stability of the 

 extracted enzyme. I refer to the accumulation of hexosediphosphate 

 in yeast preparations in contrast to the balance of formation and 



