188 



A SYMPOSIUM ON RESPIRATORY ENZYMES 



pyrophosphatase), play an important role as regulators of the con- 

 centration of inorganic phosphate. 



Table 9.— Reactions of the phosphate cycle 



Glycogen+Phosphate 



it, 



Glucose + Phosphate^ 



Glucose-1 -phosphate 



it, 



Other "P" 



acceptors 



-Glucose-6-phosphate< Glucose < 



Fructose-6-phosphate< Fructose < — ATP— 



t .! Uf 



Fructose-l-6-diphosphate ADP<^ 



it -4H it 



2 Triosephosphate j AA <■ 



If +2 phosphate j 



>2 Phosphate 



2 Phosphoglycerate 



tt 



2 Phosphopyruvate 



IT 



Lactate;=^2 Pyruvate 



Phosphate 

 transfer - 



-8H 



+4 phosphate 



Summary 



This brief presentation of the phosphate cycle is of course far 

 from a complete picture. Its elements are the individual enzymatic 

 reactions. If an enzyme has been isolated from the tissues, if the 

 reaction product or products have been identified, if the kinetics of 

 the reaction and the role of coenzymes, activators, and inhibitors 

 are known, we are, I believe, on solid ground. Several of the indi- 

 vidual enzymatic reactions comprising the phosphate cycle have 

 been studied in this manner; others remain to be studied in greater 

 detail. 



It is also possible to combine a number of individual enzymatic 

 reactions in the test tube and to reproduce overall effects, such as the 

 polymerization of glucose to glycogen or the conversion of fructose 

 to glucose. The coupling between respiration and phosphorylation 

 has given us an insight into the mechanism of energy transfer in the 

 cell. There is still an essential element lacking in this picture, which 

 in the absence of a better definition might be called the regulatory 

 function of the cell. There can be no doubt that mechanisms exist 

 in the intact cell which regulate the rate and direction of individual 

 enzymatic reactions and which lead to a high degree of integration 



