Role of TPN in Control of Glycolysis 



237 



G-6-P is the starting substrate, the system becomes dependent 

 on inorganic phosphate concentration, as is shown in Fig. 4, 

 and arsenate could replace phosphate. This unexpected situa- 

 tion conforms to the conclusion that in a multi-enzyme 

 system a sequence of reactions may be interrupted by a 

 block in a related reaction, the oxidation of 3-phospho- 

 D-glyceraldehyde in the case we are dealing with, depending 



3. 



E 

 O 



< 



_l 

 < 



2.4h 



DPN REDUCTION WITH GLUCOSE AS A SUBSTRATE 

 f ADDITION OF ORTHOPHOSPHATE (pM) 



10 20 30 

 TIME (MINUTES) 



10 



Fig. 3. Influence of phosphate concentration on the reduction 

 of DPN by brain supernatant. Each Beckman cuvette con- 

 tained in a 3-0-ml. volume, 0-4 ml. b.rain-Sg, 12 y.M glucose, 

 9 [JLM ATP, 180 |j.M NaF, 6 jam DPN, 6[j,M-MgCl2, variable amounts 

 of potassium phosphate when indicated, 40 (jlm Tris buffer pH 

 7-46. 0-15 M-KCl to complete volume. 



on the equilibrium constants and the Michaelis constants of 

 the systems that interact. We interpret our finding as the 

 consequence of a block in the splitting of F-di-P, when pro- 

 duction of F-di-P is small, due to an accumulation of a very 

 small amount of 3-phospho-D-glyceraldehyde produced by the 

 absence of inorganic phosphate. The interpretation requires 

 a low affinity of the a-glycerophosphate dehydrogenase for its 

 oxidized substrate as compared with the affinity of aldolase 



