Role of TPN in Control of Glycolysis 



241 



former is not affected by the concentration of TPN, these 

 facts were considered as an indication of an inhibition of 

 hexokinase by TPN. In another system, in which F-di-P was 

 used to reduce DPN, the addition of TPN also produced an 

 inhibition (Fig. 6). The inhibitions were of the non-competi- 

 tive type, as shown by the fact that the TPN effect depended 

 markedly on the amount of brain preparation used. This is 



TPN REDUCTION 

 (GLUCOSE SUBSTRATE) 



CONTROL 

 (L2;jM TPN) 



6pM TPN 

 SpM TPN 



01 0.2 0.3 0.4 0.5 



10% BRAIN SUPERNATANT (ml /cuvette) 



Fig. 7. Titration of the brain enzyme system by commercial 



TPN. The composition of the medium was similar to that 



used in Fig. 6 (left and central graphs). 



illustrated in Fig. 7 for the system that measures hexokinase 

 activity. 



The inhibition of the two different systems could be pre- 

 vented by the addition of cysteine or ethylenediaminetetra- 

 acetate, and also if the TPN was previously passed through 

 a Dowex-50 column, as shown i^ Fig. 6. These facts strongly 

 suggested that there was a heavy metal contaminating the 

 commercial preparations of TPN, that could explain the 

 inhibition of glycolysis. However, the effects observed in the 

 spectrophotometric studies were obtained with much higher 



