248 



Van R. Potter and Hermann Niemeyer 



results were in agreement with an inhibition of phospho- 

 hexoisomerase. 



If 6-PG plays any role in the mechanism of TPN action, one 

 has to show that there is an accumulation of 6-PG under our 

 experimental conditions. The prerequisite for such an accumu- 

 lation is present in the system, since the assays indicated that 

 the G-6-P dehydrogenase is more active than the 6-PG 



1.2 



E 

 ol.O 



fO 



>- 

 z 



UJ 



o 



^■^ 



o 



\- 



G-6-P: .113 O.U./MIN. 

 6-PG: .050 » (44%) 



GLUCOSE-6-PHOSPHATE 



SUBSTR. 



6-PHOSPHOGLUCONATE 

 (3/jM) 



^TIME (minute'^ 



Fig. 14. Glucose-6-phosphate and 6 phosphogluconic 

 dehydrogenase activities in brain supernatant fraction. 

 The curve indicated by "Glucose-6-phosphate" re- 

 presents the activity of glucose-6-phosphate dehydro- 

 genase, corrected for the activity on 6-phosphoglucon- 

 ate, as described by Clock and McLean (1954). 



dehydrogenase (Fig. 14). This relationship had also been 

 shown by Glock and McLean (1954) and more recently by 

 McNair Scott (1958). Preliminary experiments have shown 

 that 6-PG accumulates in the presence of TPN, but not in its 

 absence, with or without mitochondria. 



Discussion 



The experimental data support the conclusion that in the 

 brain system the presence of TPN can change the rate of 



