COUPLING OF REDUCED PYRIDINE NUCLEOTIDE OXIDATION- 



ITS 



readily used as substrate by the slices. It was shown with the isolated 

 enzyme system that vitamin K.j-bisulphite may also be used in stimtilating 

 the mitochondrial oxidation of TPXH presumably because the bisulphite 

 portion is readily split otf. 



In yiew of the yarious possible oxidative pathways open to the DPXH 

 which would be generated from the oxidation of glucose it was felt that 

 the DT diaphorase system in the rat liver slice might better be demon- 

 strated with a substrate which would specifically generate TPXH. 



In Fig. 5 is shown the effect of dicoumarol on the Amytal-resistant 

 respiration of rat liver slices in the presence of citrate as substrate. It can 

 be seen that while there was some stimulation of citrate respiration by 



- 10 



D Without dicoumarol 

 Q With dicoumarol 



endcgep. citrcte citrate 



Vit K,-bi5. Vit. K-,-bi 



Fig. 5. The dicoumarol sensitivity of the Amytal-resistant respiration of 

 liver slices with citrate as substrate. Conditions as in Fig. 4. The amounts of the 

 additions were 25 /tmoles citrate, 5 x 10- /^mole vitamin Ky-bisulphite, i /umole 

 TPX, and 5 x lo"^ ^(mole dicoumarol. Time measured, 60 min. [13]. 



vitamin K3, there was very little dicoumarol sensiti\ity characteristic of 

 DT diaphorase. It was therefore apparent that unless there was an im- 

 permeability of the slice to dicoumarol that there must exist alternative 

 amytal-insensitive pathways of TPNH oxidation. One such possibility 

 considered was a trans-hydrogenation to DPX and subsequent oxidation 

 of the DPXH formed. It has been suggested by various workers [11, 15, 

 17] that the dehydrogenases which are able to react with both DPXH and 

 TPXH may, in the presence of their substrate, act as transhydrogenases. 

 If this is the case it might be assumed that lactic dehydrogenase con- 

 stitutes the major portion of the transhydrogenase activity in the cytoplasm. 

 On this basis the previous experiment was repeated on rat liver slices 



