l8o T. E. CONOVER 



LowENSTEiN : What I was going to ask Dr. Ernster earlier was whether the 

 TPN diaphorase from mitochondria is the same as transhydrogenases discussed 

 by Dr. Kaplan ? 



Ernster: No. 



LowENSTEiN : What is the difference ? 



Ernster: That it doesn't transfer hydrogen between TPN and DPN. Kaplan's 

 transhydrogenase is reported to be bound to the mitochondria (J. biol. Che?n., 205, 

 I (1953)) whereas about 95 °o of our enzyme is in the soluble cytoplasm {Biochem. 

 biophys. Res. Comni., 2, 88 (i960)). Furthermore, DT diaphorase is strongly 

 inhibited by dicoumarol. 



LowENSTEiN : Can you give a figure for the comparative rates of soluble TPN 

 diaphorase and mitochondrial transhydrogenase ? 



Ernster: The activity of DT diaphorase ranges between 30 and 100 /umoles 

 reduced pyridine nucleotide oxidized per min. and per g. rat liver (wet weight). I 

 don't know what the activity of transhydrogenase is if you measure it with TPN 

 and DPN and not with the analogues. 



Singer : I am wondering if you could refresh my memory on what compelling 

 reason there is to believe that the transhydrogenase activity, as measured by Kaplan 

 and others, can be ascribed to the action of the respiratory chain DPNH-dehydro- 

 genase ? I might add, to clarify my question, that while considerable transhydro- 

 genase activity follows the respiratory chain DPNH-dehydrogenase throughout 

 purification when the transhydrogenation of DPNH with DPN analogues is used 

 as an assay, no DPNH-TPN transhydrogenation at all is shown by the purified 

 enzyme. Thus the enzyme we have isolated is obviously not the one catalyzing 

 transhydrogenations observed in mitochondria and its fragment involving TPN. 



Lowenstein: It has been purified by Kaufmann and Kaplan and has been 

 found to remain intimately associated with the DPNH-electron transport system. 



