DISCUSSION 143 



Hk. Arnon: I would like to :isk Dr. Vclick il In- would venture a guess on 

 the reasons for the spec ifu ity of TPN-linked triose phosphate dehydrogenases 

 in the photosynthetie reaitions. Isolated cidoroplasts reduce ll'N but not 

 OPN in light and so the triose phosphate dehydrogenase which is involved in 

 ilie reduction of PG.\ in light is always the TPN-linked enzyme. If we L;ike, 

 however, another photosynthetie organism, Chromatiurn, where we can show 

 that the pyridine nucleotide can be reduced with hydrogen in the dark, we 

 find that the classical, DPN-linked triose phosphate dehydrogenase is the 

 effective enzyme in PG.\ reduction, as is, of course, the case in all other 

 non-photosynthctic cells. There seems to be something peculiar about the 

 link between light and the TPN-linked enzyme. Is the reason to be sought 

 in the extra phosphate of TPN as contrasted with DPN? 



Dr. Vklick: I don't think the specificity lies in that extra phosphate group. 

 I think you will have to look for a teleological explanation. 



Dr. Arnon: Would you care to give it to me? 



Dr. Racker: Dr. Velick, before you do this may I point out that there are 

 several TPN-linked triose phosphate dehydrogenases that are found in non- 

 photosynthetic bacterial systems? 



Dr. Arnon: But why does it always apply to the photosynthetie systems? 



Dr. Arnon: Let us hear that last teleological comment before we adjourn, 

 Mr. Chairman. 



Dr. \'f.lick: I would say that in general the difference between TPNH and 

 DPNH in intermediary metabolism is that TPNH is utilized in the reduction 

 of organic substrates in animal metabolism and DPNH is in the oxidative 

 pathway through flavins and cytochromes to oxygen. There is a demarca- 

 tion of coenzyme for functional convenience and this is the way it happens 

 to come out in plants. 



