202 Discussion 



5 X 10"^ M, the percentage inhibition is related to the amount of vitamin 

 K reductase used. In other words, is it practically irreversible? 



Martins : It is a competitive inhibition. 



Potter : So it is not likely to be related to the amount of the enzyme. 



How does one explain the low yield of phosphate-bound energy with 

 TPNH as compared with DPNH? From your Fig. 1 it appears that it 

 would be much easier to get a low yield with DPN than with TPN. 

 Could vitamin K reductase act as a transhydrogenase? 



Martius: The transhydrogenase activity of vitamin K reductase is 

 under investigation. 



Lehninger: When I commented on the dicoumarol uncoupling at 

 three different levels, I suggested the possibility that the vitamin 

 K-containing substance may be "parasitic" along the chain at three 

 different points. You have said that this is not possible. Have you some 

 experimental reason for excluding a "parasitic" vitamin K-containing 

 carrier at each of the three levels? Supposing we had, at each of the 

 three coupling sites, the possibility that these vitamin-K-containing 

 "parasitic" molecules, which can change their oxidation-reduction 

 state, were in some kind of equilibrium with the real carriers, and that 

 the O-R potentials of the three different vitamin K derivatives might 

 fit those of the carriers at the coupling sites ; in other words, a kind of 

 condenser system. This could account for dicoumarol uncoupling at 

 three different sites. Recently Todd and his co-workers have speculated 

 that quinones might be involved in phosphorylation (Clark, V. M., 

 Kirby, G. W., and Todd, Sir A. (1958). Nature, Lond., 181, 1650). I 

 am just putting their suggestion together with the possibility of vitamin 

 K acting at three different sites. There may be some kind of sym- 

 pathetic "parasitism" between the O-R states of these three different 

 vitamin K compounds at three different sites here, which may be 

 connected mechanistically with phosphorylation. 



Martius : I do not believe that vitamin K acts in three different steps, 

 but only in the first step, because its reduction potential is between 

 cytochrome b and the flavoproteins. The naphthoquinone group is the 

 functional group of vitamin K. It must be this group which is oxidized 

 and reduced. 



Potter: In 1940 I proposed a definition of a hydrogen transport 

 mechanism, namely, that if it is a hydrogen transport mechanism it has 

 one system which reduces it, and a different system which oxidizes it 

 (1940, Medicine, 19, 441). The difference in your parasitic system, which 

 is analogous to a condenser, depends on whether there is only one 

 system that can oxidize it and reduce it. The crucial experiment for the 

 argument is whether or not the reduced vitamin K is readily oxidized 

 by cytochrome b. 



Martius : It is, and very rapidly. 



Potter : If it is, then you have fulfilled the requirements for hydrogen 

 transport, and it is not a dead-end street, which I gather is Prof. 

 Lehninger's definition of a parasite. 



Lehninger: The condenser analogy was unfortunate. I meant these 

 things to be in equilibrium, not necessarily in series, but in parallel; 



