148 LARS ERNSTER 



It is obvious that any type of quinone specificity involved in the function 

 of DT diaphorase is a specificity on the acceptor and not on the donor 

 side. In other words, vitamin K may be the specific coupler of DT dia- 

 phorase to the cytochrome system in the living cell ; however, 7iot because 

 DT diaphorase requires vitamin K specifically but because cytochrome b 

 does. Whatever the reason for this requirement may be, it is clear that 

 the name "vitamin K reductase" for the flavoenzyme as such is hardly 

 adequate. 



Fig. 5. Quinone-catalyzed coupling between DT diaphorase and cytochromes. 

 NQ = naphthoquinone; BQ = benzoquinone; Me = methyl. 



CONCLUSIONS AND COMMENTS 



Figure 6 summarizes in a schematic form our conclusions as to the 

 relation of DT diaphorase to the main pathway of mitochondrial DPNH 

 oxidation. It seems to be clear from the data presented above that the 

 amytal-sensitive, DPNH specific pathway represents the main, if not 

 exclusive, route of DPNH oxidation in normal, intact rat-liver mitochondria. 

 The dicoumarol-sensitive DT diaphorase, which very probably is identical 

 with Martins and collaborators' vitamin K reductase, is present in these 

 mitochondria without any apparent functional link to the terminal electron 

 transport system. In order to establish such a link, the external supply of a 

 catalytic amount of vitamin K3 (or any other 2-methylnaphthoquinone 

 without a long carbon-chain substituent in the 3-position) is needed for 

 electrons from DT diaphorase to enter the respiratory chain at or before 

 the level of cytochrome b. Alternatively, DT diaphorase can be coupled to 

 the respiratory chain by way of naphthoquinones without a 2-methyl 

 substituent, and to some extent also by way of 2-methylbenzoquinones. 

 In these cases, external cytochrome c is required in addition to the quinone, 

 and the entrance of the electrons takes place beyond the antimycin A 

 sensitive site of the chain, probably at the level of cytochrome a. 



There are only two further brief comments I would like to add to these 

 conclusions. First, it should be pointed out that although Martius's 

 vitamin K reductase does not seem to participate in the main pathway of 



