MITOCHONDRIAL STRICTI RU AND FUNCTION lOI 



More recently my colleagues, Daniel Ziegler and Howard Tisdalc, and I have 

 been able to isolate DPNH dehydrogenase in the form of a lipid complex which is 

 at least ten times more active in enzymatic assays than the classical lipid-free de- 

 hydrogenase. The lipid-free DPNH dehydrogenase was previously shown by 

 Mahler and co-workers in our laboratory to react with cytochrome c and to contain 

 as prosthetic group a flavin dinucleotide other than flavin adenine dinucleotide. 

 It has been possible to show quite unambiguously that flavin adenine dinucleotide 

 is indeed the prosthetic group of the DPNH dehydrogenase. In the course of iso- 

 lation by the older method the flavin prosthetic group becomes modified, and 

 coincident with this modification of the fla\in the capacity to interact with cyto- 

 chrome c emerges. Apparently both of these properties are artifacts of prepara- 

 tion. More important, the physiological form of the dehydrogenase which con- 

 tains FAD as prosthetic group is linketl to lipid — one of the components of which 

 may well be the physiological electron acceptor for reduced flavin. 



I hope that my account so far has at least acquainted you with the broad 

 strategy of our efforts to deduce the structural pattern of the electron transfer 

 chain. Vitamin K, vitamin E, and carotenoids are other lipid components which 

 may eventually find a place in the electron transport system, though we ourselves 

 have made relatively litde contribution in that area. Vitamin E has been shown 

 by Nason (28) and Slater (5) to be present in mitochondria, and a requirement 

 for the vitamin in the oxidation of DPNH by cytochrome c can be shown with 

 isooctane-extracted particles according to Nason. A role for vitamin K in the 

 electron transfer chain of Mycobacterium phlei has been indicated by the work of 

 Brodie et al. (6). 



Martins was first to claim a role for vitamin K in oxidative phosphorylation re- 

 actions of animal mitochondria. However, it should be stressed that the occurrence 

 of vitamin K as such in animal mitochondria has not been established. An intrigu- 

 ing possibility is that coenzyme Q is either the functional form of vitamin K 

 which is operative in mitochondrial reactions, or that coenzyme Q plays a role 

 analogous to the role of vitamin K in microorganisms. 



Finally, I would like to remind you that it is no longer possible to entertam 

 seriously the classical picture of the electron transfer chain as a series of free-mov- 

 ing cytochromes interacting with one another serially by thermal collisions. The 

 cytochromes, with the possible exception of cytochrome c, are rigidly positioned in 

 the particle. They do not react directly with other cytochromes or flavoproteins, 

 and they are separated from one another by lipoproteins or lipid components. It is 

 presumably only the small molecules which react by thermal collision and even 

 this degree of freedom is limited by the spatial restrictions imposed by the lipid 

 core of the lipoproteins in which they are located. 



