Function of Flavoenzymes in Electron Transport 

 and Oxidative Phosphorylation*! 



Lars Erxster 



Tlie Wcuner-Cjven Institute for Expen'inenta/ Biology, 

 L tiirersitv of Stocklio/iii, Szvedeu 



Questions concerned with reaction pathways and mechanisms bv which 

 flavoenzymes hnk substrate oxidation to the terminal respiratory chain 

 currently occupy an important position in the research field of mito- 

 chondrial electron transport and oxidative phosphorylation. In the present 

 paper two topics relevant to these problems are discussed. 



In the first section, data relating to the problem of involvement of 

 quinone reductases, and in particular of vitamin K reductase, in the 

 mitochondrial oxidation of reduced pyridine nucleotides will be presented. 

 Cogent evidence will be put forward that the dicoumarol-sensitive flavo- 

 protein, described by Martius and collaborators [14] under the name 

 phylloquinone reductase or vitamin K reductase, does not participate in 

 the main pathway of the mitochondrial oxidation of DPXH. It does 

 constitute the major pathway of direct oxidation of TPNH in the mito- 

 chondria, and presumably in the cell. Dr. Conover in this Svmposium will 

 present data bearing on this latter point [5]. 



Recently the concept was developed in our laboratory [6, 7] that the 

 aerobic oxidation of succinate in mitochondria involves an investment of 

 high-energy phosphate, i.e. a type of " activation reaction ". The basic lines 

 of evidence underlying this concept will be summarized by Dr. Azzone 

 later this afternoon [8]. Some recent data bearing on the relation of this 

 activation mechanism to the phenomenon of the succinate-linked reduction 

 of mitochondrial pyridine nucleotides, described some time ago by Chance 

 and HoUunger [9, 10] and by Klingenberg and co-workers [11, 12], are the 

 subject of the second section of this paper. 



* This work has been supported by grants from the Swedish Cancer Society 

 and the Swedish Aledical Research Council. 



t The following abbreviations are used: AcAc, acetoacetate ; AMP, ADP, 

 ATP, adenosine-5'-mono, di-, and tri-phosphate ; DCPIP, 2,6-dichlorophenolin- 

 dophenol; DPX.^DPXH, TPN, TPNH, oxidized and reduced di- and triphos- 

 phopyridine nucleotide; EDTA, ethylenediaminetetraacetate ; P,, inorganic 

 orthophosphate. 



