Pyridine Nucleotides in Mitochondria 



E. C. Slater, AI. J. Bailie and J. Bouman 



Laboratory of Physiological Chemistry, 

 University of Amsterdam, Xetherlands 



One of the experimental difficulties in studying the mechanism of 

 respiratory-chain phosphorylation is that the concentration of the high- 

 energv phosphate compounds, which must surely be intermediates in this 

 reaction, will be of the same order as that of the mitochondrial enzyme- 

 coenzvme system which catalyzes the reaction. This situation may be con- 

 trasted with that appertaining to glycolytic phosphorylation, where the 

 high-energy phosphate intermediate (diphosphoglyceric acid) can accumu- 

 late in amounts of the same order as the substrate concentration. 



This difficulty can be decreased by about one order of magnitude by 

 studying those components of the respiratory chain which are present in 

 the greatest concentration in the mitochondria, viz. the pyridine nucleo- 

 tides and ubiquinone (coenzyme Q). This lecture is concerned with the 

 pyridine nucleotides. 



It is only during the last lo years or so that the pyridine nucleotides 

 have been considered to be constituents of the mitochondria. In the 

 nineteen-thirties, diphosphopyridine nucleotide at least was thought of as 

 a dissociable coenzyme catalyzing anaerobic oxido-reductions in the 

 "soluble" fraction of the cytoplasm. The first indication that it was much 

 more firmly bound came from the isolation by Cori in 1948 of crystalline 

 phosphoglyceraldehyde dehydrogenase containing firmly bound DPN + 

 [i]. Even more important for the topic of this lecture was the observation 

 of Huennekens and Green [2] that rabbit-liver and rabbit-kidney " cyclo- 

 phorase" preparations, consisting largely of mitochondria, contained 

 considerable amounts of firmly bound pyridine nucleotide, sufficient for 

 maximal respiration in the absence of added pyridine nucleotide. The 

 amounts of the pyridine nucleotide were little changed after prolonged 

 incubation in the presence of substrates. The importance of the mito- 

 chondrial pyridine nucleotides was further stressed by Lehninger's [3] 

 observation that extra-mitochondrial DPNH was only very slowly oxidized 



