Coupling of Reduced Pyridine Nucleotide Oxidation to 

 the Terminal Respiratory Chain 



T. E. CONOVER* 



The Wenner-Gren Institute for Experimental Biology, 

 Lhiiversity of Stockholm, Szceden 



As was reported previously [9], a soluble enzyme has been isolated 

 which can couple the oxidation of both TPNH and DPNHf to the 

 reduction of a number of dyes and other electron acceptors. This enzyme, 

 which has been called " DT diaphorase " from its lack of specificity towards 

 the pyridine nucleotides, has been found in all cellular fractions examined, 

 but exists predominately in the cytoplasm. The properties of this enzyme 

 and its interaction with various quinones and with the respiratory chain of 

 mitochondria have been studied at some length. In this paper some of 

 these studies will be reported with the hope of drawing a possible inference 

 as to the role of this abundant enzyme in the cell. 



The importance of the level of extramitochondrial or cytoplasmic 

 reduced pyridine nucleotide in the control and regulation of metabolism 

 and svnthesis has been pointed out by Krebs [14], Dickens [6], and others. 

 The mechanism of the regulation of the levels of these reduced pyridine 

 nucleotides, however, remains incompletely understood. This is particularly 

 true in the case of TPNH, a substance which is essential for cellular syn- 

 thetic reactions. It is without question that a most important point in the 

 regulation of these levels of reduced pyridine nucleotide is the control of 

 their mitochondrial oxidation. Since the DT diaphorase reacts with both 

 DPNH and TPXH and is abundantly present in the cytoplasm, the ques- 

 tion of whether this enzyme functions in the oxidation of cytoplasmic 

 reduced pvridine nucleotide was carefullv considered. 



If freshly prepared mitochondria from rat liver were incubated with 

 soluble cytoplasm from rat liver prepared by centrifugation at 105 000 x g 



* Fellow of the National Foundation, New York, N.Y. Present address : 

 Division of Nutrition and Physiology, The Public Health Research Institute of the 

 City of New York, Inc., New York, N.Y. 



t Abbreviations : TPNH, reduced triphosphopyridine nucleotide ; TPN, 

 triphosphnpyridine nucleotide; DPNH, reduced diphosphopyridine nucleotide; 

 DPN, diphosphopyridine nucleotide; ATP, adenosine triphosphate; DCPIP, 

 2,6-dichlorophenolindophenol. 



