20 OLOV LINDBERG et al. 



with 2,6-dichlorophenoHndophenol as the terminal electron acceptor. At 

 the same time desaminothyroxine only shghtly inhibited the DPNH 

 oxidase activity obtained in the presence of a catalytic amount of cyto- 

 chrome c and the DPNH-cytochrome c reductase activity as measured 

 with cytochrome c as terminal electron acceptor. As shown in Table VI, 

 the sensitivity to desaminothyroxine of the diaphorase reaction was 

 roughly equal to that of the Mg + +-activated ATPase and that the des- 

 aminothyroxine sensitivity of the latter reaction was not influenced by the 

 presence of DPNH and cytochrome c. Conversely, addition of ATP and 

 Mg^ + to the DPNH-cytochrome c reductase system did not increase the 

 sensitivity of this system to desaminothyroxine. 



TABLE VI 



Comparison of Effects of Desaminothyroxine on DPNH Diaphorase, DPNH- 

 Cytochrome c Reductase and ATPase Activities of Mitochondrial Frag- 

 ments Prepared According to Kiellev and Kielley' [8o]. 



For experimental conditions see Figs. 2 and 9. 



",, inhibition 

 Reaction by 10 * M 



desaminothyroxine 



DPNH diaphorase (in presence of ATP and Mg + +) 81 



DPNH-cyt. c red. (in absence of ATP and Mg + +) 18 



DPNH-cyt. c red. (in presence of ATP and Mg + +) 24 



ATPase (in absence of DPNH and cyt. c) 84 



ATPase (in presence of DPNH and cyt. c) 76 



It would appear to follow from these data that the DPNH diaphorase 

 component of the amytal- and antimycin A-sensitive mitochondrial DPNH 

 oxidase, which probably represents the main phosphorylative pathway of 

 terminal electron transport in the intact liver mitochondria, is inhibited 

 by desaminothyroxine to the same extent as the mitochondrial ATPase 

 reactions. In contrast, the non-phosphorylating amytal- and antimycin 

 A-insensitive DPNH-cytochrome c reductase appears to be much less 

 sensitive to this agent. 



Another pyridine nucleotide oxidizing flavoprotein which shows a 

 relatively high sensitivity to thyroxine analogues is the so-called DT 

 diaphorase. This enzyme, the detection [96, 97] and purification [98] of 

 which was reported some time ago, and which now [53] appears to be 

 identical with the vitamin K reductase of Martins and collaborators 

 [99-102], catalyzes the oxidation of both DPNH and TPNH by various 

 dyestuffs and quinones. The enzyme occurs mainly in the soluble cytoplasm 

 but is present to a small extent also in mitochondria and microsomes 



