1 8 OLOV LINDBERG et al. 



(c) ATP-SPLITTING REACTIONS OF NON-MITOCHONDRIAL ORIGIN 



At this Stage is was of interest to test the effect of desaminothyroxine 

 on ATP-spHtting reactions of non-mitochondrial origin. Myosin ATPase, 

 muscle myokinase, potato apyrase, and yeast hexokinase, were all un- 

 affected by a concentration of desaminothyroxine of io~^ m (Table V), 

 indicating that desaminothyroxine is not a general inhibitor of ATP- 

 splitting enzymes. A similar correlation was previously [89] reached 

 concerning atebrin and chlorpromazine. It may be of interest on the other 

 hand that a liver microsomal ATPase recently studied in our laboratory 

 [90] seems to be sensitive both to atebrin and chlorpromazine and to 

 thyroxine analogues. 



TABLE V 

 Effect of Desaminothyroxine on a Number of ATP-Splitting Enzymes 



The mitochondrial ATPase, myosin, potato apyrase, and hexokinase were 

 assayed in the manner described by Low [89]. Myokinase was assayed by measuring 

 the decrease in 7 min.-P in the presence of enzyme, hexokinase and glucose. 



(d) DIAPHORASE REACTIONS 



Previous work in this laboratory [76-78, 85] has given rise to the con- 

 cept that the mitochondrial ATPase reactions, both that induced by 

 dinitrophenol in intact phosphorylating mitochondria, and the Mg^ ^- 

 activated ATPase reaction appearing in structurally damaged mito- 

 chondrial preparations, involve the diaphorase flavoprotein as intermediate 

 phosphate carrier. A possible explanation for the sensitivity of these 

 reactions to thyroxine and related compounds would seem therefore to be 

 that these compounds interfere in some way with the mitochondrial 

 diaphorase. The Kielley and Kielley preparation proved to be a suitable 

 system for investigating this question, since it was found [53] that this 

 preparation exhibited besides a high Mg + +-activated ATPase activity 

 a DPNH diaphorase reaction also. It was found, moreover, that the dia- 

 phorase present in this preparation was an integral part of a mitochondrial 

 DPNH oxidase system, as indicated by its sensitivity to both amytal and 



