264 DANIEL M. ZIEGLER 



of bleaching is not commensurate with the flavin undergoing such a complete 

 cycle, although per mole of flavin the turnover number of the enzyme is exactly 

 the same as in intact phosphorylating mitochondria, I think we must entertain the 

 possibility that the enzyme shuttles between oxidized form and semiquinone and 

 not between the oxidized and the reduced form which make all such measurements 

 highly dubious. I think we should bear in mind that measurements at 450 m/x do 

 not indicate participation of flavoprotein in the respiratory chain. 



ZlEGLER : I agree with Dr. Singer that a considerable amount of reduction we 

 observe in the 450 m/x region is not due to the flavin. 



EsTABROOK : A few years ago you reported a very powerful inhibition by 

 propionyl-CoA indicating that it acts in the flavin region of the respiratory chain. 

 Is the iron reduced or not reduced, is the Q reduced or not reduced by propionyl- 

 CoA? 



ZlEGLER : We have not studied the effect of propionyl-CoA on non-haem iron 

 reduction. 



