THE MECHANISM OF COENZYME Q REDUCTION IX HEART MITOCHONDRIA 263 



ZiEGLER : No, the cytochrome bound to the enzyme is, of course, not reduced 

 and the enzyme will not catalyze the reduction of a number purified cytochrome 

 h's we have tested. 



HoLTON : Could I ask you whether your conclusion that the succinate-Q 

 reductase does not reduce cytochrome b is based on a difference spectrum of the 

 reductase in the presence and absence of succinate ? 



Ziegler: This is one piece of evidence, yes. 



HoLTOX: Is it not just possible that your isolated reductase has cytochrome b 

 present in the reduced form already without there being succinate present, and 

 that is why the only change you show on addition of succinate is the reduction of Q. 

 It seems to me to be very odd that succinate does not reduce cytochrome b in the 

 presence of an enzyme which catalyzes the oxidation of cytochrome b by added 

 fumarate. 



ZlEGLER : No. This is not a possibility. Most of the spectra we have of this 

 enzyme are direct spectra and in no instance have we been able to keep the haem 

 in the reduced form. Cytochrome h is quite auto-oxidizable. 



Ch.\nce : This cytochrome b which comes along with the succinate-Q reductase 

 has an absorption band at an appreciably different wavelength from the cyto- 

 chrome b in the particle. I think it is very close to what Dr. Holton and I call 

 "inactive" cytochrome 6, because it is reduced only by dithionite, so I think it is 

 a little premature to say that the properties of this kind of cytochrome h identify 

 it with a particular pathway of electron transfer or phosphorylation. 



HoLTOX : Its reoxidation by fumarate indicates that this cytochrome b is in 

 direct connection with the succinate-fumarate system. 



Chance: But it may be by a different pathway. 



Ernster : The mechanism we have been thinking about in connection with 

 this activation of succinic oxidase in phosphorylating systems is this : 



Succ. '^ ^ Fps-^ 



^^ -\ Noii-p/iosp/iory/dting system 



phusplwrylating system ^ ATP "~-- 



DPNH . Fpi, ^ cyt. b > CoQ 



(Fps = succinic dehydrogenase; Fpi, = DPNH dehydrogenase.) 



Ziegler: I agree with Dr. Ernster that this is one possibility we have to 

 consider. However, as Dr. Chance pointed out, the haem attached to the enzyme 

 may have been modified during isolation of the enzyme since all of the bands have 

 been shifted to slightly lower wavelengths. 



Slater: Is this cytochrome b reduced by succinate in the presence of 

 antimycin ? 



Ziegler: No, it is not. 



Singer: Much is made in discussions of this type of the reduction of flavin as 

 measured at 450 or 460 m/t with or without a reference label. Perhaps I am merely 

 voicing Prof. Keilin's recent caution in stating that the reduction of flavin in 

 succinic dehydrogenase, etc., shuttles between the oxidized and reduced forms in 

 its normal catalytic action. Since the isolated dehydrogenase does not undergo 

 anything like a full bleaching even after activation by succinate and since its rate 



