256 DANIEL M. ZIEGLER 



present in significant amounts in the isolated enzyme, we have determined, 

 by a chemical method, the oxidation-reduction state of the enzyme bound 

 non-haem iron in the isolated flavoproteins and a number of submito- 

 chondrial particles (Table III). 



In agreement with the earlier work of Massey [8], we have found that 

 the non-haem iron associated with the primary succinic fiavoprotein is not 

 reduced by succinate, but approximately 30"',, of the non-haem iron in the 

 isolated Q reductase is reduced by substrate. Succinate, but not DPNH, 

 reduces significant amounts of the iron in the succinic-cytochrome c 

 reductase particle prepared by the method of Green and Burkhard [9]. 

 This particle does not contain a functional DPNH chain and cannot 



400 



450 500 



Wavelength (m/i) 



600 



Fig. 2. Difference spectra (oxidized vs. reduced) of the succinic-CoQ reduc- 

 tase. The enzyme was first reduced with Umiting amounts of dithionite and then 

 fumarate (10 /xmoles/ml.) was added. 



catalyze the reduction of CoQ by DPNH ; whereas, in the DPNH cyto- 

 chrome c reductase particle [10] which is essentially free from the succinic 

 fiavoprotein, only DPNH reduces significant amounts of the non-haem 

 iron. Either substrate can reduce approximately the same amount of the 

 non-haem iron in ETP where both the DPNH and succinate electron 

 transport chains are intact. All the iron potentially reducible in these 

 preparations is reduced in a few seconds at 5". Our studies on the rates of 

 iron reduction, indicate that at 5' the non-haem iron is reduced as rapidly 

 as CoQ. 



Under the conditions given in Table III, the reduction of enzyme- 

 bound iron is strictly substrate-dependent. The possible non-specific 



