128 



BRITTON CHANCE 



The succinate-linked mitochondrial pyridine nucleotide is postulated to 

 be separated from the remainder of the DPN by a compartment — possibly 

 the cristae and matrices of the mitochondria are involved. This com- 

 partmentation implies that electrons donated by a DPN-linked substrate 

 cannot readily enter the succinate-linked pyridine-nucleotide pool while 

 those from succinate can. 



Succinate -^ 



Fumarate 



Malate -^ 



3X~P+3I 

 3ATP+ 3X 



dbc 



Fig. 12. Schematic diagram of electron-transfer pathways in respiratory chain 

 involving succinate-linked pyridine-nucleotide reduction. This diagram is similar 

 to those presented earlier [i, i8, 19] and includes the quinone component [20]. 

 (Reproduced with permission of the Jotirnal of Biological Chemistry.) 



2. Energy-linked cytochrome oxidation 



Since the preceding considerations demonstrate reverse electron trans- 

 fer in a branch of the respiratory chain, we have actively considered the 

 possibility that flavoprotein may be oxidized in DPN reduction as indi- 

 cated in Fig. II, provided experimental conditions could be arranged so 

 that pyridine nucleotide was oxidized and flavoprotein reduced. A suitable 

 condition for this can be obtained by antimycin-A or quinoline oxide 

 inhibition of the respiratory chain, reinforced by hydrosulphide inhibition 

 of the oxidase. The plan for such an experiment is indicated by Fig. 8. If 

 electrons have already been transferred up to the level of cytochrome h 

 and flavoprotein, so that the flavoprotein involved in DPN reduction is 

 already reduced, then indeed addition of ATP should be all that is needed 

 to cause pyridine-nucleotide reduction with a concomitant oxidation of 

 flavoprotein. It has been observed in pigeon-heart mitochondria that 

 treatment of the aerobic suspension with 4 niM succinate and sufficient 



